We already have been usingI/O since the first chapter, but only to get going.It is time to dig a little bit deeper, so we can write nicer programs.

In its heydays Pascal was so smart and defined a minimal common, yet convenient interface to interact withI/O.Despite various standardization effortsI/O operations differ among every singleOS, yet – as part of the language – Pascal defines a set of operations to be present, regardless of the utilized compiler orOS.

In thefirst chapter it was already mentioned thatinput andoutput are special program parameters.If you list them in the program parameter list, you can use these identifiers to write and read from the terminal, theCLI you are using.

In fact,input andoutput are variables.Their data type istext.We call a variable that has the data typetext atext file.

The data of atext file are composed oflines.Aline is a (possibly empty) sequence of characters (e. g. letters, digits, spaces or punctuation) until and including a terminating “newline character”.

A file – in general – has the following properties:

• It can be associated with an external entity.External means “outside” of your program. A suitableentity can be, for instance, your console window, a device such as your keyboard, or a file that resides in your file system.
• If a file is associated with an external entity, it is consideredbound.
• A file has amode. Every file can be ingeneration orinspection mode, none or both. If a file is in generationand inspection mode at the same time, this can also be calledupdate mode.^{[fn 1]}
• Every file has abuffer. This buffer is a temporary storage for writing or reading data, so virtually another variable. This buffer variable exists due to reasons howI/O on computers works.

All this information isimplicitly available to you, you do not need to take care of it.You can query and alter some information in predefined ways.

All you have to keep in mind in order to successfully use files is that a filehas a mode.The text filesinput andoutput are, once they are listed in the program parameter list, in inspection and generation mode respectively.You canonlyread data from files that are inspection mode.And it is only possible towrite datato files that are generation mode.

Note, due to their special nature the mode ofinput andoutput cannot be changed.

Pascal defines the following routines to read and write to files:

• get,
• put,
• read/readLn, and
• write/writeLn.

The routinesreadLn andwriteLn can only be used in conjunction with text files, whereas all other routines work withany kind of file.In the following sections we will focus onread andwrite.These routines build upon the “low-level”get andput.In thechapter “Files” we will take a look at them, though.

Let’s look at a simple program:

programwriteDemo(output);varx:integer;beginx:=10;writeLn(output,x:20);end.

Copy the program and see what it does.

First, we will learn a new statement, the assignment.Colon equals (:=) is read as “becomes”.In the linex := 10 the variable’s valuebecomes ten.On the left hand side you write a variable name.On the right hand side you put a value.The value has to be valid for the variable’s data type.For instance, you couldnot assign'Hello world!' to the variablex, because it is not a validinteger, i. e. the data typex has.

The power ofwrite/writeLn is that – for text files – it converts the parameters into a human-readable form.On modern computers theinteger valueten is stored in a particularbinary form.00001010 is a visual representation of the bits set (1) and unset (0) for storing “ten”.Yet, despite the binary storage the characters you see on the screen are10.This conversion, from zeroes and ones into a human-readable representation, the character sequence “10”, is doneautomatically.

If the destination ofwrite/writeLn is a text file, all parameters are converted into ahuman-readable form provided such conversion is necessary and makes sense.

Furthermore, after the parameterx comes :20.As you might have noticed, when you run the program the value ten is printed right-aligned making the 0 in 10 appear at the 20^th column (position from the left margin).

The:20 is aformat specifier.It ensures that the given parameter has aminimum width of that many characters and it may fill missing “width” with spaces to left.

Format specifiers in awrite/writeLn call can only be specified where a human-readable representation is necessary, in other words if the destination is a text file.

Look at this program:

programiceCream(input,output);varresponse:char;beginwriteLn('Do you like ice cream?');writeLn('Type “y” for “yes” (“Yummy!”) and “n” for “no”.');writeLn('Confirm your selection by hitting Enter.');readLn(input,response);ifresponse='y'thenbeginwriteLn('Awesome!');end;end.

All parameters given toread/readLn have to be variables.The first parameter, thesource, has to be a file variable which is currently in inspection mode.We ensure that by puttinginput into the program parameter list.If the source parameter isinput, you are allowed to omit it, thusreadLn(response) is equivalent toreadLn(input, response).

If the source is a text file, you can only read values for variables having the data typechar,integer,real, or “string types”. Other variables not compatible to these types cannot be read from a text file. (The term “compatible” will be explained later.)

A new language construct which we will cover in detail in the next chapter is theif-then-branch.The code afterthen that is surrounded bybegin andend; is only executed ifresponse equals to the character value 'y'.Otherwise, we are polite and do not express our strong disagreement.

programvalentine(output);constwidth=49;beginwriteLn('   ####     ####   ':width);writeLn(' ######## ######## ':width);writeLn('##     ####      ##':width);writeLn('##       #       ##':width);writeLn('##      ILY      ##':width);writeLn(' ##   sweetie   ## ':width);writeLn('  ###         ###  ':width);writeLn('    ###     ###    ':width);writeLn('      ### ###      ':width);writeLn('        ###        ':width);writeLn('         #         ':width);end.

programvalentine(output);constwidth=49;beginwriteLn('####     ####   ':width);writeLn('######## ######## ':width);writeLn('##     ####      ##':width);writeLn('##       #       ##':width);writeLn('##      ILY      ##':width);writeLn('##   sweetie   ## ':width);writeLn('###         ###  ':width);writeLn('###     ###    ':width);writeLn('### ###      ':width);writeLn('###        ':width);writeLn('#         ':width);end.

programvalentine(output);constwidth=49;beginwriteLn('   ####     ####   ':width);writeLn(' ######## ######## ':width);writeLn('##     ####      ##':width);writeLn('##       #       ##':width);writeLn('##      ILY      ##':width);writeLn(' ##   sweetie   ## ':width);writeLn('  ###         ###  ':width);writeLn('    ###     ###    ':width);writeLn('      ### ###      ':width);writeLn('        ###        ':width);writeLn('         #         ':width);end.

programvalentine(output);constwidth=49;beginwriteLn('####     ####   ':width);writeLn('######## ######## ':width);writeLn('##     ####      ##':width);writeLn('##       #       ##':width);writeLn('##      ILY      ##':width);writeLn('##   sweetie   ## ':width);writeLn('###         ###  ':width);writeLn('###     ###    ':width);writeLn('### ###      ':width);writeLn('###        ':width);writeLn('#         ':width);end.

o--------------------------------------o|                                      ||                                      ||                                      ||                                      |o--------------------------------------o

programbox(output);constspace=38;beginwriteLn('o--------------------------------------o');writeLn('|','':space,'|');writeLn('|','':space,'|');writeLn('|','':space,'|');writeLn('|','':space,'|');writeLn('o--------------------------------------o');end.

programbox(output);constspace=38;topAndBottomEdge='o--------------------------------------o';beginwriteLn(topAndBottomEdge);writeLn('|','':space,'|');writeLn('|','':space,'|');writeLn('|','':space,'|');writeLn('|','':space,'|');writeLn(topAndBottomEdge);end.

programbox(output);constspace=38;beginwriteLn('o--------------------------------------o');writeLn('|','':space,'|');writeLn('|','':space,'|');writeLn('|','':space,'|');writeLn('|','':space,'|');writeLn('o--------------------------------------o');end.

programbox(output);constspace=38;topAndBottomEdge='o--------------------------------------o';beginwriteLn(topAndBottomEdge);writeLn('|','':space,'|');writeLn('|','':space,'|');writeLn('|','':space,'|');writeLn('|','':space,'|');writeLn(topAndBottomEdge);end.

Notes:

• Book:Pascal Programming