This is theprint version ofLearning Python 3 with the Linkbot You won't see this message or any elements not part of the book's content when you print orpreview this page.

The original authors and contributors for the original text as of this writing are:

• Mike Challender
• Ryan Uebner
• David Ko (User:Davidko_barobo)
• Natalie Ryland
• Graham Ryland
• Aaron Cooper
• Dylan Besk

• Josh Cogliati (User:Jrincayc), jjcogliati-jan2007 AT yahoo.com
• Mitchell Aikens, LGIT, WSIT, AAS (User:msaikens)
• Kiah Morante (User:greenmanwitch)
• Elizabeth Cogliati
• James A. Brown
• Joe Oppegaard
• Benjamin Hell (User:Siebengang)

For Python programming, you need a working Python installation and a text editor. Python comes with its own editorIDLE, which is quite nice and sufficient for the beginning programmer. As you get more into programming, you will probably switch to some other editor likeemacs,vi or another editor.

The Python download page is:https://www.python.org/downloads/. The most recent version is Python 3.10.6 (as of 8th August 2022);Python 2.7 and older versions will not work with this tutorial. There are various different installation files for different computer platforms available on the download site. Here are some specific instructions for the most common operating systems:

To install everything you need to run this curriculum, which includes Python 3, Python 3 setup tools, and the Barobo Python package, make sure you have an internet connection and run these commands in a terminal window:

sudoapt-getinstallpython3python3-setuptoolspython3-numpyidle3sudoeasy_install3pybarobo

You will also have to add your user account to the "dialout" group. You can do this by using the command:

sudousermod-a-Gdialout$USER

The above command will add whatever user you are logged in as to the "dialout" group. If you wish to add a user account that is not the one that is logged in, replace "$USER" with the user name of the other account. You may have to log out and log back in for the change to take effect.

For other distributions of Linux, follow the instructions provided by either your distribution or on the Python website to getPython 3,NumPy, andPyBarobo installed.

Raspbian comes with Python 3 and numpy installed by default. To get the Raspberry Pi ready to control the Linkbot, make sure your Pi is connected to the internet and type the following commands:

sudousermod-a-Gdialout$USERsudoapt-getinstallpython3-setuptoolssudoeasy_install3pybarobo

You may have to log out and log back in again for the changes to take effect.

Starting from Mac OS X (Tiger), Python ships by default with the operating system, but you will need to update to Python 3 until OS X starts including Python 3 (check the version by startingpython3 in a command line terminal). Also IDLE (the Python editor) might be missing in the standard installation. If you want to (re-)install Python, get the MacOS installer from thePython download site.

Download the appropriate Windows installer (thex86 MSI installer, if you do not have a 64-bit AMD or Intel chip). Even if you do have a 64-bit chip, I would still recommend downloading the 32-bit version because some of the Barobo Linkbot libraries are pre-built for 32-bit system by default. Start the installer by double-clicking it and follow the prompts. Be sure to include the optional package "pip", which can come in handy for installing additional Python modules and add-ons.

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Once Python is installed and the path is configured, open a command prompt and type the following command:

pipinstallpybarobo

Seehttps://docs.python.org/3/using/windows.html#installing-python for more information.

The PATH environment variable is a list of folders, separated by semicolons, in which Windows will look for a program whenever you try to execute one by typing its name at a Command Prompt. You can see the current value of your PATH by typing this command at a Command Prompt:

echo %PATH%

The easiest way to permanently change environment variables is to bring up the built-in environment variable editor in Windows. How you get to this editor is slightly different on different versions of Windows.

On Windows 8: Press the Windows key and typeControl Panel to locate the Windows Control Panel. Once you've opened the Control Panel, select View by: Large Icons, then click onSystem. In the window that pops up, click theAdvanced System Settings link, then click theEnvironment Variables... button.

On Windows 7 or Vista: Click the Start button in the lower-left corner of the screen, move your mouse overComputer, right-click, and selectProperties from the pop-up menu. Click theAdvanced System Settings link, then click theEnvironment Variables... button.

On Windows XP: Right-click theMy Computer icon on your desktop and selectProperties. Select theAdvanced tab, then click theEnvironment Variables... button.

Once you've brought up the environment variable editor, you'll do the same thing regardless of which version of Windows you're running. UnderSystem Variables in the bottom half of the editor, find a variable calledPATH. If there is one, select it and clickEdit.... Assuming your Python root is C:\Python34, add these two folders to your path (and make sure you get the semicolons right; there should be a semicolon between each folder in the list):

C:\Python34C:\Python34\Scripts

Note:If you want to double-click and start your Python programs from a Windows folder and not have the console window disappear, you can add the following code to the bottom of each script:

print("Hello World")#stops console from exitingend_prog=""whileend_prog!="q":end_prog=input("type q to quit")

The Advanced Python 3 lessons before you are geared toward furthering your knowledge and abilities with programming using Python. The tutorials are designed to allow you to interact with the Linkbot robots by programming them to move in very specific ways. By seeing the Linkbot respond to your programming commands via code that you write, your abilities to use Python coding will motivate you to learn even more. First, you will just type in the code that is displayed in the tutorial to see how the Linkbot responds. Then, you will make your own changes to that code to play around with the Linkbot's abilities. The worst thing that can happen is that the program won't work and nothing will happen with the robot, so feel free to play with the code. When you are expected to type in code, it will look like this:

##Python is easy to learnprint("Hello, World!")

The reason that it will be formatted this way is to make code you need to type easy to distinguish from other text. Additionally, the code will be in color with different parts in different colors to help you see the code's distinct parts. When you enter code, it will not necessarily be in color, but it won't matter as long as you type it the same way that it is written here.

If the computer prints something out it will be formatted like this:

Hello, World!

(Note that printed text goes to your screen, and does not involve paper. Before computers had screens, the output of computer programs would be printed on paper.)

Please notice that these Advanced Python lessons are set up for Python 3, which means that most of the examples will not work in Python 2.7 and previous versions. Additionally, some of the extra lessons created by other programmers like you may not have been converted to Python 3. However, the differences between one version of Python and another are not really large, so if you learn how to code in one version, you should be able to read programs written for the other version without too much difficulty. At some point, you might want to look at the Non-Programmer's Tutorial for Python 2.6.

There will often be a mixture of the text you type (which is shown inbold) and the text the program prints on the screen, which would look like this:

Halt!Who Goes there?LinkbotYou may pass, Linkbot

These lessons will introduce you to the terminology or vocabulary of programming. For example, programming is also often calledcoding orhacking. By learning the special vocabulary of programming, you will be able to understand what programmers are talking about and sound like a programmer yourself.

Finally, it's very important for your success with these lessons that you have Python 3 software. If you don't already have the Python software, go tohttp://www.python.org/download/ and get the Python 3 version for your computer platform. Most likely if you are learning in a classroom, your teacher will already have done this for you beforehand. If not, download it, read the instructions, and install the program.

For Python programming, you need a working Python installation and a text editor. Python comes with its own editorIDLE, which is quite nice and sufficient for the beginning programmer. As you get more into programming, you will probably switch to some other editor likeemacs,vi or another editor.

The Python download page is:http://www.python.org/download. The most recent version is Python 3.4.2 (as of 18th October 2014);Python 2.7 and older versions will not work with this tutorial. There are various different installation files for different computer platforms available on the download site. Here are some specific instructions for the most common operating systems:

To install everything you need to run this curriculum, which includes Python 3, Python 3 setup tools, and the Barobo Python package, make sure you have an internet connection and run these commands in a terminal window:

sudo apt-get install python3 python3-setuptools python3-numpy idle3sudo easy_install3 pybarobo

You will also have to add your user account to the "dialout" group. You can do this by using the command:

sudo usermod -a -G dialout $USER

The above command will add whatever user you are logged in as to the "dialout" group. If you wish to add a user account that is not the one that is logged in, replace "$USER" with the user name of the other account. You may have to log out and log back in for the change to take effect.

For other distributions of Linux, follow the instructions provided by either your distribution or on the Python website to getPython 3,NumPy, andPyBarobo installed.

Raspbian comes with Python 3 and numpy installed by default. To get the Raspberry Pi ready to control the Linkbot, make sure your Pi is connected to the internet and type the following commands:

sudo usermod -a -G dialout $USERsudo apt-get install python3-setuptoolssudo easy_install3 pybarobo

You may have to log out and log back in again for the changes to take effect.

Starting from Mac OS X (Tiger), Python ships by default with the operating system, but you will need to update to Python 3 until OS X starts including Python 3 (check the version by startingpython3 in a command line terminal). Also IDLE (the Python editor) might be missing in the standard installation. If you want to (re-)install Python, get the MacOS installer from thePython download site.

Download the appropriate Windows installer (thex86 MSI installer, if you do not have a 64-bit AMD or Intel chip). Even if you do have a 64-bit chip, I would still recommend downloading the 32-bit version because some of the Barobo Linkbot libraries are pre-built for 32-bit system by default. Start the installer by double-clicking it and follow the prompts. Be sure to include the optional package "pip", which can come in handy for installing additional Python modules and add-ons.

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Next, you will need to install a driver, which is available here:Installer. Follow the link, download the file and run it.Once Python is installed and the path is configured, open a windows command prompt and type the following command:

pip install pybarobo

Seehttps://docs.python.org/3/using/windows.html#installing-python for more information.

The PATH environment variable is a list of folders, separated by semicolons, in which Windows will look for a program whenever you try to execute one by typing its name at a Command Prompt. You can see the current value of your PATH by typing this command at a Command Prompt:

echo %PATH%

The easiest way to permanently change environment variables is to bring up the built-in environment variable editor in Windows. How you get to this editor is slightly different on different versions of Windows.

On Windows 8: Press the Windows key and typeControl Panel to locate the Windows Control Panel. Once you've opened the Control Panel, select View by: Large Icons, then click onSystem. In the window that pops up, click theAdvanced System Settings link, then click theEnvironment Variables... button.

On Windows 7 or Vista: Click the Start button in the lower-left corner of the screen, move your mouse overComputer, right-click, and selectProperties from the pop-up menu. Click theAdvanced System Settings link, then click theEnvironment Variables... button.

On Windows XP: Right-click theMy Computer icon on your desktop and selectProperties. Select theAdvanced tab, then click theEnvironment Variables... button.

Once you've brought up the environment variable editor, you'll do the same thing regardless of which version of Windows you're running. UnderSystem Variables in the bottom half of the editor, find a variable calledPATH. If there is one, select it and clickEdit.... Assuming your Python root is C:\Python34, add these two folders to your path (and make sure you get the semicolons right; there should be a semicolon between each folder in the list):

C:\Python34C:\Python34\Scripts

Note:If you want to double-click and start your Python programs from a Windows folder and not have the console window disappear, you can add the following code to the bottom of each script:

print("Hello World")#stops console from exitingend_prog=""whileend_prog!="q":end_prog=input("type q to quit")

Go into IDLE (also called the Python GUI). You should see a window that has some text like this:

Python 3.0 (r30:67503, Dec 29 2008, 21:31:07) [GCC 4.3.2 20081105 (Red Hat 4.3.2-7)] on linux2Type "copyright", "credits" or "license()" for more information.    ****************************************************************    Personal firewall software may warn about the connection IDLE    makes to its subprocess using this computer's internal loopback    interface.  This connection is not visible on any external    interface and no data is sent to or received from the Internet.    ****************************************************************    IDLE 3.0      >>>

The>>> is Python's way of telling you that you are ininteractive mode. In interactive mode what you type is immediatelyrun. Try typing1+1 in. Python will respond with2.Interactive mode allows you to test out and see what Python will do.If you ever feel you need to play with new Python statements, go intointeractive mode and try them out.

Go into IDLE if you are not already. In the menu at the top, selectFile thenNew Window. In the new window that appears, type the following:

print("Hello, World!")

Now save the program: selectFile from the menu, thenSave. Save it as "hello.py" (you can save it in any folder you want). Now that it is saved it can be run. Just a Rookies note here, if you change a program, save it with a version notation likehelloV1.py that way you won't change your original program that worked.

Next run the program by going toRun thenRun Module (or if you have an older version of IDLE useEdit thenRun script). This will outputHello, World! on the*Python Shell* window.

For a more in-depth introduction to IDLE, a longer tutorial with screenshots can be found athttp://hkn.eecs.berkeley.edu/~dyoo/python/idle_intro/index.html

It is very useful to stick to some rules regarding the file names of Python programs. Otherwise some thingsmight go wrong unexpectedly. These don't matter as much for programs, but you can have weird problems if you don't follow them for module names (modules will be discussed later).

1. Always save the program with the extension.py. Do not put another dot anywhere else in the file name.
2. Only use standard characters for file names: letters, numbers, dash (-) and underscore (_).
3. White space ("") should not be used at all (use underscores instead).
4. Do not use anything other than a letter (particularly no numbers!) at the beginning of a file name.
5. Do not use "non-english" characters (such asä,ö,ü,å orß) in your file names—or, even better, do not use them at all when programming.

If you don't want to use Python from the command line, you don't have to, just use IDLE. To get into interactive mode just typepython3 without any arguments. To run a program, create it with a text editor (Emacs has a good Python mode) and then run it withpython3 program_name.

Additionally, to use Python within Vim, you may want to visitPython wiki page about VIM

If you are using Unix (such as Linux, Mac OS X, or BSD), if you make the program executable withchmod, and have as the first line:

#!/usr/bin/env python3

you can run the python program with./hello.py like any other command.

At some point in your Python career you will probably get stuck and have no clue about how to solve the problem you are supposed to work on. This tutorial only covers the basics of Python programming, but there is a lot of further information available.

First of all, Python is very well documented. There might even be copies of these documents on your computer, which came with your Python installation:

• The officialPython 3 Tutorial by Guido van Rossum is often a good starting point for general questions.
• For questions about standard modules (you will learn what this is later), thePython 3 Library Reference is the place to look at.
• If you really want to get to know something about the details of the language, thePython 3 Reference Manual is comprehensive but quite complex for beginners.

There are a lot of other Python users out there, and usually they are nice and willing to help you. Before you ask the Python user community a question, everyone will appreciate it if youdo a web search for a solution to your problem before contacting the community. This very active user community is organized mostly through mailing lists and a newsgroup:

• Thetutor mailing list is for people who want to ask questions regarding how to learn computer programming with the Python language.
• Thepython-help mailing list is python.org's help desk. You can ask a group of knowledgeable volunteers questions about all your Python problems.
• The Python newsgroupcomp.lang.python (Google groups archive) is the place for general Python discussions, questions, and the central meeting point of the community.
• Python wiki has alist of local user groups, you can join the group mailing list and ask questions. You can also participate in the user group meetings.

Lesson Information**To Be Added**Vocabulary:Python Editor: a program used to create a block of text that will be sent to Python and interpreted. In thisbook we recommend IDLE 3.Line: a single sentence of programming code.  Lines of code can be used to measure the size of a program andcompare it to other programs.Function: a type of procedure or routine, a set of instructions to follow.Argument: data provided to a function as input.  This is like thedomain in algebra.String: a sequence of characters representing either a constant or a variable.Function Call:Necessary Materials and Resources:Computer Science Teachers Association Standards: L1:3.CT.4:Recognize that software is created to controlcomputer operationsCommon Core Math Practice Standards: CCSS.MATH.PRACTICE.MP5 Use appropriate tools strategically.

Once you've completed this chapter, you will know how to edit programs in aPython Editor, IDLE 3, or some other text editor, save them, and run them.

Since the beginning of programming tutorials, "Hello World!"^[1] has been the first foray into creating something on the screen, here is how to do it with Python:

print("Hello, World!")

If you are using the command line to run this program, then type it in a text editor, save it ashello.py and run it withpython3.0 hello.py

Otherwise go into your Python Editor, create a new file (Ctrl + N for IDLE), and create the program asin sectionCreating and Running Programs.

When this program is run here's what it prints:

Hello, World!

Remember, whenever you see code as an example, it's a good idea for you to type it in and run it. Learning by doing can be a powerful tool! It is worth the time it takes.

Here is a modified 'Hello World' using the Linkbot to say hello. Comments are added to help you understand what the code is doing step by step.

importbarobo#loads 'barobo' moduleimporttime#loads 'time' moduledongle=barobo.Dongle()#uses commands from 'barobo' to connect Linkbotdongle.connect()robotID=input('Enter Linkbot ID: ')#prompts user for Linkbot IDrobot=dongle.getLinkbot(robotID)print('Hello Linkbot Programmer')#prints greetingrobot.setBuzzerFrequency(1047)#calls for pizo buzzer to make a tonetime.sleep(0.25)#sets duration of tonerobot.setBuzzerFrequency(1567)#changes tonetime.sleep(0.5)robot.setBuzzerFrequency(0)#turns off tone.

Feel free to chance the frequency of the buzzer to change the tone, and the duration of time.sleep to see what that does to the greeting.

Now here is are a few more program:

print("I pledge allegiance to the flag")print("of the United States of America,")print("and to the republic, for which it stands,")

When you run this program it prints out:

I pledge allegiance to the flagof the United States of America,and to the republic, for which it stands,

When the computer runs this program it first sees theline (like one sentence of code):

print("I pledge allegiance to the flag")

so the computer prints:

I pledge allegiance to the flag

Then the computer goes down to the next line and sees:

print("of the United States of America,")

So the computer prints to the screen:

of the United States of America,

The computer keeps looking at each line, follows the command and then goes on to the next line. The computer keeps running commands until it reaches the end of the program.

Linkbots are equipped with a multi-color LED, which is basically just a light that can change colors.Lets try a simple program to control the LED color on a Linkbot. To get started, follow these steps:

• Turn on your Linkbot by holding down the power button until the Linkbot flashes a bright red light.
• After about 5 seconds, the Linkbot will beep and show a blue light. Your Linkbot is now on!
• Take a Micro-USB cable and connect the Linkbot to your computer.
• Open your text editor and try the following code:https://en.wikibooks.org/wiki/Learning_Python_3_with_the_Barobo_Linkbot

importbarobomyDongle=barobo.Dongle()myDongle.connect()myLinkbot=myDongle.getLinkbot()myLinkbot.setLEDColor(0,255,0)

Template:Message boxWhen you run this program, you should notice that your Linkbot turned green! In the code that you wrote, there were three numbers: 0, 255, and 0. These three numbers determine the brightness of the red, green, and blue LEDs inside the linkbot, where 0 denotes the lowest brightness setting and 255 the maximum setting. If you want to try making your Linkbot a brilliant purple color, try setting the color numbers to "255, 0, 255". This tells the Linkbot to turn on its red and blue LED's to their maximum brightness. When the red light blends with the blue light, it appears purple!

Now is probably a good time to give you a bit of an explanation of what is happening - and a little bit of programming terminology.

What we were doing above was using afunction calledprint. The function's name -print - is followed by parentheses containing zero or morearguments. So in this example

print("Hello, World!")

there is one argument, which is"Hello, World!". Note that this argument is a group of characters enclosed in double quotes (""). This is commonly referred to as a string of characters, orstring, for short. Another example of a string is"Jack and Jill went up a hill". The combination of a function and parentheses with the arguments is afunction call.

A function and its arguments are one type of statement that python has, so

print("Hello, World!")

is an example of a statement. Basically, you can think of a statement as a single line in a program.

That's probably more than enough terminology for now.

Here is another program:

print("2 + 2 is",2+2)print("3 * 4 is",3*4)print("100 - 1 is",100-1)print("(33 + 2) / 5 + 11.5 is",(33+2)/5+11.5)

And here is theoutput when the program is run:

2 + 2 is 43 * 4 is 12100 - 1 is 99(33 + 2) / 5 + 11.5 is 18.5

As you can see, Python can turn your thousand-dollar computer into a five-dollar calculator.

In this example, the print function is followed by two arguments, with each of the arguments separated by a comma. So with the first line of the program

print("2 + 2 is",2+2)

The first argument is the string"2 + 2 is" and the second argument is themathematical expression2 + 2, which is commonly referred to as anexpression.

What is important to note is that a string is printed as is (without the enclosing double quotes), but anexpression isevaluated, or converted to its actual value.

Python has seven basic operations for numbers:

Notice that there are two ways to do division, one that returns the repeating decimal, and the other that can get the remainder and the whole number. The order of operations is the same as in math:

• parentheses()
• exponents**
• multiplication*, division/, integer division//, and remainder%
• addition+ and subtraction-

So use parentheses to structure your formulas when needed.

Often in programming you are doing something complicated and may not in the future remember what you did. When this happens the program should probably be commented. Acomment is a note to you and other programmers explaining what is happening. For example:

# Not quite PI, but a credible simulationprint(22/7)

Which outputs

3.14285714286

Notice that the comment starts with a hash:#. Comments are used to communicate with others who read the program and your future self to make clear what is complicated.

Note that any text can follow a comment, and that when the program is run, the text after the# through to the end of that line is ignored. The# does not have to be at the beginning of a new line:

# Output PI on the screenprint(22/7)# Well, just a good approximation

Each chapter (eventually) will contain examples of the programming features introduced in the chapter. You should at least look over them and see if you understand them. If you don't, you may want to type them in and see what happens. Mess around with them, change them and see what happens.

Denmark.py

print("Something's rotten in the state of Denmark.")print("                -- Shakespeare")

Output:

Something's rotten in the state of Denmark.                -- Shakespeare

School.py

# This is not quite true outside of USA# and is based on my dim memories of my younger yearsprint("Firstish Grade")print("1 + 1 =",1+1)print("2 + 4 =",2+4)print("5 - 2 =",5-2)print()print("Thirdish Grade")print("243 - 23 =",243-23)print("12 * 4 =",12*4)print("12 / 3 =",12/3)print("13 / 3 =",13//3,"R",13%3)print()print("Junior High")print("123.56 - 62.12 =",123.56-62.12)print("(4 + 3) * 2 =",(4+3)*2)print("4 + 3 * 2 =",4+3*2)print("3 ** 2 =",3**2)

Output:

Firstish Grade1 + 1 = 22 + 4 = 65 - 2 = 3Thirdish Grade243 - 23 = 22012 * 4 = 4812 / 3 = 413 / 3 = 4 R 1Junior High123.56 - 62.12 = 61.44(4 + 3) * 2 = 144 + 3 * 2 = 103 ** 2 = 9

1. Write a program that prints your full name and your birthday as separate strings.
2. Write a program that shows the use of all 7 math functions.

print("Ada Lovelace","born on","November 27, 1852")

print("Albert Einstein","born on","14 March 1879")

print(("John Smith"),("born on"),("14 March 1879"))

print("5**5 = ",5**5)print("6*7 = ",6*7)print("56/8 = ",56/8)print("14//6 = ",14//6)print("14%6 = ",14%6)print("5+6 = ",5+6)print("9-0 = ",9-0)

1. ↑Here is a great list of the famous "Hello, world!" program in many programming languages. Just so that you know how simple Python can be...

Lesson Information**To Be Added**Vocabulary:Necessary Materials and Resources:Computer Science Teachers Association Standards: 5.3.A.CD.4: Compare various forms of input and output.Common Core Math Content Standards:Common Core Math Practice Standards:Common Core English Language Arts Standards:

You are now ready to create a more complicated program. Type the following into your Python Editor:

print("Halt!")user_input=input("Who goes there?")print("You may pass,",user_input)

Run your program to ensure that it displays the following:

Halt!Who goes there?LinkbotYou may pass, Linkbot

Note: After running the code by pressing F5 on your keyboard, the Python Shell will only give the output:

Halt!Who goes there?

Enter your name in the Python Shell and press enter for the remainder of the output.

Your program will look different because of theinput() statement. Notice that, when running the program, you must type your name and press 'Enter'. The program responds by displaying text that includes your name. This is an example ofinput. The program reaches a certain point, and then waits for the user to type more data for the program to include.

The user's data that the program includes is stored as avariable. In the previous programuser_input is avariable. A variable is like a box that can store a piece of data. This program demonstrates the use of variables:

(Helpful blurb, Aaron, "This may not make a lot of sense right now, but refer back to it as you read the paragraph about variables.")

a=123.4b23='Barobo'first_name="Linkbot"b=432c=a+bprint("a + b is",c)print("first_name is",first_name)print("The Linkbot was created by ",b23)

And here is the output:

a + b is 555.4first_name is LinkbotThe Linkbot was created by Barobo

Variables store data, such as a number, a word, a phrase, or anything you choose to make it. The variables in the above program area,b23,first_name,b, andc. The two basic types of variables arenumbers andstrings.Numbers are simply a mathematical, numerical number, such as 7 or 89 or -3.14. In the above example,a,b, and evenc are number variables.c is considered a number variable because the result is a number.Strings are a sequence of letters, numbers and other characters. In this exampleb23 andfirst_name are variables that store strings.Barobo,Linkbot,a + b is,first_name is, andThe Linkbot was created by are the strings in this program. The characters are surrounded by" or'. The other types of symbols used in the program are variables representing numbers. Remember that variables are used to store a value, they do not use punctuation marks (" and'). If you want to use an actualvalue, youmust use those punctuation marks. See the next example.

value1==Pimvalue2=="Pim"

Both lines of code look the same at first glance, but they are different. In the first one, Python checks if the value stored in the variablevalue1 is the same as the value stored in thevariablePim. In the second one, Python checks if the string (the actual lettersP,i, andm) are the same as invalue2 (More explanation about strings and about the== come later).

In review, there are boxes called variables, and data goes into those boxes. Recall the previous examples. The computer sees a line such asfirst_name = "Linkbot", and it reads it as "put the stringLinkbot into the box, or variable,first_name". Then the computer sees the statementc = a + b, and it reads it as "put the sum ofa + b or123.4 + 432 which equals555.4 intoc". The right hand side of the statementa + b isevaluated, meaning that the two terms are equal, and the result is stored in the variable on the left hand sidec. In other words, one side of the statement isassigned to the other side. This process is calledassignment.

One single equal sign in coding makes the line of code a grammatical statement. For example,apple = banana reads "apple is banana." By writing the code in this way, apple is assigned to banana, and they are now considered equal by the program.You are basically telling the program that the two terms are equal, even if that is not in actuality the case.

Two equal signs together makes the line of code into a question that the computer will answer. For example,apple == banana reads "is apple equal to banana?" This answer would be "no," of course, unless you have previously and incorrectly assigned apple to banana.

(Helpful blurb, Aaron, "Be careful how many= you use because it can change your program entirely!")

Here is another example of variable usage:

a=1print(a)a=a+1print(a)a=a*2print(a)

Here is the output:

124

Even if the same variable appears on both sides of the equals sign (e.g., spam = spam), the computer still reads it as, "First find out the data to store, and then find out where the data goes."

Try another program:

number=float(input("Type in a number: "))integer=int(input("Type in an integer: "))text=input("Type in a string: ")print("number =",number)print("number is a",type(number))print("number * 2 =",number*2)print("integer =",integer)print("integer is a",type(integer))print("integer * 2 =",integer*2)print("text =",text)print("text is a",type(text))print("text * 2 =",text*2)

The output should be:

Type in a number:12.34Type in an integer:-3Type in a string:Hellonumber = 12.34number is a <class 'float'>number * 2 = 24.68integer = -3integer is a <class 'int'>integer * 2 = -6text = Hellotext is a <class 'str'>text * 2 = HelloHello

Notice thatnumber was created withfloat(input()) whiletext was created withinput().input() returns a string while the functionfloat returns a number from a string.int returns an integer, that is a number with no decimal point. When you want the user to type in a decimal usefloat(input()), if you want the user to type in an integer useint(input()), but if you want the user to type in a string useinput().

The second half of the program uses thetype() function which identifies a variable's type. Numbers are of typeint orfloat, which areshort forinteger andfloating point (mostly used for decimal numbers), respectively. Text strings are of typestr, short forstring. Integers and floats can be worked on by mathematical functions, strings cannot. Notice how when pythonmultiplies a number by an integer the expected thing happens. Howeverwhen a string is multiplied by an integer the result is that multiplecopies of the string are produced (i.e.,text * 2 = HelloHello).

Operations with strings do different things than operations with numbers. As well, some operations only work with numbers (both integers and floating point numbers) and will give an error if a string is used. Here are some interactive mode examplesto show that some more.

>>> print("This" + " " + "is" + " joined.")This is joined.>>> print("Ha, " * 5)Ha, Ha, Ha, Ha, Ha, >>> print("Ha, " * 5 + "ha!")Ha, Ha, Ha, Ha, Ha, ha!>>> print(3 - 1)2>>> print("3" - "1")Traceback (most recent call last):  File "<stdin>", line 1, in <module>TypeError: unsupported operand type(s) for -: 'str' and 'str'>>>

Here is the list of some string operations:

Each Linkbot has two joints that it can move to do a variety of tasks. Sometimes, there might be wheels attached to the joints so that the Linkbot can drive around like a two-wheeled car. Or, the joints might be attached to a gripper, which would allow a Linkbot to pick up objects.

Each joint on a Linkbot has a number printed into the plastic. Looking at the Linkbot directly from a "top down" view, joint 1 is on the left side, joint 2 faces you, and joint 3 is on the right hand side. The Linkbot-L only has joints 1 and 2 and the Linkbot-I only has joints 1 and 3. Lets learn how to move a joint on a Linkbot!

moveMotor.py

# This program moves Joint 1 on a Linkbot by an amount# specified by the userimportbarobomyDongle=barobo.Dongle()myDongle.connect()robotID=input('Enter Linkbot ID: ')myLinkbot=myDongle.getLinkbot(robotID)degrees=float(input("Degrees to rotate Joint 1: "))myLinkbot.moveJoint(1,degrees)

When you run this example, you should notice joint 1 move. If you typed "90" for the input, you should see the joint rotate counter-clockwise 90 degrees. If you typed "-90", you should see the joint move 90 degrees in the clockwise direction. When you run this program, be careful about entering very large numbers or else you might have to wait a while for the joint to stop moving.

Rate_times.py

# This program calculates rate and distance problemsprint("Input a rate and a distance")rate=float(input("rate: "))distance=float(input("Distance: "))print("Time:",(distance/rate))

Sample runs:

Input a rate and a distanceRate:5Distance:10Time: 2.0

Input a rate and a distanceRate:3.52Distance:45.6Time: 12.9545454545

Area.py

# This program calculates the perimeter and area of a rectangleprint("Calculate information about a rectangle")length=float(input("Length: "))width=float(input("Width: "))print("Area:",length*width)print("Perimeter:",2*length+2*width)

Sample runs:

Calculate information about a rectangleLength:4Width:3Area: 12.0Perimeter: 14.0

Calculate information about a rectangleLength:2.53Width:5.2Area: 13.156Perimeter: 15.46

Temperature.py

# This program converts Fahrenheit to Celsiusfahr_temp=float(input("Fahrenheit temperature: "))print("Celsius temperature:",(fahr_temp-32.0)*5.0/9.0)

Sample runs:

Fahrenheit temperature:32Celsius temperature: 0.0

Fahrenheit temperature:-40Celsius temperature: -40.0

Fahrenheit temperature:212Celsius temperature: 100.0

Fahrenheit temperature:98.6Celsius temperature: 37.0

Write a program that gets 2 string variables and 2 number variablesfrom the user, concatenates (joins them together with no spaces) anddisplays the strings, then multiplies the two numbers on a new line.

string1=input('String 1: ')string2=input('String 2: ')float1=float(input('Number 1: '))float2=float(input('Number 2: '))print(string1+string2)print(float1*float2)

Write a program that gets 2 number variables from the user; each one an angle in degrees. After the program gets the two numbers, it makes the Linkbot rotate joint 1 by the amount in the first number, and then joint 3 (or 2, if you have a Linkbot-L) by the second number.

# This program moves Joints 1 and 3 on a Linkbot by an amount# specified by the userimportbarobomyDongle=barobo.Dongle()myDongle.connect()robotID=input('Enter Linkbot ID: ')myLinkbot=myDongle.getLinkbot(robotID)degrees1=float(input("Degrees to rotate Joint 1: "))degrees3=float(input("Degrees to rotate Joint 3: "))myLinkbot.moveJoint(1,degrees1)myLinkbot.moveJoint(3,degrees3)

Lesson Information

**To Be Added**Vocabulary:Necessary Materials and Resources:Computer Science Teachers Association Standards: 5.2.CPP.5: Implement problem solutions using a programming language, including: looping behavior, conditional statements, logic, expressions, variables, and functions.Common Core Math Content Standards:Common Core Math Practice Standards:Common Core English Language Arts Standards:

Ordinarily the computer starts with the first line and then goes down from there.Control structures change the order that statements are executed or decide if a certain statement will be run. Here's the source for a program that uses the while control structure:

a=0# FIRST, set the initial value of the variable a to 0(zero).whilea<10:# While the value of the variable a is less than 10 do the following:a=a+1# Increase the value of the variable a by 1, as in: a = a + 1!print(a)# Print to screen what the present value of the variable a is.# REPEAT! until the value of the variable a is equal to 9!? See note.# NOTE:# The value of the variable a will increase by 1# with each repeat, or loop of the 'while statement BLOCK'.# e.g. a = 1 then a = 2 then a = 3 etc. until a = 9 then...# the code will finish adding 1 to a (now a = 10), printing the# result, and then exiting the 'while statement BLOCK'.#              --# While a < 10: |#     a = a + 1 |<--[ The while statement BLOCK ]#     print (a) |#              --

And here is the extremely exciting output:

12345678910

(And you thought it couldn't get any worse after turning your computer into a five-dollar calculator?)

So what does the program do? First it sees the linea = 0 and setsa to zero. Then it seeswhile a < 10: and so the computer checks to see ifa < 10. The first time the computer sees this statement,a is zero, so it is less than 10. In other words, as long asa is less than ten, the computer will run the tabbed in statements. This eventually makesa equal to ten (by adding one toa again and again) and thewhile a < 10 is not true any longer. Reaching that point, the program will stop running the indented lines.

Always remember to put a colon ":" at the end of thewhile statement line!

Here is another example of the use ofwhile:

a=1s=0print('Enter Numbers to add to the sum.')print('Enter 0 to quit.')whilea!=0:print('Current Sum:',s)a=float(input('Number? '))s=s+aprint('Total Sum =',s)

Enter Numbers to add to the sum.Enter 0 to quit.Current Sum: 0Number?200Current Sum: 200.0Number?-15.25Current Sum: 184.75Number?-151.85Current Sum: 32.9Number?10.00Current Sum: 42.9Number?0Total Sum = 42.9

Notice howprint 'Total Sum =', s is only run at the end. Thewhile statement only affects the lines that are indented with whitespace. The!= means does not equal sowhile a != 0: means as long asa is not zero run the tabbed statements that follow.

Note thata is a floating point number, and not all floating point numbers can be accurately represented, so using!= on them can sometimes not work. Try typing in 1.1 in interactive mode.

Now that we have while loops, it is possible to have programs that run forever. An easy way to do this is to write a program like this:

while1==1:print("Help, I'm stuck in a loop.")

The "==" operator is used to test equality of the expressions on the two sides of the operator, just as "<" was used for "less than" before (you will get a complete list of all comparison operators in the next chapter).

This program will outputHelp, I'm stuck in a loop. until the heat death of the universe or you stop it, because 1 will forever be equal to 1. The way to stop it is to hit the Control (orCtrl) button andC (the letter) at the same time. This will kill the program. (Note: sometimes you will have to hit enter after the Control-C.) On some systems, nothing will stop it, short of killing the process--so avoid!

Fibonacci-method1.py

# This program calculates the Fibonacci sequencea=0b=1count=0max_count=20whilecount<max_count:count=count+1print(a,end=" ")# Notice the magic end=" " in the print function arguments# that keeps it from creating a new line.old_a=a# we need to keep track of a since we change it.a=bb=old_a+bprint()# gets a new (empty) line.

Output:

0 1 1 2 3 5 8 13 21 34 55 89 144 233 377 610 987 1597 2584 4181

Note that the output is on a single line because of the extra argumentend=" " in theprint arguments.

Fibonacci-method2.py

# Simplified and faster method to calculate the Fibonacci sequencea=0b=1count=0max_count=10whilecount<max_count:count=count+1print(a,b,end=" ")# Notice the magic end=" "a=a+bb=a+bprint()# gets a new (empty) line.

Output:

0 1 1 2 3 5 8 13 21 34 55 89 144 233 377 610 987 1597 2584 4181

Password.py

# Waits until a password has been entered. Use Control-C to break out without# the password#Note that this must not be the password so that the# while loop runs at least once.password=str()# note that != means not equalwhilepassword!="unicorn":password=input("Password: ")print("Welcome in")

Sample run:

Password:auoPassword:y22Password:passwordPassword:open sesamePassword:unicornWelcome in

The Linkbot has a small buzzer on board that can play one note at a time. We can control the frequency of the buzzer on the Linkbot to generate different tones. The equation${\displaystyle \mathrm{p}\mathrm{i}\mathrm{a}\mathrm{n}\mathrm{o}\mathrm{k}\mathrm{e}\mathrm{y}\mathrm{F}\mathrm{r}\mathrm{e}\mathrm{q}\mathrm{u}\mathrm{e}\mathrm{n}\mathrm{c}\mathrm{y}=2^{((\mathrm{k}\mathrm{e}\mathrm{y}-49)/12)}\ast 440}$ shows how to calculate the frequency of a key on a piano. There are 88 keys on a real piano, but lets see if we can make our robot play from the 34th key all the way up to the 73rd key.

In Python, to calculate the "power" of a number, we can use the** operator. For instance,${\displaystyle 2^3}$ can be calculated in Python using2**3.

importbarobodongle=barobo.Dongle()dongle.connect()robotID=input('Enter Linkbot ID: ')robot=dongle.getLinkbot(robotID)importtime# imports the Python "time" module because we want to use "time.sleep()" later.key=34# Set key=34 which is the 34th key on a piano keyboard.whilekey<74:# While the key number is less than 74.robot.setBuzzerFrequency(2**((key-49)/12.0)*440)# Play the frequency to the corresponding note.time.sleep(.25)# Play the note for 0.25 seconds.robot.setBuzzerFrequency(0)# Turn off the notekey=key+1# Increase the key number by 1. This is the end of the loop. At this point,# Python will check to see if the condition in the "while" statement,# "key < 74", is true. If it is still true, Python will go back to the# beginning of the loop. If not, the program exits the loop.

Write a program that asks the user for a Login Name and password. Then when they type "lock", they need to type in their name and password to unlock the program.

name=input("What is your UserName: ")password=input("What is your Password: ")print("To lock your computer type lock.")command=Noneinput1=Noneinput2=Nonewhilecommand!="lock":command=input("What is your command: ")whileinput1!=name:input1=input("What is your username: ")whileinput2!=password:input2=input("What is your password: ")print("Welcome back to your system!")

name=input('Set name: ')password=input('Set password: ')while1==1:nameguess=""passwordguess=""key=""while(nameguess!=name)or(passwordguess!=password):nameguess=input('Name? ')passwordguess=input('Password? ')print("Welcome,",name,". Type lock to lock.")whilekey!="lock":key=input("")

Modify the Linkbot Buzzer program to play every third key on the piano from the 34th key up to the 74th key.

importbarobodongle=barobo.Dongle()dongle.connect()robotID=input('Enter Linkbot ID: ')robot=dongle.getLinkbot(robotID)importtime# imports the Python "time" module because we want to use "time.sleep()" later.key=34# Set key=34 which is the 34th key on a piano keyboard.whilekey<74:# While the key number is less than 74.robot.setBuzzerFrequency(2**((key-49)/12.0)*440)# Play the frequency to the corresponding note.time.sleep(.25)# Play the note for 0.25 seconds.robot.setBuzzerFrequency(0)# Turn off the notekey=key+3# Increase the key number by 3. This is the end of the loop. At this point,# Python will check to see if the condition in the "while" statement,# "key < 74", is true. If it is still true, Python will go back to the# beginning of the loop. If not, the program exits the loop.

As always I believe I should start each chapter with a warm-up typing exercise, so here is a short program to compute the absolute value of an integer:

n=int(input("Number? "))ifn<0:print("The absolute value of",n,"is",-n)else:print("The absolute value of",n,"is",n)

Here is the output from the two times that I ran this program:

Number?-34The absolute value of -34 is 34

Number?1The absolute value of 1 is 1

So what does the computer do when it sees this piece of code? First it prompts the user for a number with the statement "n = int(input("Number? "))". Next it reads the line "if n < 0:". Ifn is less than zero Python runs the line "print("The absolute value of", n, "is", -n)". Otherwise it runs the line "print("The absolute value of", n, "is", n)".

More formally Python looks at whether theexpressionn < 0 is true or false. Anif statement is followed by an indentedblock of statements that are run when the expression is true. Optionally after theif statement is anelse statement and another indentedblock of statements. This second block of statements is run if the expression is false.

There are a number of different tests that an expression can have. Here is a table of all of them:

Another feature of the if command is the elif statement. It stands for else if and means if the original if statement is false but the elif part is true, then do theelif part. And if neither theif orelif expressions are true, then do what's in theelse block. Here's an example:

a=0whilea<10:a=a+1ifa>5:print(a,">",5)elifa<=3:print(a,"<=",3)else:print("Neither test was true")

and the output:

1 <= 32 <= 33 <= 3Neither test was trueNeither test was true6 > 57 > 58 > 59 > 510 > 5

Notice how theelif a <= 3 is only tested when theif statement fails to be true. There can be more than oneelif expression, allowing multiple tests to be done in a singleif statement.

In the previous chapter,Who Goes There?, we saw that you can move a linkbot's motor with themoveJoint() function.It is typically used like:

myLinkbot.moveJoint(1,180)

where the first argument is the motor you want to move and the second argument is the angle in degrees you want to move it. The above code will move motor "1" in the positive direction by 180 degrees.

What if you want to move more than one motor at a time? The linkbot also has amove() function that does just that. It looks something like this:

myLinkbot.move(90,180,270)

Themove() function expects three numbers inside the parentheses. The first number is the angle to move motor 1, the second number is the angle to move motor 2, and the third number is the angle to move motor 3. If your Linkbot only has two motors, the valuefor the face that does not have a motor is ignored. For example, if I have a Linkbot-I and I ran the above sample code, the "180" is ignored because motor 2 is not a movable motor. Instead, the Linkbot will move motor 1 forward 90 degrees and motor 3 forward 270 degrees.

When themove() function is executed, the Linkbot will move any/all of its motors simultaneously.

In this example, we'd like to create a program that we can use to move our Linkbot around. This demo will use the pythonbreak command. In the previous chapter, we saw that Python can get stuck in "infinite loops". Thebreak command can be used to exit loops even if the loop condition is still true.

Lets start with just moving the Linkbot forward and backward first.To run this demo, you'll need a Linkbot-I and two wheels. A caster is also recommended but optional.

importbarobodongle=barobo.Dongle()dongle.connect()myLinkbot=dongle.getLinkbot('abcd')# Replace abcd with your Linkbot's serial IDwhileTrue:# This is an infinite loop. Get out by using the "break" statementcommand=input('Please enter a robot command or "quit" to exit the program: ')ifcommand=='forward':# Rotate the wheels 180 degrees to move the robot forwardmyLinkbot.move(180,0,-180)# Motor 3 has to spin in the negative direction to move the robot forwardelifcommand=='backward':myLinkbot.move(-180,0,180)elifcommand=='quit':breakelse:print('I\'m sorry. I don\'t understand that command.')print('Goodbye!')

A Note on Strings

You might have noticed that towards the end of the program, there is a line that readsprint('I\'m sorry. I don\'t understand that command.') with funny backslashes in the code. When you run the program though, it doesn't print those backslashes. These backslashes actually serve an important purpose.

When Python encounters either a single quote ' or double quote " , it knows that what follows will be a string that is terminated by the same type of quotation mark that started it. But, what if we want to include some quotation marks as part of the string? Consider the following piece of code:

mystring="Hello there. My dog says "woof!" when he's bored."

When Python sees the first Quotation mark at"Hello th..., it says "Ok, this is going to be a string and as soon as I see another quotation mark, that will be the end of the string". Soon, Python sees another quotation mark at...says "woof! and thinks "I've found the end of the string!", but this is not the end of our string! We wanted Python to go all the way to the end at...bored." . How do we tell Python that the quotation marks around"woof!" are special, and that our string doesn't actually end there?

The answer is backslashes. If there is a backslash preceding a quotation mark, that is a special indicator to Python to include that quotation mark as part of the string, rather than terminating the string. Therefore, the fixed code should be:

mystring="Hello there. My dog says\"woof!\" when he's bored."

In this example, the single quote inhe's is automatically included as part of the string because the string started with a double-quotation mark, and thus Python is looking for a double-quotation mark to terminate the string.

Please enter a robot command or 'quit' to exit the program:forwardPlease enter a robot command or 'quit' to exit the program:backwardPlease enter a robot command or 'quit' to exit the program:aoeuI'm sorry. I don't understand that command.Please enter a robot command or 'quit' to exit the program:quitGoodbye!

equality.py

# This Program Demonstrates the use of the == operator# using numbersprint(5==6)# Using variablesx=5y=8print(x==y)

And the output

FalseFalse

high_low.py

# Plays the guessing game higher or lower# This should actually be something that is semi random like the# last digits of the time or something else, but that will have to# wait till a later chapter.  (Extra Credit, modify it to be random# after the Modules chapter)number=7guess=-1print("Guess the number!")whileguess!=number:guess=int(input("Is it... "))ifguess==number:print("Hooray! You guessed it right!")elifguess<number:print("It's bigger...")elifguess>number:print("It's not so big.")

Sample run:

Guess the number!Is it...2It's bigger...Is it...5It's bigger...Is it...10It's not so big.Is it...7Hooray! You guessed it right!

even.py

# Asks for a number.# Prints if it is even or oddnumber=float(input("Tell me a number: "))ifnumber%2==0:print(int(number),"is even.")elifnumber%2==1:print(int(number),"is odd.")else:print(number,"is very strange.")

Sample runs:

Tell me a number:33 is odd.

Tell me a number:22 is even.

Tell me a number:3.48953.4895 is very strange.

average1.py

# keeps asking for numbers until 0 is entered.# Prints the average value.count=0sum=0.0number=1# set to something that will not exit the while loop immediately.print("Enter 0 to exit the loop")whilenumber!=0:number=float(input("Enter a number: "))ifnumber!=0:count=count+1sum=sum+numberifnumber==0:print("The average was:",sum/count)

Enter 0 to exit the loopEnter a number:3Enter a number:5Enter a number:0The average was: 4.0

Enter 0 to exit the loopEnter a number:1Enter a number:4Enter a number:3Enter a number:0The average was: 2.66666666667

average2.py

# keeps asking for numbers until count numbers have been entered.# Prints the average value.#Notice that we use an integer to keep track of how many numbers,# but floating point numbers for the input of each numbersum=0.0print("This program will take several numbers then average them")count=int(input("How many numbers would you like to average: "))current_count=0whilecurrent_count<count:current_count=current_count+1print("Number",current_count)number=float(input("Enter a number: "))sum=sum+numberprint("The average was:",sum/count)

Sample runs:

This program will take several numbers then average themHow many numbers would you like to average:2Number 1Enter a number:3Number 2Enter a number:5The average was: 4.0

This program will take several numbers then average themHow many numbers would you like to average:3Number 1Enter a number:1Number 2Enter a number:4Number 3Enter a number:3The average was: 2.66666666667

Write a program that asks the user their name, if they enter your namesay "That is a nice name", if they enter "John Cleese" or "MichaelPalin", tell them how you feel about them ;), otherwise tell them "Youhave a nice name."

name=input('Your name: ')ifname=='Ada':print('That is a nice name.')elifname=='John Cleese':print('... some funny text.')elifname=='Michael Palin':print('... some funny text.')else:print('You have a nice name.')

Modify the higher or lower program from this section to keep track of how many times the user has entered the wrong number. If it is more than 3 times, print "That must have been complicated." at the end, otherwise print "Good job!"

number=7guess=-1count=0print("Guess the number!")whileguess!=number:guess=int(input("Is it... "))count=count+1ifguess==number:print("Hooray! You guessed it right!")elifguess<number:print("It's bigger...")elifguess>number:print("It's not so big.")ifcount>3:print("That must have been complicated.")else:print("Good job!")

Write a program that asks for two numbers. If the sum of the numbers is greater than 100, print "That is a big number."

number1=float(input('1st number: '))number2=float(input('2nd number: '))ifnumber1+number2>100:print('That is a big number.')

Modify the Linkbot program presented earlier to include the commands "turnright", "turnleft", and "beep". Make the "turnright" command turn the robot to the right, "turnleft" turn the robot to the left, and "beep" beep the robot buzzer for 1 second.

importbaroboimporttime# So we can use time.sleep() laterdongle=barobo.Dongle()dongle.connect()myLinkbot=dongle.getLinkbot('abcd')# Replace abcd with your Linkbot's serial IDwhileTrue:# This is an infinite loop. Get out by using the "break" statementcommand=input('Please enter a robot command or "quit" to exit the program: ')ifcommand=='forward':# Rotate the wheels 180 degrees to move the robot forwardmyLinkbot.move(180,0,-180)# Motor 3 has to spin in the negative direction to move the robot forwardelifcommand=='backward':myLinkbot.move(-180,0,180)elifcommand=='turnright':myLinkbot.move(180,0,180)elifcommand=='turnleft':myLinkbot.move(-180,0,-180)elifcommand=='beep':myLinkbot.setBuzzerFrequency(440)time.sleep(1)myLinkbot.setBuzzerFrequency(0)elifcommand=='quit':breakelse:print('I\'m sorry. I don\'t understand that command.')print('Goodbye!')

Lets take an inventory of all of the functions that we've used so far to control various aspects of the Linkbot.

Using these functions, we can move our robot around, change the robot's LED color, and make the robot beep or play simple melodies. However, using only these functions, it is not possible to make a robot play a tune and change its LED colors while the robot is moving. For example, lets say we want to make our robot beep 2 times _while_ our robot is moving. We can try something like this:Template:Message box

myLinkbot.setBuzzerFrequency(440)# 1time.sleep(0.25)# 2myLinkbot.setBuzzerFrequency(0)# 3time.sleep(0.25)# 4myLinkbot.setBuzzerFrequency(440)time.sleep(0.25)myLinkbot.setBuzzerFrequency(0)time.sleep(0.25)myLinkbot.move(180,0,-180)# 5

Lets reason our way through each major point of this program.

1. First, we turn on the buzzer on the Linkbot. It starts buzzing
2. Pause the program for 0.25 seconds
3. Turn off the Linkbot's buzzer.
4. Pause the program for another 0.25 seconds. The end result of steps 1-4 is that the Linkbot buzzes its buzzer for 0.25 seconds. The next four lines do the same exact thing.
5. At this point, the Linkbot has buzzed twice, and now themove() function makes the Linkbot roll forward.

This isalmost what we want to do, except we want the robot to beep twicewhile the robot is moving; not before it starts moving. Lets try again:

myLinkbot.move(180,0,-180)# 1myLinkbot.setBuzzerFrequency(440)# 2time.sleep(0.25)myLinkbot.setBuzzerFrequency(0)time.sleep(0.25)myLinkbot.setBuzzerFrequency(440)time.sleep(0.25)myLinkbot.setBuzzerFrequency(0)time.sleep(0.25)

This program is almost exactly the same as the program before except we relocated themove() function to the beginning of the program. Will this program make the robot beep twice as the robot is moving?

1. First, we call themove() function which moves the robot's motors. The robot begins moving. However, the program stays at item 1 until themove() function is completed. After the robot finishes moving, the program continues.
2. Here, the robot turns the buzzer on similar to the previous example. UsingsetBuzzerFrequency() andtime.sleep(), it makes the buzzer beep twice.

As we can see, we have failed yet again to make the robot beep twice while moving. In order to make the robot do these things simultaneously, we must use a similar but new type of function: Non-blocking functions. Lets take a look at an example:

myLinkbot.moveNB(180,0,-180)# 1myLinkbot.setBuzzerFrequency(440)# 2time.sleep(0.25)myLinkbot.setBuzzerFrequency(0)time.sleep(0.25)myLinkbot.setBuzzerFrequency(440)time.sleep(0.25)myLinkbot.setBuzzerFrequency(0)time.sleep(0.25)

1. Here, instead of using themove() function, we use themoveNB() function.

The "NB" in the function name stands for "non-blocking". Both functions move the motors on the Linkbot, but there is one important difference: The NB version of the function "returns" immediately. What this means is Python continues on to the next line of code immediately without waiting for the movement to finish first. In other words, themoveNB() function does not "block" Python from continuing on immediately after the function is called.

1. Now, the Linkbot begins playing its buzzer while the Linkbot is still moving. We've accomplished our task! This program will move the robot and beep twice while the robot is moving.

Now lets try one more example. Lets say we want to make the robot move its wheels 180 degrees to roll forward while beeping twice, and then change the LED color to greenafter the movement is done. To accomplish this, we introduce themoveWait() function. Lets take a look and see how it works:

myLinkbot.moveNB(180,0,-180)# 1myLinkbot.setBuzzerFrequency(440)# 2time.sleep(0.25)myLinkbot.setBuzzerFrequency(0)time.sleep(0.25)myLinkbot.setBuzzerFrequency(440)time.sleep(0.25)myLinkbot.setBuzzerFrequency(0)time.sleep(0.25)myLinkbot.moveWait()# 3myLinkbot.setLEDColor(0,255,0)# 4

1. As before, this line tells the robot to begin moving motors 1 and 3. Since we used the "NB" version of the function, Python continues directly on to the next line of code even though the robot is still moving.
2. The next several lines of code beep the robot twice, similar to before.
3. Here, we call a function calledmoveWait(). This function blocks until all motor movements on the robot are finished. In effect, Python will wait at # 3 until the motors are done moving.
4. Here, we set the LED color to green.

If we had omittedmoveWait() at # 3, the LED color would've been set whether the motors were still moving or not. By using themoveWait() function, we force Python to wait for the motors to stop moving before setting the LED color.

Almost all Linkbot movement commands have a non-blocking version with the "NB" suffix. The ones that do not have a non-blocking version are the ones that move a Linkbot's motor continuously forever. We haven't explored any of them yet, but they are out there. These functions do not have non-blocking versions because they would block forever.

All of the functions that begin with the prefix "set", such assetBuzzerFrequency() andsetLEDColor(), can be considered non-blocking because of how fast the buzzer and LED colors are set. For instance, if you run the following snippet of code:

myLinkbot.setBuzzerFrequency(440)myLinkbot.setLEDColor(0,255,0)

The Linkbot would appear to start buzzing and change the LED color to green simultaneously. Technically, the robot is actually doing one before the other, but the two things happen so fast (typically within 5 milliseconds of each other) that it is practically simultaneous.

"As soon as we started programming, we found to our surprise that it wasn't as easy to get programs right as we had thought. Debugging had to be discovered. I can remember the exact instant when I realized that a large part of my life from then on was going to be spent in finding mistakes in my own programs." —Maurice Wilkes discovers debugging, 1949

By now if you have been messing around with the programs you have probably found that sometimes the program does something you didn't want it to do. This is fairly common. Debugging is the process of figuring out what the computer is doing and then getting it to do what you want it to do. This can be tricky. It surprisingly common to spend weeks tracking down and fixing a bug that was caused by someone putting anx where ay should have been.

This chapter will be more abstract than previous chapters.

The first thing to do (this sounds obvious) is to figure out what theprogram should be doing if it is running correctly. Come up with sometest cases and see what happens.

For instance, lets try to write a program that rolls a Linkbot-I with two wheels forward one rotation while making the LED green, and then rolls the Linkbot backward with the LED red. Here's a first try at this program:

importbarobodongle=barobo.Dongle()dongle.connect()myLinkbot=dongle.getLinkbot('ABCD')# Change 'ABCD' to your Linkbot's Serial IDmyLinkbot.moveNB(360,0,-360)# Roll ForwardmyLinkbot.setLEDColor(0,255,0)# Set LED to greenmyLinkbot.moveNB(-360,0,360)# Roll BackwardmyLinkbot.setLEDColor(255,0,0)# Set the LED color to red

At first glance, the above program may appear to be correct. When you actually try it though, you'll see that the robot never turns its LED green, and it never moves forward. It immediately changes its LED red and moves backward. What happened to the two lines that are supposed to make the robot roll forward and turn the LED green?

One easy and powerful method of debugging problems like this is to use theprint() function to track the progress of the program while it's running. Lets try adding some print statements to our original program and see if we can track down the problem:

importbarobodongle=barobo.Dongle()dongle.connect()myLinkbot=dongle.getLinkbot('ABCD')# Change 'ABCD' to your Linkbot's Serial IDprint("Moving forward...")myLinkbot.moveNB(360,0,-360)# Roll Forwardprint("Done. Setting LED to green...")myLinkbot.setLEDColor(0,255,0)# Set LED to greenprint("Done. Moving backward...")myLinkbot.moveNB(-360,0,360)# Roll Backwardprint("Done. Setting LED to red...")myLinkbot.setLEDColor(255,0,0)# Set the LED color to redprint("Done.")

When you run this program, pay attention not only to what it's printing, but also how and when it's printing. When you run the program, the output you should get is:

Moving forward...Done. Setting LED to green...Done. Moving backward...Done. Setting LED to red...Done.

However, you'll notice that all of those statements printimmediately; there is no delay between the commands that tell the robot to move forward and the commands that tell it to move backward. That should immediately give us an indication for what is wrong: the program should wait until the robot is finished moving forward before moving backward and turning red. You'll notice that we used non-blocking movement functions in our program so that we can move and change the LED color at the same time. However, we also know that Python doesn't stop and wait at non-blocking and "set" functions. From the previous chapter, we also have a function that makes Python wait until a non-blocking motion is completed calledmoveWait().Lets try fixing our broken program:

importbarobodongle=barobo.Dongle()dongle.connect()myLinkbot=dongle.getLinkbot('ABCD')# Change 'ABCD' to your Linkbot's Serial IDmyLinkbot.moveNB(360,0,-360)# Roll ForwardmyLinkbot.setLEDColor(0,255,0)# Set LED to greenmyLinkbot.moveWait()# Wait until the robot is finished rolling forwardmyLinkbot.moveNB(-360,0,360)# Roll BackwardmyLinkbot.setLEDColor(255,0,0)# Set the LED color to red

AmoveWait() statement has been added in-between the two steps. This forces Python to wait until the robot is done moving forward before telling it to move backward and set the LED color to red.

Without themoveWait() statement, Python will go through all of those statementsso fast that the robot rolling forward and green LED won't be noticeable to human eyes and ears. Only the last commands, rolling backward and the red LED, "stay" on the robot and are observable, making it seem like Python was skipping the moving forward and green LED altogether.

1. This may sound obvious, but the first thing to ensure is that you know what the program is supposed to do.
2. Check to make sure the program flow is correct by usingprint() statements.
3. Check to make sure variable values are correct usingprint() statements.
4. For complex programs that might use loops and variables, try "stepping" through the program, line by line, keeping track of variable values and program outputs using pen and paper.
5. If you have a long program that does many calculations on a variable and the value of the variable is wrong at the end of the program, try using the "binary-search" method to find the bug: Use aprint() statement to check the value of the variable in the middle of the program. If the value is correct, you know the bug is somewhere in the second half of your program. Put anotherprint() statement in the middle of the second half of the program and check its value. Repeat until you've found the incorrect line of code that is incorrectly setting the variable's value. This technique is highly useful for findingwhere a bug might be occurring, and it is useful for many types of bugs.

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To start off this chapter I am going to give you an example of what you could do but shouldn't (so don't type it in):

a=23b=-23ifa<0:a=-aifb<0:b=-bifa==b:print("The absolute values of",a,"and",b,"are equal.")else:print("The absolute values of",a,"and",b,"are different.")

with the output being:

The absolute values of 23 and 23 are equal.

The program seems a little repetitive. Programmers hate to repeat things -- that's what computers are for, after all! (Note also that finding the absolute value changed the value of the variable, which is why it is printing out 23, and not -23 in the output.) Fortunately Python allows you to create functions to remove duplication. Here is the rewritten example:

a=23b=-23defabsolute_value(n):ifn<0:n=-nreturnnifabsolute_value(a)==absolute_value(b):print("The absolute values of",a,"and",b,"are equal.")else:print("The absolute values of",a,"and",b,"are different.")

with the output being:

The absolute values of 23 and -23 are equal.

The key feature of this program is thedef statement.def(short for define) starts a function definition.def isfollowed by the name of the functionabsolute_value. Next comes a '(' followed by the parametern (n is passed from the program into the function when the function is called). The statements after the ':' are executed when the function is used. The statements continue until either the indented statements end or areturn is encountered. Thereturn statement returns a value back to the place where the function was called. We already have encountered a function in our very first program, theprint function. Now we can make new functions.

Notice how the values ofa andb are not changed.Functions can be used to repeat tasks that don't returnvalues. Here are some examples:

defhello():print("Hello")defarea(width,height):returnwidth*heightdefprint_welcome(name):print("Welcome",name)hello()hello()print_welcome("Fred")w=4h=5print("width =",w," height =",h," area =",area(w,h))

with output being:

HelloHelloWelcome Fredwidth = 4  height = 5  area = 20

That example shows some more stuff that you can do withfunctions. Notice that you can use no arguments or two or more.Notice also when a function doesn't need to send back a value, areturn is optional.

When eliminating repeated code, you often have variables in the repeated code. In Python, these are dealt with in a special way. So far all variables we have seen are global variables. Functions have a special type of variable called local variables. These variables only exist while the function is running. When a local variable has the same name as another variable (such as a global variable), the local variable hides the other. Sound confusing? Well, these next examples (which are a bit contrived) should help clear things up.

a=4defprint_func():a=17print("in print_func a = ",a)print_func()print("a = ",a)

When run, we will receive an output of:

in print_func a = 17a = 4

Variable assignments inside a function do not override global variables, they exist only inside the function. Even thougha was assigned a new value inside the function, this newly assigned value was only relevant toprint_func, when the function finishes running, and thea's values is printed again, we see the originally assigned values.

Here is another more complex example.

a_var=10b_var=15e_var=25defa_func(a_var):print("in a_func a_var = ",a_var)b_var=100+a_vard_var=2*a_varprint("in a_func b_var = ",b_var)print("in a_func d_var = ",d_var)print("in a_func e_var = ",e_var)returnb_var+10c_var=a_func(b_var)print("a_var = ",a_var)print("b_var = ",b_var)print("c_var = ",c_var)print("d_var = ",d_var)

 in a_func a_var =  15 in a_func b_var =  115 in a_func d_var =  30 in a_func e_var =  25 a_var =  10 b_var =  15 c_var =  125 d_var =   Traceback (most recent call last):  File "C:\def2.py", line 19, in <module>    print("d_var = ", d_var)NameError: name 'd_var' is not defined

In this example the variablesa_var,b_var, andd_var are all local variables when they are inside the functiona_func. After the statementreturn b_var + 10 is run, they all cease to exist. The variablea_var is automatically a local variable since it is a parameter name. The variablesb_var andd_var are local variables since they appear on the left of an equals sign in the function in the statementsb_var = 100 + a_var andd_var = 2 * a_var .

Inside of the functiona_var has no value assigned to it. When the function is called withc_var = a_func(b_var), 15 is assigned toa_var since at that point in timeb_var is 15, making the call to the functiona_func(15). This ends up settinga_var to 15 when it is inside ofa_func.

As you can see, once the function finishes running, the local variablesa_var andb_var that had hidden the global variables of the same name are gone. Then the statementprint("a_var = ", a_var) prints the value10 rather than the value15 since the local variable that hid the global variable is gone.

Another thing to notice is theNameError that happens at the end. This appears since the variabled_var no longer exists sincea_func finished. All the local variables are deleted when the function exits. If you want to get something from a function, then you will have to usereturn something.

One last thing to notice is that the value ofe_var remains unchanged insidea_func since it is not a parameter and it never appears on the left of an equals sign inside of the functiona_func. When a global variable is accessed inside a function it is the global variable from the outside.

Functions allow local variables that exist only inside the function and can hide other variables that are outside the function.

We've seen now that we can pass numbers and variables to a function in the function parameters. It turns out that you can pass just about anything into a function, including Linkbot objects.In the Chapter "Decisions", we wrote some code that could move a two-wheels linkbot around. In the code, we specified the angle to rotate the wheels, but it would be much cooler if we could tell the Linkbot to move some distance on the ground. When you're writing new functions, it's common to prototype how the function might be used before actually writing it, so lets try that now.We want our function to be used something like this:

driveDistance(myLinkbot,10)# Drive a wheeled Linkbot 10 inches forward

Now that we're satisfied with how we want touse our function, now we have to worry about how to actually write it so that it does what we want it to do. First, let us catalog what we know and what we have. So far we know about 2 functions that we can use to move motors on the Linkbot:moveJoint() andmove(). Of the two, we have found thatmove() is probably better for driving two-wheeled Linkbots. However, themove() function takes angles as arguments. That leaves the question: How do we turn a distance into an angle?

It turns out there is an equation that you can use to figure out how many degrees a wheel has to turn to travel a certain distance. The equation is:${\displaystyle \mathrm{d}\mathrm{e}\mathrm{g}\mathrm{r}\mathrm{e}\mathrm{e}\mathrm{s}=\frac{360}{2\pi r}\ast \mathrm{d}\mathrm{i}\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{n}\mathrm{c}\mathrm{e}}$

If you would like to see a derivation where that equation comes from, click on the "derivation" link below.

Derivation

To solve this question, we can consider how wheels roll on a surface. Let us consider a wheel rolling on a surface that doesn't slip on the surface because it will make our calculations easier. Let us consider the "circumference" of a wheel. If you took a string and wrapped it all the way around a circle, the length of that string is the "circumference".The circumference of a circle with radius "r" is

${\displaystyle \mathrm{c}\mathrm{i}\mathrm{r}\mathrm{c}\mathrm{u}\mathrm{m}\mathrm{f}\mathrm{e}\mathrm{r}\mathrm{e}\mathrm{n}\mathrm{c}\mathrm{e}=2\pi r}$

The following figure illustrates a blue wheel with a green string wrapped around it. As the wheel rolls, the string unrolls off of the wheel.

As we can see from the figure, if the wheel rolls one full revolution, it travels one circumference of distance. Knowing that one revolution is 360 degrees, we can write a ratio:

${\displaystyle \frac{360}{2\pi r}=\frac{\mathrm{d}\mathrm{e}\mathrm{g}\mathrm{r}\mathrm{e}\mathrm{e}\mathrm{s}}{\mathrm{d}\mathrm{i}\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{n}\mathrm{c}\mathrm{e}}}$

So if we want to travel a distance "distance", we can do

${\displaystyle \mathrm{d}\mathrm{e}\mathrm{g}\mathrm{r}\mathrm{e}\mathrm{e}\mathrm{s}=\frac{360}{2\pi r}\ast \mathrm{d}\mathrm{i}\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{n}\mathrm{c}\mathrm{e}}$

Using this equation, if we know the wheel radius and we know what distance we want to travel, we can calculate the degrees to turn the wheel! Now we can write our function.

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Now, we can include that equation in our function. This allows us to re-use the function for any distance and we don't have to type in the equation over and over again.

importbaroboimportmath# So that we can use math.pidongle=barobo.Dongle()dongle.connect()myLinkbot=dongle.getLinkbot('abcd')# Change abcd to your Linkbot's serial IDdefdriveDistance(linkbot,distance):r=3.5/2# If you have a wheel that's not 3.5 inches in diameter, change "3.5" to the diameter of your wheeldegrees=(360)/(2*math.pi*r)*distancelinkbot.move(degrees,0,-degrees)driveDistance(myLinkbot,10)# Drives the Linkbot 10 inches forwarddriveDistance(myLinkbot,-5)# Drives the Linkbot 5 inches backward

The program shown above uses thedriveDistance() function to drive a robot forward 10 inches and then backward 5 inches. You might wondering why we went through all the trouble of defining a function, which took four lines of code, when we could accomplish the same task without functions.

• Consider if the task was much more complex than just two movements. If you have to drive the robot forwards and backwards more than 4 times, you are actually saving time and code by using a function.
• Writing repeated code via copy/paste can be very hard to debug. Imagine if you copy and pasted the equation 20 times for 20 robot movements, and then you found a bug in the copy-pasted code. You would have to correct the bug in each one of the 20 pasted code blocks. If you had written a function with a bug in it, you would only have to fix the equation inside the function.
• If you are writing code for someone else to use, it would make sense to encapsulate your code in a function. Imagine if you are working with a team of people and your job is to write a function that moves the robot forward and backward, another person's job is to write a function that turns the robot, and a third person has to write a function that changes the LED color. You could then take all three functions and put them into a single program and have a robot that accurately drives a certain distance, turns, and changes LED color.

temperature2.py

#! /usr/bin/python#-*-coding: utf-8 -*-# converts temperature to Fahrenheit or Celsiusdefprint_options():print("Options:")print(" 'p' print options")print(" 'c' convert from Celsius")print(" 'f' convert from Fahrenheit")print(" 'q' quit the program")defcelsius_to_fahrenheit(c_temp):return9.0/5.0*c_temp+32deffahrenheit_to_celsius(f_temp):return(f_temp-32.0)*5.0/9.0choice="p"whilechoice!="q":ifchoice=="c":c_temp=float(input("Celsius temperature: "))print("Fahrenheit:",celsius_to_fahrenheit(c_temp))choice=input("option: ")elifchoice=="f":f_temp=float(input("Fahrenheit temperature: "))print("Celsius:",fahrenheit_to_celsius(f_temp))choice=input("option: ")elifchoice=="p":#Alternatively choice != "q": so that print when anything unexpected inputedprint_options()choice=input("option: ")

Sample Run:

Options: 'p' print options 'c' convert from Celsius 'f' convert from Fahrenheit 'q' quit the programoption:cCelsius temperature:30 Fahrenheit: 86.0option:fFahrenheit temperature:60Celsius: 15.5555555556option:q

area2.py

#! /usr/bin/python#-*-coding: utf-8 -*-# calculates a given rectangle areadefhello():print('Hello!')defarea(width,height):returnwidth*heightdefprint_welcome(name):print('Welcome,',name)defpositive_input(prompt):number=float(input(prompt))whilenumber<=0:print('Must be a positive number')number=float(input(prompt))returnnumbername=input('Your Name: ')hello()print_welcome(name)print()print('To find the area of a rectangle,')print('enter the width and height below.')print()w=positive_input('Width: ')h=positive_input('Height: ')print('Width =',w,' Height =',h,' so Area =',area(w,h))

Sample Run:

Your Name:JoshHello!Welcome, JoshTo find the area of a rectangle,enter the width and height below.Width:-4Must be a positive numberWidth:4Height:3Width = 4  Height = 3  so Area = 12

Rewrite the area2.py program from the Examples above to have a separate function for the area of a square, the area of a rectangle, and the area of a circle (3.14 * radius**2). This program should include a menu interface.

defsquare(side):returnside*sidedefrectangle(width,height):returnwidth*heightdefcircle(radius):return3.14159*radius**2defoptions():print()print("Options:")print("s = calculate the area of a square.")print("c = calculate the area of a circle.")print("r = calculate the area of a rectangle.")print("q = quit")print()print("This program will calculate the area of a square, circle or rectangle.")choice="x"options()whilechoice!="q":choice=input("Please enter your choice: ")ifchoice=="s":side=float(input("Length of square side: "))print("The area of this square is",square(side))options()elifchoice=="c":radius=float(input("Radius of the circle: "))print("The area of the circle is",circle(radius))options()elifchoice=="r":width=float(input("Width of the rectangle: "))height=float(input("Height of the rectangle: "))print("The area of the rectangle is",rectangle(width,height))options()elifchoice=="q":print(" ",end="")else:print("Unrecognized option.")options()

Some people find this section useful, and some find it confusing. If you find it confusing you can skip it. Now we will do a walk through for the following program:

defmult(a,b):ifb==0:return0rest=mult(a,b-1)value=a+restreturnvalueprint("3 * 2 = ",mult(3,2))

Basically this program creates a positive integer multiplication function(that is far slower than the built in multiplication function) and thendemonstrates this function with a use of the function. This program demonstrates the use of recursion, that is a form of iteration (repetition) in which there is a function that repeatedly calls itself until an exit condition is satisfied. It uses repeated additions to give the same result as mutiplication: e.g. 3 + 3 (addition) gives the same result as 3 * 2 (multiplication).

Question: What is the first thing the program does?

Answer: The first thing done is the function mult is defined with the lines:

defmult(a,b):ifb==0:return0rest=mult(a,b-1)value=a+restreturnvalue

This creates a function that takes two parameters and returns a value when it is done. Later this function can be run.

What happens next?

The next line after the function,print("3 * 2 = ", mult(3, 2)) is run.

And what does this do?

It prints3 * 2 = and the return value ofmult(3, 2)

And what doesmult(3, 2) return?

We need to do a walkthrough of themult function to find out.

What happens next?

The variablea gets the value 3 assigned to it and the variableb gets the value 2 assigned to it.

And then?

The lineif b == 0: is run. Sinceb has the value 2 this is false so the linereturn 0 is skipped.

And what then?

The linerest = mult(a, b - 1) is run. This line sets the local variablerest to the value ofmult(a, b - 1). The value of a is 3 and the value of b is 2 so the function call ismult(3,1)

So what is the value ofmult(3, 1) ?

We will need to run the function mult with the parameters 3 and 1.

So what happens next?

The local variables in the new run of the function are set so thata has the value 3 and b has the value 1. Since these are local values these do not affect the previous values ofa andb.

And then?

Since b has the value 1 the if statement is false, so the next line becomesrest = mult(a, b - 1).

What does this line do?

This line will assign the value ofmult(3, 0) to rest.

So what is that value?

We will have to run the function one more time to find that out. This timea has the value 3 andb has the value 0.

So what happens next?

The first line in the function to run isif b == 0:. b has the value 0 so the next line to run isreturn 0

And what does the linereturn 0 do?

This line returns the value 0 out of the function.

So?

So now we know thatmult(3, 0) has the value 0. Now we know what the linerest = mult(a, b - 1) did since we have run the functionmult with the parameters 3 and 0. We have finished runningmult(3, 0) and are now back to running mult(3, 1). The variablerest gets assigned the value 0.

What line is run next?

The linevalue = a + rest is run next. In this run of the function,a = 3 andrest = 0 so nowvalue = 3.

What happens next?

The linereturn value is run. This returns 3 from the function. This also exits from the run of the function mult(3, 1). Afterreturn is called, we go back to running mult(3, 2).

Where were we in mult(3, 2)?

We had the variablesa = 3 andb = 2 and were examining the linerest = mult(a, b - 1).

So what happens now?

The variablerest get 3 assigned to it. The next linevalue = a + rest setsvalue to3 + 3 or 6.

So now what happens?

The next line runs, this returns 6 from the function. We are now back to running the lineprint("3 * 2 = ", mult(3, 2)) which can now print out the 6.

What is happening overall?

Basically we used two facts to calculate the multiple of the two numbers. The first is that any number times 0 is 0 (x * 0 = 0). The second is that a number times another number is equal to the first number plus the first number times one less than the second number (x * y = x + x * (y - 1)). So what happens is3 * 2 is first converted into 3 + 3 * 1. Then3 * 1 is converted into3 + 3 * 0. Then we know that any number times 0 is 0 so3 * 0 is 0. Then we can calculate that3 + 3 * 0 is3 + 0 which is3. Now we know what3 * 1 is so we can calculate that3 + 3 * 1 is3 + 3 which is6.

This is how the whole thing works:

3 * 23 + 3 * 13 + 3 + 3 * 03 + 3 + 03 + 36

Programming constructs solving a problem by solving a smaller version of the same problem are calledrecursive. In the examples in this chapter, recursion is realized by defining a function calling itself. This facilitates implementing solutions to programming tasks as it may be sufficient to consider the next step of a problem instead of the whole problem at once. It is also useful as it allows to express some mathematical concepts with straightforward, easy to read code.

Any problem that can be solved with recursion could be re-implemented with loops. Using the latter usually results in better performance. However equivalent implementations using loops are usually harder to get done correctly.

Probably the most intuitive definition ofrecursion is:

Recursion

If you still don't get it, seerecursion.

Try walking through the factorial example if the multiplication example did not make sense.

factorial.py

#defines a function that calculates the factorialdeffactorial(n):ifn<=1:return1returnn*factorial(n-1)print("2! =",factorial(2))print("3! =",factorial(3))print("4! =",factorial(4))print("5! =",factorial(5))

Output:

2! = 23! = 64! = 245! = 120

countdown.py

defcount_down(n):print(n)ifn>0:returncount_down(n-1)count_down(5)

Output:

543210

You have already seen ordinary variables that store a single value. However other variable types can hold more than one value. These are called containers because they can contain more than one object. The simplest type is called a list. Here is an example of a list being used:

which_one=int(input("What month (1-12)? "))months=['January','February','March','April','May','June','July','August','September','October','November','December']if1<=which_one<=12:print("The month is",months[which_one-1])

and an output example:

What month (1-12)?3The month is March

In this example themonths is a list.months is defined with the lines months = ['January', 'February', 'March', 'April', 'May', 'June', 'July', and'August', 'September', 'October', 'November', 'December'] (note that a\ could also be used to split a long line, but that is not necessary in this case because Python is intelligent enough to recognize that everything within brackets belongs together). The[ and] start and end the list with commas (,) separating the list items. The list is used inmonths[which_one - 1]. A list consists of items that are numbered starting at 0. In other words, if you wanted January you would usemonths[0]. Give a list a number and it will return the value that is stored at that location.

The statement if 1 <= which_one <= 12: will only be true if which_one is between one and twelve inclusive (in other words it is what you would expect if you have seen that in algebra).

Lists can be thought of as a series of boxes. Each box has a different value. For example, the boxes created bydemolist = ['life', 42, 'the universe', 6, 'and', 9] would look like this:

Each box is referenced by its number so the statementdemolist[0] would get'life',demolist[1] would get42 and so on up todemolist[5] getting9.

The next example is just to show a lot of other stuff lists can do (for once I don't expect you to type it in, but you should probably play around with lists in interactive mode until you are comfortable with them.). Here goes:

demolist=["life",42,"the universe",6,"and",9]print("demolist = ",demolist)demolist.append("everything")print("after 'everything' was appended demolist is now:")print(demolist)print("len(demolist) =",len(demolist))print("demolist.index(42) =",demolist.index(42))print("demolist[1] =",demolist[1])# Next we will loop through the listforcinrange(len(demolist)):print("demolist[",c,"] =",demolist[c])deldemolist[2]print("After 'the universe' was removed demolist is now:")print(demolist)if"life"indemolist:print("'life' was found in demolist")else:print("'life' was not found in demolist")if"amoeba"indemolist:print("'amoeba' was found in demolist")if"amoeba"notindemolist:print("'amoeba' was not found in demolist")another_list=[42,7,0,123]another_list.sort()print("The sorted another_list is",another_list)

The output is:

demolist =  ['life', 42, 'the universe', 6, 'and', 9]after 'everything' was appended demolist is now:['life', 42, 'the universe', 6, 'and', 9, 'everything']len(demolist) = 7demolist.index(42) = 1demolist[1] = 42demolist[ 0 ] = lifedemolist[ 1 ] = 42demolist[ 2 ] = the universedemolist[ 3 ] = 6demolist[ 4 ] = anddemolist[ 5 ] = 9demolist[ 6 ] = everythingAfter 'the universe' was removed demolist is now:['life', 42, 6, 'and', 9, 'everything']'life' was found in demolist'amoeba' was not found in demolistThe sorted another_list is [0, 7, 42, 123]

This example uses a whole bunch of new functions. Notice that you canjustprint a whole list. Next theappend function is usedto add a new item to the end of the list.len returns how manyitems are in a list. The valid indexes (as in numbers that can beused inside of the[]) of a list range from 0 tolen - 1. Theindex function tells where the first location of an item islocated in a list. Notice howdemolist.index(42) returns 1, andwhendemolist[1] is run it returns 42. To get help on all the functions a list provides for you, typehelp(list) in the interactive Python interpreter.

The line# Next we will loop through the list is a just a reminder to the programmer (also called acomment). Python ignores everything that is written after a# on the current line. Next the lines:

forcinrange(len(demolist)):print('demolist[',c,'] =',demolist[c])

create a variablec, which starts at 0 and is incremented until it reaches the last index of the list. Meanwhile, theprint statement prints out each element of the list.

A much better way to do the above is:

forc,xinenumerate(demolist):print("demolist[",c,"] =",x)

Thedel command can be used to remove a given element in a list. The next few lines use thein operator to test if an element is in or is not in a list. Thesort function sorts the list. This is useful if you need alist in order from smallest number to largest or alphabetical. Notethat this rearranges the list. In summary, for a list, the following operations occur:

This next example uses these features in a more useful way:

menu_item=0namelist=[]whilemenu_item!=9:print("--------------------")print("1. Print the list")print("2. Add a name to the list")print("3. Remove a name from the list")print("4. Change an item in the list")print("9. Quit")menu_item=int(input("Pick an item from the menu: "))ifmenu_item==1:current=0iflen(namelist)>0:whilecurrent<len(namelist):print(current,".",namelist[current])current=current+1else:print("List is empty")elifmenu_item==2:name=input("Type in a name to add: ")namelist.append(name)elifmenu_item==3:del_name=input("What name would you like to remove: ")ifdel_nameinnamelist:# namelist.remove(del_name) would work just as fineitem_number=namelist.index(del_name)delnamelist[item_number]# The code above only removes the first occurrence of# the name.  The code below from Gerald removes all.# while del_name in namelist:#       item_number = namelist.index(del_name)#       del namelist[item_number]else:print(del_name,"was not found")elifmenu_item==4:old_name=input("What name would you like to change: ")ifold_nameinnamelist:item_number=namelist.index(old_name)new_name=input("What is the new name: ")namelist[item_number]=new_nameelse:print(old_name,"was not found")print("Goodbye")

And here is part of the output:

--------------------1. Print the list2. Add a name to the list3. Remove a name from the list4. Change an item in the list9. QuitPick an item from the menu:2Type in a name to add:JackPick an item from the menu:2Type in a name to add:JillPick an item from the menu:10 . Jack1 . JillPick an item from the menu:3What name would you like to remove:JackPick an item from the menu:4What name would you like to change:JillWhat is the new name:Jill PetersPick an item from the menu:10 . Jill PetersPick an item from the menu:9Goodbye

That was a long program. Let's take a look at the source code. The linenamelist = [] makes the variablenamelist a list with no items (or elements). The next important line iswhile menu_item != 9:. This line starts a loop that allows the menu system for this program. The next few lines display a menu and decide which part of the program to run.

The section

current=0iflen(namelist)>0:whilecurrent<len(namelist):print(current,".",namelist[current])current=current+1else:print("List is empty")

goes through the list and prints each name.len(namelist) tells how many items are in the list. Iflen returns0, then the list is empty.

Then, a few lines later, the statementnamelist.append(name) appears. It uses theappend function to add an item to the end of the list. Jump down another two lines, and notice this section of code:

item_number=namelist.index(del_name)delnamelist[item_number]

Here theindex function is used to find the index value that will be used later to remove the item.del namelist[item_number] is used to remove an element of the list.

The next section

old_name=input("What name would you like to change: ")ifold_nameinnamelist:item_number=namelist.index(old_name)new_name=input("What is the new name: ")namelist[item_number]=new_nameelse:print(old_name,"was not found")

usesindex to find theitem_number and then putsnew_name where theold_name was.

Congratulations, with lists under your belt, you now know enough of the languagethat you could do any computations that a computer can do (this is technically known asTuring-Completeness). Of course, there are still many features thatare used to make your life easier.

test.py

## This program runs a test of knowledge# First get the test questions# Later this will be modified to use file io.defget_questions():# notice how the data is stored as a list of listsreturn[["What color is the daytime sky on a clear day? ","blue"],["What is the answer to life, the universe and everything? ","42"],["What is a three letter word for mouse trap? ","cat"]]# This will test a single question# it takes a single question in# it returns True if the user typed the correct answer, otherwise Falsedefcheck_question(question_and_answer):# extract the question and the answer from the list# This function takes a list with two elements, a question and an answer.question=question_and_answer[0]answer=question_and_answer[1]# give the question to the usergiven_answer=input(question)# compare the user's answer to the tester's answerifanswer==given_answer:print("Correct")returnTrueelse:print("Incorrect, correct was:",answer)returnFalse# This will run through all the questionsdefrun_test(questions):iflen(questions)==0:print("No questions were given.")# the return exits the functionreturnindex=0right=0whileindex<len(questions):# Check the question#Note that this is extracting a question and answer list from the list of lists.ifcheck_question(questions[index]):right=right+1# go to the next questionindex=index+1# notice the order of the computation, first multiply, then divideprint("You got",right*100/len(questions),\"% right out of",len(questions))# now let's get the questions from the get_questions function, and# send the returned list of lists as an argument to the run_test function.run_test(get_questions())

The valuesTrue andFalse point to 1 and 0, respectively. They are often used in sanity checks, loop conditions etc. You will learn more about this a little bit later (chapterBoolean Expressions).Please note that get_questions() is essentially a list because even though it's technically a function, returning a list of lists is the only thing it does.

Sample Output:

What color is the daytime sky on a clear day?greenIncorrect, correct was: blueWhat is the answer to life, the universe and everything?42CorrectWhat is a three letter word for mouse trap?catCorrectYou got 66 % right out of 3

LinkbotMelody.py

We can also create our own lists of items and use for loops to loop through them. Lets try making a list of keyboard keys to play in order to play a simple tune.We'll write this program using the knowledge that middle-C is the 40th key on our keyboard.When you run this program, it should play the beginning of a very familiar tune. Can you guess what it is?

importbarobodongle=barobo.Dongle()dongle.connect()robot=dongle.getLinkbot('6wbn')# Replace '6wbn' with the serial ID on your Linkbotimporttime# Need to "import time" so we can use time.sleep()myNotes=[44,42,40,42,44,44,44]# Put some notes into a listt=0.5# Set a value to be used for the duration of the noteforiinmyNotes:# Select which keys on a piano keyboard to use for the tonesk=pow(2,(i-49)/12)*440# Determines the frequency of the note to be playedrobot.setBuzzerFrequency(k)# Directs the Linkbot to play this frequencytime.sleep(t)# Pauses the program while the note is playedrobot.setBuzzerFrequency(0)# Turns off the piezo speaker at the end of each note

Expand the test.py program so it has a menu giving the option of takingthe test, viewing the list of questions and answers, and an option toquit. Also, add a new question to ask, "What noise does a trulyadvanced machine make?" with the answer of "ping".

## This program runs a test of knowledgequestions=[["What color is the daytime sky on a clear day? ","blue"],["What is the answer to life, the universe and everything? ","42"],["What is a three letter word for mouse trap? ","cat"],["What noise does a truly advanced machine make?","ping"]]# This will test a single question# it takes a single question in# it returns True if the user typed the correct answer, otherwise Falsedefcheck_question(question_and_answer):# extract the question and the answer from the listquestion=question_and_answer[0]answer=question_and_answer[1]# give the question to the usergiven_answer=input(question)# compare the user's answer to the testers answerifanswer==given_answer:print("Correct")returnTrueelse:print("Incorrect, correct was:",answer)returnFalse# This will run through all the questionsdefrun_test(questions):iflen(questions)==0:print("No questions were given.")# the return exits the functionreturnindex=0right=0whileindex<len(questions):# Check the questionifcheck_question(questions[index]):right=right+1# go to the next questionindex=index+1# notice the order of the computation, first multiply, then divideprint("You got",right*100/len(questions),"% right out of",len(questions))#showing a list of questions and answersdefshowquestions():q=0whileq<len(questions):a=0print("Q:",questions[q][a])a=1print("A:",questions[q][a])q=q+1# now let's define the menu functiondefmenu():print("-----------------")print("Menu:")print("1 - Take the test")print("2 - View a list of questions and answers")print("3 - View the menu")print("5 - Quit")print("-----------------")choice="3"whilechoice!="5":ifchoice=="1":run_test(questions)elifchoice=="2":showquestions()elifchoice=="3":menu()print()choice=input("Choose your option from the menu above: ")

And here is the new typing exercise for this chapter:

onetoten=range(1,11)forcountinonetoten:print(count)

and the ever-present output:

12345678910

The output looks awfully familiar but the program code looks different. The first line uses therange function. Therange function uses two arguments like thisrange(start, finish).start is the first number that is produced.finish is one larger than the last number. Note that this program could have been done in a shorter way:

forcountinrange(1,11):print(count)

The range function returns an iterable. This can be converted into a list with thelist function. Here are some examples to show what happens with therange command:

>>>range(1, 10)range(1, 10)>>>list(range(1, 10))[1, 2, 3, 4, 5, 6, 7, 8, 9]>>>list(range(-32, -20))[-32, -31, -30, -29, -28, -27, -26, -25, -24, -23, -22, -21]>>>list(range(5,21))[5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20]>>>list(range(5))[0, 1, 2, 3, 4]>>>list(range(21, 5))[]

The next linefor count in onetoten: uses thefor control structure. Afor control structure looks likefor variable in list:.list is gone through starting with the first element of the list and going to the last. Asfor goes through each element in a list it puts each intovariable. That allowsvariable to be used in each successive time thefor loop is run through. Here is another example (you don't have to type this) to demonstrate:

demolist=['life',42,'the universe',6,'and',7,'everything']foritemindemolist:print("The current item is:",item)

The output is:

The current item is: lifeThe current item is: 42The current item is: the universeThe current item is: 6The current item is: andThe current item is: 7The current item is: everything

Notice how thefor loop goes through and sets item to each element in the list. So, what isfor good for? The first use is to go through all the elements of a list and do something with each of them. Here's a quick way to add up all the elements:

list=[2,4,6,8]sum=0fornuminlist:sum=sum+numprint("The sum is:",sum)

with the output simply being:

The sum is: 20

Or you could write a program to find out if there are any duplicates in a list like this program does:

list=[4,5,7,8,9,1,0,7,10]list.sort()prev=Noneforiteminlist:ifprev==item:print("Duplicate of",prev,"found")prev=item

and for good measure:

Duplicate of 7 found

Okay, so how does it work? Here is a special debugging version to help you understand (you don't need to type this in):

l=[4,5,7,8,9,1,0,7,10]print("l = [4, 5, 7, 8, 9, 1, 0, 7, 10]","\t\tl:",l)l.sort()print("l.sort()","\t\tl:",l)prev=l[0]print("prev = l[0]","\t\tprev:",prev)dell[0]print("del l[0]","\t\tl:",l)foriteminl:ifprev==item:print("Duplicate of",prev,"found")print("if prev == item:","\t\tprev:",prev,"\titem:",item)prev=itemprint("prev = item","\t\tprev:",prev,"\titem:",item)

with the output being:

l = [4, 5, 7, 8, 9, 1, 0, 7, 10]        l: [4, 5, 7, 8, 9, 1, 0, 7, 10]l.sort()                l: [0, 1, 4, 5, 7, 7, 8, 9, 10]prev = l[0]             prev: 0del l[0]                l: [1, 4, 5, 7, 7, 8, 9, 10]if prev == item:        prev: 0         item: 1prev = item             prev: 1         item: 1if prev == item:        prev: 1         item: 4prev = item             prev: 4         item: 4if prev == item:        prev: 4         item: 5prev = item             prev: 5         item: 5if prev == item:        prev: 5         item: 7prev = item             prev: 7         item: 7Duplicate of 7 foundif prev == item:        prev: 7         item: 7prev = item             prev: 7         item: 7if prev == item:        prev: 7         item: 8prev = item             prev: 8         item: 8if prev == item:        prev: 8         item: 9prev = item             prev: 9         item: 9if prev == item:        prev: 9         item: 10prev = item             prev: 10        item: 10

The reason I put so manyprint statements in the code was so that you can see what is happening in each line. (By the way, if you can't figure out why a program is not working, try putting in lots of print statements in places where you want to know what is happening.) First the program starts with a boring old list. Next the program sorts the list. This is so that any duplicates get put next to each other. The program then initializes aprev(ious) variable. Next the first element of the list is deleted so that the first item is not incorrectly thought to be a duplicate. Next afor loop is gone into. Each item of the list is checked to see if it is the same as the previous. If it is a duplicate was found. The value ofprev is then changed so that the next time thefor loop is run throughprev is the previous item to the current. Sure enough, the 7 is found to be a duplicate. (Notice how\t is used to print a tab.)

The other way to usefor loops is to do something a certain number of times. Here is some code to print out the first 9 numbers of the Fibonacci series:

a=1b=1forcinrange(1,10):print(a,end=" ")n=a+ba=bb=n

with the surprising output:

1 1 2 3 5 8 13 21 34

Everything that can be done withfor loops can also be done withwhile loops butfor loops give an easy way to go through all the elements in a list or to do something a certain number of times.

In this example the Linkbot will play changing tones a fixed number of times while the duration remains constant. The Linkbot can also change tone and duration a fixed number of times. Here we are using a for loop running with in a fixed range of values.

setBuzzerFrequency.py

importbarobodongle=barobo.Dongle()dongle.connect()robot=dongle.getLinkbot('6wbn')# Replace '6wbn' with the serial ID on your Linkbotimporttime# For time.sleep()t=1# Set a value to be used for the duration of the noteforiinrange(33,43):# Select which keys on a piano keyboard to use for the tonesk=pow(2,(i-49)/12)*440# Determines the frequency of the note to be playedrobot.setBuzzerFrequency(k)# Directs the Linkbot to play this frequencytime.sleep(t)# Pauses the program while the note is playedrobot.setBuzzerFrequency(0)# Turns off the piezo speaker at the end of the program