Python, like many other computer programming languages, uses Boolean logic for its decision control. That is, the Python interpreter compares one or more values in order to decide whether to execute a piece of code or not, given the proper syntax and instructions.

Decision control is then divided into two major categories, conditional and repetition. Conditional logic simply uses the keywordif and a Boolean expression to decide whether or not to execute a code block. Repetition builds on the conditional constructs by giving us a simple method in which to repeat a block of code while a Boolean expression evaluates totrue.

Here is a little example of boolean expressions (you don't have to type it in):

a=6b=7c=42print(1,a==6)print(2,a==7)print(3,a==6andb==7)print(4,a==7andb==7)print(5,nota==7andb==7)print(6,a==7orb==7)print(7,a==7orb==6)print(8,not(a==7andb==6))print(9,nota==7andb==6)

With the output being:

1 True2 False3 True4 False5 True6 True7 False8 True9 False

What is going on? The program consists of a bunch of funny lookingprint statements. Eachprint statement prints a number and an expression. The number is to help keep track of which statement I am dealing with. Notice how each expression ends up being either True or False; these are built-in Python values.The lines:

print(1,a==6)print(2,a==7)

print out True and False respectively, just as expected, since the first is true and the second is false. The third print,print (3, a == 6 and b == 7), is a little different. The operator and means if both the statement before and the statement after are true then the whole expression is true otherwise the whole expression is false. The next line,print (4, a == 7 and b == 7), shows how if part of an and expression is false, the whole thing is false. The behavior of and can be summarized as follows:

Note that if the first expression is false Python does not check the second expression since it knows the whole expression is false.

The next line,print (5, not a == 7 and b == 7), uses thenot operator.not just gives the opposite of the expression (The expression could be rewritten asprint (5, a != 7 and b == 7)). Here's the table:

The two following lines,print (6, a == 7 or b == 7) andprint (7, a == 7 or b == 6), use the or operator. The or operator returns true if the first expression is true, or if the second expression is true or both are true. If neither are true it returns false. Here's the table:

Note that if the first expression is true Python doesn't check the second expression since it knows the whole expression is true. This works since or is true if at least one of the expressions are true. The first part is true so the second part could be either false or true, but the whole expression is still true.

The next two lines,print (8, not (a == 7 and b == 6)) andprint (9, not a == 7 and b == 6), show that parentheses can be used to group expressions and force one part to be evaluated first. Notice that the parentheses changed the expression from false to true. This occurred since the parentheses forced the not to apply to the whole expression instead of just the a == 7 portion.

Here is an example of using a boolean expression:

list=["Life","The Universe","Everything","Jack","Jill","Life","Jill"]# Make a copy of the list.copy=list[:]# Sort the copycopy.sort()prev=copy[0]delcopy[0]count=0# Go through the list searching for a matchwhilecount<len(copy)andcopy[count]!=prev:prev=copy[count]count=count+1# If a match was not found then count can't be < len# since the while loop continues while count is < len# and no match is foundifcount<len(copy):print("First Match:",prev)

See theLists chapter for an explanation of the slice operator, [:], occurring incopy = list[:].

Here is the output:

First Match: Jill

This program works by continuing to check for a match while count < len(copy) and copy[count] != prev. When either count is greater than the last index of copy or a match has been found the and is no longer true so the loop exits. The if simply checks to make sure that the while exited because a match was found.

The other 'trick' of and is used in this example. If you look at the table for and notice that the third entry is "false and won't check". If count >= len(copy) (in other words count < len(copy) is false) then copy[count] is never looked at. This is because Python knows that if the first is false then they both can't be true. This is known as a short circuit and is useful if the second half of the and will cause an error if something is wrong. I used the first expression ( count < len(copy)) to check and see if count was a valid index for copy. (If you don't believe me remove the matches `Jill' and `Life', check that it still works and then reverse the order of count < len(copy) and copy[count] != prev to copy[count] != prev and count < len(copy).)

Boolean expressions can be used when you need to check two or more different things at once.

password1.py

## This programs asks a user for a name and a password.# It then checks them to make sure that the user is allowed in.# Note that this is a simple and insecure example,# real password code should never be implemented this way.name=raw_input("What is your name? ")password=raw_input("What is the password? ")ifname=="Josh"andpassword=="Friday":print("Welcome Josh")elifname=="Fred"andpassword=="Rock":print("Welcome Fred")else:print("I don't know you.")

Sample runs

What is your name? JoshWhat is the password? FridayWelcome JoshWhat is your name? BillWhat is the password? SaturdayI don't know you.

1. Write a program that has a user guess your name, but they only get 3 chances to do so until the program quits.

Solutions

• Book:Python Programming