Pascal Tutorial

By Daniel D'Agostino

Contents

Pascal Language Tutorial

Pascal Reference

Pascal Language Tutorial

Introduction to Pascal

In the late 1950s, an international committee led by Peter Naur developed ALGOL, a high-level language intended for scientific computing. This language was designed to be platform-independent, making it more flexible, but also making it harder to write compilers for it on the various platforms it supported. ALGOL was mostly abandoned by programmers, except to describe algorithms (hence the name, ALGOrithmic Language).

During the next decade, several computer scientists worked to develop ALGOL to improve it and eliminate its drawbacks. Among them was Dr. Niklaus Wirth of the Swiss Federal Institute of Technology, who in 1971 published his specification of Pascal, a new high-level language named after Blaise Pascal, the 17th-century mathematician and philosopher who in 1642 invented an adding and subtracting machine, i.e. one of the first forms of computer.

Nowadays, Pascal has mostly withered from the professional programming scene. However, although obsolete compared to the languages there are today, Pascal is nowhere as forgotten as ALGOL, from which it was forged. Its syntax, mainly based on simple English, makes it very easy to understand, and thus Pascal is mainly used nowadays to introduce students to programming.

Overview of the Programming Process

The Compiler

What you will be writing in Pascal is called the source code, i.e. it is the code written by the programmer in a particular programming language.

The source code needs to be translated into machine language (executional code or object code) that the computer can understand in order to be executable. The source code alone is just text; the actual program is the translated code brought to life in machine language.

A compiler is a program which translates source code to object code. Therefore you will need a compiler to program in Pascal. There are a number of free compilers around on the internet (such as Free Pascal or Dev-Pascal), but the one used as standard is Borland Turbo Pascal. Pascal code that works with Turbo Pascal is not guaranteed to work with other compilers.

Borland has released up to version 5.5 of Turbo Pascal for free at their software museum, but you will need to create a Borland user account to be able to download it. The latest version of Turbo Pascal is 7.0 (which is obviously much better than 5.5), but it is not free.

Once you write your source code in the compiler, you need to compile the source code to translate it into machine language. After compilation, you may run the program to test it. If compilation is not successful and an error is shown, you will need to debug the code to make the program work.

Note that if you get a message like "Error 200: Division by Zero", then the problem is with the compiler, not with your code. Search for "Pascal patch" in your favourite search engine and download the patch. Run it to fix the error.

Errors in Programming

There are three kinds of errors encountered when programming:

When any error is encountered, you will need to debug the program, finding and eliminating errors. The compiler will usually tell you the line in which the error occurred, and it will also tell you what went wrong. That will help you find errors and fix them.

Comments

Any computer language allows you to write comments in your code, i.e. to make notes about what each part of the code does. This will not increase the size of the output program, and is very useful. If you go back to look at your code after a long time, it will be very difficult to understand what you did; comments help you to recognise what your code does.

There are two ways to write comments in Pascal:

{this is a comment}
(*
   this is another comment
*)

Both methods work the same way and may be used on single or multiple lines alike. { or (* begins the comment, and } or *) ends it.

Basic Input and Output

The basic program begins with actually giving the program a name (optional), using the program keyword followed by the name you want:

program calculator;

Almost every statement ends with a semicolon.

The actual instructions of the program go between the begin and end keywords:

program calculator;

begin

end.

Putting a semicolon after begin is optional, because it is the beginning of the main function; it is not a statement or command that is used on one line. The program always ends with a full stop.

Output

The write() or writeln() functions are used to write text on the screen. The only difference between them is that writeln() skips a line after what it displays, while write() keeps going on the same line.

To display text, put a text string in the brackets, enclosed by single quotes:

writeln('What is your favourite song?');
write('Nevermind. I do not want to know.');

You can use writeln to skip lines. Writing it without the brackets will skip a line without writing anything to the screen:

writeln;

The fact that text is enclosed in single quotes means that you cannot use an apostrophe within the string, or the string will end prematurely and give an error.

Write the apostrophe twice if you want to use it within the string:

write('It''s Raining!');

You may concatenate or join two separate strings within the write() or writeln() functions using a comma or plus sign. It is best to use the comma though, because while the plus sign will concatenate text, it will add if you are writing a numeric variable.

Constants

In the space between the program name and the begin point go many important declarations about variables, constants and other things to be used by the program. Among these are, of course, constants. These are numbers of text strings whose value never changes, but is simply used without changing.

Constants go in the const block, and have a value assigned to them using the 'equals' sign. They may later be used with write() or other functions.

program constdemo;

const
    pi = 3.141592654;
    e  = 2.718281828;

begin
  writeln('The value of pi is ',pi);
end.

Note that in the writeln() or write() functions, single quotes are only used to contain text. Numbers, constants, variables and other data types should not be enclosed in quotes or will be treated as text.

Variables

Variables are much like constants, with the difference that their value is subject to change.

Like constants, they are declared in the area between program and begin, and what you define is not their content but their type.

A Pascal variable can be of any of the following types:

Note that these data types can also be specified for constants:

const
    users_age:integer=20;
    pi       :real   =3.141592654

The following example demonstrates how to declare variales and use them in output.

program outputpi;

var
  author_name:string[4];
  pi         :real;

begin
  author_name := 'John';
  pi          := 3.141592654;

  write('Hi, my name is ', author_name, '!');
  write('The value of pi is ', pi, '.');
end.

As you can see above, values are assigned to variables with the assignation operator (:=).

Note that you can save space by declaring multiple variables of the same type on the same line, using a comma to separate them.

var
  day, year, age : integer;
  pi, e          : real;
  name, month    : string;

Note also that you do not have to stay declaring π or e. A predefined function for π exists so all you have to do is write() it as follows. Another function exists for e, which we will see later.

write(pi);

Input

The read() and readln() functions work for input pretty much like how write() and writeln() work for output. A variable is specified in the brackets, and the user input is stored in that variable:

var name : string;
begin
  writeln('What is your name?');
  readln(name);
  writeln('Pleased to meet you, ', name, '.');
end.

Note that if the program vanishes before showing you the final output, add another read statement to ask for input before ending the program.

With numerical input, you can have several numbers inputted at a time; the user will enter each number and press ENTER. This is done by entering multiple variables in the read() or readln() function, separating them with commas. Here is a practical example of its use:

program average_of_three_numbers;
var x,y,z:integer;

begin
  writeln('Please enter three numbers:');
  readln(x,y,z);
  writeln('Their average is ', ((x+y+z)/3));
end.

Arithmetic Operations

Like any real programming language, Pascal can be used to perform arithmetic operations. There are several symbols involved; we have already seen the use of the assignation operator (:=) and two of the arithmetic operations are shown in the previous example. These are the arithmetic operators available in Pascal:

OperatorOperationData typeExampleAnswer to Example
+AdditionReal/Integer5+38
-SubtractionReal/Integer15-69
*MultiplicationReal/Integer4*520
/Real DivisionReal/Integer2.2/1.12.0000000000E+00
divInteger DivisionInteger81 div 99
modModulus (Remainder)Integer5 mod 21

The first three operators should be very straightforward. The Real Division (/) divides two numbers and gives the answer as a decimal, irrespective of whether they were integers or decimals. The Integer Division (div), on the other hand, is used to divide two integers and gives the answer as an integer. The Modulus (mod) operator returns the remainder of the division of two numbers.

You may use brackets to perform some operations before others. The following example illustrates the use of arithmetic operations within a program.

program tracknums;
var a,b,c:integer;

begin
  a := 3;                           {a=3, b=0,  c=0 }
  c := (5*a);                       {a=3, b=0,  c=15}
  b := (c-a);                       {a=3, b=12, c=15}
  a := (c mod b);                   {a=0, b=12, c=15}
  c := ((c div 5) + (b-9));         {a=0, b=12, c=6 }
end.

Note that if you are outputting numbers which are to undergo arithmetical operations, putting the operations in brackets in the writeln() function is just as good as using the assignation operator:

  a := 5;
  b := (a+5);
  writeln(b);      {b=10}
  a:=5;
  writeln((a+5));  {10 is written}

Ordinal Functions

integer and char are called ordinal data types. Each integer or character has another before it (predecessor) and another after it (successor). For example, the successor of 'a' is 'b' and the predecessor of 20 is 19.

For the purpose of finding these predecessors and successors, you may use the following functions:

FunctionDescriptionExamplesOutput of Examples
chr()Displays the character corresponding to the ASCII value specified
chr(97);
chr(43);
chr(234);
a
+
Ω
ord()Displays the ordinal value of the specified character or integer. In the case of integers, the integer itself is returned. In the case of characters, the corresponding ASCII value is returned.
ord(5);
ord('a');
ord(25);
ord('r');
5
97
25
114
pred()Returns the predecessor
pred(39);
pred('b');
38
a
succ()Returns the successor
succ(8);
succ('x');
9
y

Advanced Mathematical Functions

A number of advanced mathematical functions are available in Pascal, as illustrated below. Don't expect to understand them without post-secondary Mathematical knowledge.

FunctionParameterGeneral OutputExampleResult
sin()angle in radianssin xsin(pi);0
cos()angle in radianscos xcos(pi);-1
arctan()angle in radianstan-1 xarctan(0);0
exp()numberexexp(1);2.7182818285E+00
ln()positive numberln xln(exp(1));1.0000000000E+00
sqr()numberx2sqr(-5);25
sqrt()positive numberxsqrt(390625);625
round()numberx rounded to nearest integerround(2.7);3
trunc()numberx rounded down to previous integertrunc(2.7);2
abs()number|x|abs(-5);5

Random Numbers

It is often necessary to generate random numbers, especially when games are involved. To generate random numbers, first initialise the generation of random numbers using the randomize function, then use the random() function to generate a random number between zero and one less than the parameter of random(), i.e. if the parameter is n, then the range in which the generated random number lies is 0≤x<n and it is between zero and (n-1).

program guessnum;

var a,b:integer;

begin
  randomize;
  a:=(random(6)+1);  {random number between 1 and 6}
  writeln('Random number generated: ',a);
  readln;
end.

Setting Output Width

You can specify a width for whatever you write() by putting a colon after the output:

writeln('Hi':10);

The above example will make the word 'Hi' take up ten spaces, so that eight extra spaces will be added before the word:

        Hi

If you are using decimal (real type) numbers, you can specify the number of decimal places to show with an additional colon:

writeln(52.234567:1:3);

The 'whole number' part of the real number is displayed as is, no matter whether it exceeds the first width (in this case 1) or not, but the decimal part is truncated to the number of decimal places specified in the second width. The previous example will result in 52.234 being written.

The CRT Unit

One of the best things about high-level programming languages is that the range of functions available to you can be extended by using one or more libraries which contain the definitions of the additional functions they let you use.

In Pascal, these libraries are called units, and they are actually .TPU files in the UNITS folder of your Turbo Pascal compiler.

You can use units with the uses keyword, which goes in the area between program and begin:

uses crt;

Once the unit us used, you may then use its functions.

One of the easiest units to use is the CRT unit. CRT stands for 'Cathode Ray Tube', and refers to the screen; it allows you to manipulate some features of the text screen such as text colour, text background colour, etc.

Functions of the CRT Unit

Clearing the screen

You can use the clrscr function to clear the screen (equivalent to the DOS cls command):

clrscr;

Simple as that. This statement clears the screen.

Most of the next functions we will be seeing use one or more parameters. A parameter is what goes in the function's brackets, for example 'Hi' is the parameter in writeln('Hi').

Text Colour and Background Colour

The textcolor() and textbackground() functions are used to set the colour and background colour respectively of the text you output. As a parameter you can specify either the name (doesn't need quotes) or the corresponding number of the colour you want. There are sixteen colours you can choose from. Notes:

  1. You can only use the first 8 colours (dark colours) for the background.
  2. If you exceed the greatest colour number (7 for background or 15 for foreground), the colours start over (e.g. 0, 16 and 32 are all black).
  3. Adding 128 to the foreground colour will make it blink. However, I wouldn't encourage the use of blinking text... it can be quite annoying.

The following is an example of the use of these two functions.

program colourexample;

uses crt;

var a,b:integer;

begin
  clrscr;
  writeln('Enter text colour number you want');
  readln(a);
  textcolor(a);       {Text colour changes to the value of 'a'}
  writeln('Now enter background colour number');
  readln(b);
  textbackground(b);  {Background colour changes to 'b'}
  readln;
end.

The textattr variable is like a combination of textcolor() and textbackground(). Its general format is thus:

textattr := (backgroundcolour*16)+textcolour

backgroundcolour and textcolour represent the background and text colour numbers you want.

Going to a specified (x,y) coordinate in output

The gotoxy() function will move to a particuar position on the screen, so that whatever you write will be written there. Its syntax is gotoxy(x,y), where x is the distance from the left side of the screen and y is the distance from the top of the screen.

gotoxy(12,9);

This will move 12 spaces right and 9 spaces down from the top-left point of the screen.

Delaying

The delay() function will cause the program to wait for s milliseconds between one command and the next, where s is the parameter of the delay() function:

delay(3000); {Wait for 3000 milliseconds (3 seconds)}
Changing text size

You can change the size of text using the textmode() function. This does not allow you to change the font size as you please, but rather manipulates the width and height in characters of the screen, resulting in stretched text.

The normal dimensions of the screen are 80 characters (wide) by 25 characters (high). Thus textmode(co80) is the normal text mode. Using textmode(co40) will make only 40 characters fit in one line, so text will be wider. You can also use textmode(font8x8), which normally fits 50 lines instead of 25, resulting in text being squashed vertically. Finally, textmode(lastmode) switches to the last mode you used. Here is a summary of parameters available for the textmode() function:

Note that the width of font8x8 depends on which other mode was previously set.

Reading keypresses

readkey is a kind of variable that is given a value when a key is pressed.

uses crt;
var c:char;
begin
  c := readkey;
  writeln(c);
end.

When the user presses a key, the corresponding ASCII value of that key is passed to readkey, and is then assigned to the character variable we declared earlier. When we write it, the ASCII value is converted back to character format and written to the screen.

Halting program flow

The halt command can be used to end the program before it reaches the end of the instructions. Whatever is after halt will not be executed.

halt may also be used as a function, with an integer as a parameter. This parameter is used as an exitcode, so that it is returned when the program halts. Zero is the default exitcode if you use it as a command (i.e. without brackets or parameters).

halt;     {Halt with exitcode 0}
halt(1);  {Halt with exitcode 1}

Note that halt does not require the CRT unit to work.

Sounds

With the CRT Unit, you can play with sounds. Not complex sound and breathtaking music, but simple DOS beeps. You just have to use the sound() function with a frequency as its parameter, delay() the sound for a number of milliseconds specified as its parameter, and then use nosound to end the sound (otherwise it will continue beeping until you close the compiler).

It's fun. I tried making the 'Ready, get set, go' beeps they used to do in old racing games:

program racesnd;

uses crt;

begin
  sound(100); delay(200); nosound;
  delay(1000);
  sound(100); delay(200); nosound;
  delay(1000);
  sound(400); delay(400); nosound;
  readln;
end.

Conditional Statements

If

The if statement allows you to determine the program's next course of action depending on whether one or more conditions are true.

Imagine the program asks the user for his age, and displays a message depending on what he entered. If his age is zero, then write "Are you sure?" If the age is between 0 and 40, then write "You're still young!" Otherwise, write "You're really old!"

program askage;

var age, absage:integer;

begin
  writeln('Enter your age below.');
  readln(age);
  absage := abs(age);  {To handle negative age values}
  if (absage=0) then writeln('Are you sure?');
  else if ((absage > 0) and (absage < 40))
    then writeln('You''re still young!')
  else writeln('You''re really old!');
  readln;
end.

You should think of the conditional statement as one big command, so that a semicolon is only used to end it all in the final else statement, and not the statements after each then.

Each single if statement as it is only accepts one command. If we were to use other commands besides writeln(), you would need to have a begin..end block enclosing them.

if (absage=0) then
  begin
    writeln('Are you sure?');
    writeln('Enter your age again');
    readln(age)
  end;

Note that in this case, each statement except the last one in the begin..end block uses a semiconol, and a semicolon after end ends the block of commands. This end ends with a semicolon because it is only ending a block of commands. The end that ends with a full stop only ends like that because it is ending the whole program.

Logical/Comparative Operators

As we have seen, commands are executed depending on whether a condition is true. The condition usually depends on whether one or more variables are equal to a given value (like absage = 0) or within a given range (like absage > 39). The variable, in each case, is compared with the given value using a logical operator.

The following logical operators may be used:

OperatorDescription
<is less than
>is greater than
<=is less than or equal to
>=is greater than or equal to
=is equal to
<>is not equal to

Note that the logical operator = is different from the assignation operator :=. One of the most common mistakes in every programming language (apart from forgetting semicolons) is to mistake one for the other. := is used to assign a value to a variable; = compares a given value with that of a variable.

Boolean Operators

Four other boolean operators may be used to check whether a condition is true. They may be used to join multiple conditions or invert a condition, and the conditions they are associated with depend on whether the variable matches the specified value. In the following examples, numbers are used instead of variables to make things clearer.

program askage2;

var mage, fage : integer;

begin
  writeln('Enter your mother''s age below.');
  readln(mage);
  mage := abs(mage);
  writeln('Now enter your father''s age.');
  readln(fage);
  fage := abs(fage);
  if ((fage<20) and (mage<20)) then
    writeln('Your parents are both really young.');
  else if ((fage>=20) xor (mage>=20)) then
    writeln('One of your parents is really young.');
  else writeln('None of your parents are really young.');
  readln;
end.

Case

Using the if statement can sometimes be cumbersome, especially if there are lots of different possivilities of output for each instance of the variable.

The case statement is much neater in such cases, as it lists instructions for each instance or range of the variable. Its general syntax is:

case variable of
  instance1 : instruction1;
  instance2 : instruction2;
  else default_instruction;
end;

Here's an example of its use:

var mark:integer;
begin
  writeln('What was your Physics mark?');
  readln(mark);
  case mark of
    0:      writeln('Hopeless!');
    1..19:  writeln('You''re serious?');
    20..39: writeln('That''s a bad fail.');
    40..49: writeln('You failed, but it was close.');
    50:     writeln('You just passed!');
    51..59: writeln('You got a D.');
    60..69: writeln('You got a C.');
    70..79: writeln('Well done! That''s a B.'); 
    80..99: writeln('Excellent!');
    100:    writeln('Perfect!');
    else writeln('Huh?');
  end;
  readln;
end.

There are three ways to specify values in the case statement:

The else part is optional, but is useful to handle input outside the range we want it in (e.g. a mark greater than 100).

Note that if you want more than one instruction to be executed for each case, you need to use a begin..end block, just like with if statements.

Loops

Sometimes it is necessary to repeat a command for a number of times. This is called looping, and there are three kinds of loop.

The for..do Loop

The for..do loop repeats a command while counting from one number to another. Its general syntax is:

for variable:=value1 to value2 do
  instruction;

The program will count from value1 to value2 while executing the instruction.

Let's say we want to list all of the ASCII characters from zero to 255. Along with the for..do loop, we are going to use the chr() function. We have already said that its parameter is an integer, and it returns the corresponding ASCII character of its parameter.

var charnum : integer;
begin
  for charnum:=0 to 255 do
    writeln(chr(charnum));
  readln;
end.

If you want to count down (e.g. from 255 to 0), replace to with downto.

Note that a begin..end block is required for multiple commands, just like with conditional statements.

The while..do Loop

The while..do loop is similar to the for..do loop. It repeats the given instructions while a condition is true. This condition is usually associated with a variable lying within a certain range, and it is up to you to increase or decrease the variable in order to eventually break out of the range and end the loop. If you never break out of the range, the loop will be infinite.

program whiledoexample;

var x,c:integer;

begin
  writeln('Enter a number');
  readln(x);
  x:=abs(x);      {To take negative 'x' as positive}
  c:=0;
  while c<x do
    begin
      writeln('*');
      c:=c+1;
    end;
  readln;
end.

This simple program takes an integer, x, as input, and outputs x asterisks.

The repeat..until Loop

The repeat..until loop is similar to the while..do loop, with the difference that the condition you specify is that which should break the loop (as opposed to that which should keep the loop going, which you specify with the while..do loop), and that this condition is written after the loop's instructions.

The while..do block in the previous example could be rewritten with repeat..until as follows:

repeat
  begin
    write('*');
    c:=c+1;
  end;
until c>=x;

File Handling

To read from or write to text files, there are four main things you need to do:

  1. assign() a text file to a variable.
  2. Open the text file in a specific mode, depending on whether and how you want to read/write.
  3. Read/write file.
  4. close() the file.

So you first need to declare a variable of type text. This is a special data type purposely used for text files. You will be directly using this variable when opening, reading/writing and closing the file.

var datafile:text;

Once your variable is declared, use the assign() function to associate your variable with a specific file. This function takes two parameters; the first is the variable and the second is the name of the file.

assign(datafile,'data.txt');

Okay, so the first step is done. The next thing to do is decide what you are going to do with the file, because you will be using a different function depending on that (for all three functions the parameter is the file variable):

After using one of these functions to open the file in a specific mode, you will want to do whatever you planned to do with the file (read/write data). This is done using the same functions you used for writing text to the screen (read(), readln(), write() and writeln()), with the only difference that as a first parameter they will take the file variable. Note also that reading functions can only be used if the file was opened with reset(), and writing functions can only be used if either rewrite() or append() was used.

write(datafile,'Hello!');

Once you're done, you need to close() the file. Here's a full example.

program email_address_book;

uses crt;

var datafile:text;
    counter:integer;
    name,email:string;

begin;
  clrscr;
  assign(datafile,'data.txt');
  rewrite(datafile);
  for counter:=1 to 3 do
    begin;
      {user inputs data}
      writeln('Enter name ',counter,' out of 3');
      readln(name);
      writeln('Enter that person''s email.');
      readln(email);
      {data is written to file}
      writeln(datafile,name);
      writeln(datafile,email);
    end;
  close(datafile);
end.

This program asks you to input 3 people's name and email address. These are written to a file in successive lines. Organising them by line makes it easier to read them, as you can read by line too. Note that since rewrite() is used, the contents of the data file are erased and replaced with new data. If you want to read the entries, you can use the following example program.

program readfile;

var datafile:text;
    counter:integer;
    line:string;

begin;
  assign(datafile,'data.txt');
  reset(datafile);
  for counter:=1 to 3 do
    begin;
      readln(datafile,line);
      writeln('Name  ',counter,': ',line);
      readln(datafile,line);
      writeln('Email ',counter,': ',line);
      writeln;
    end;
  close(datafile);
end.

The eof() function

What if my program allows new entries to be appended to the file so that I don't know how many records there are? In such a case, using a for loop between two constant values (line 1 and 3 above) is useless. Instead, you can read line by line until you reach the end of the file, which is where the eof() (end of file) function returns true. The example program below is a rewrite of the program above, using the eof() function.

program readfile2;

var datafile:text;
    line:string;
    linecounter:integer;

begin;
  assign(datafile,'data.txt');
  reset(datafile);
  linecounter := 1;
  while eof(datafile) = false do
    begin;
      readln(datafile,line);
      linecounter := linecounter + 1;
      {Odd-numbered lines are names; even-numbered lines are addresses}
      if ((linecounter mod 2) = 0) then
        write('Name  ')
      else
        write('Email ');
      {Write line number followed by the actual name/email}
      writeln((linecounter div 2),': ',line);
      {Skip a line after each email}
      if ((linecounter mod 2) = 1) then
        writeln;
    end;
  close(datafile);
  readln;
end.

The eoln() function

eoln() is similar to eof(), but instead marks the end of a line. The following program is an example of its use. Try using it with the sample text file shown after it. After compiling and running the code as it is, replace eoln with eof and see the difference.

program eoln_example;

uses crt;

var thefile:text;
    filechar:char;

begin;
  clrscr;
  assign(thefile,'file.txt');
  reset(thefile);
  while eoln(thefile) = false do
    begin;
      read(thefile,filechar);
      write(filechar);
    end;
  close(thefile);
  readln;
end.
file.txt
This line will be read.
This line, on the other hand, will not.

Subprograms

In a program, there are several lines of code you might want to use more than once. Simply copying and pasting that code will bloat your source code, making it harder to develop the program. Code that needs to be reused may be written in subprograms (which are functions and procedures). Calling the required function/procedure will execute the necessary instructions, and save copying and pasting lots of lines of code.

Procedures

If all you want is to execute multiple instructions in a given order, the subprogram you need to use is the procedure. A procedure may have its own variables and constants, and consists of a begin..end block, which is where you put the instructions you want. Note that any variables or constants belonging to the subprogram are not available in the program's main begin..end block - variables (and constants) are restricted to a particular scope (the particular subprogram, loop or main program). Any subprogram must be defined at the beginning of a program before they can be used, and this is how it is done (in general):

procedure Proc_Name

const
  {constants go here}
var
  {variables go here}
begin;
  {do something}
end;

The definition goes before the main begin..end block. Proc_Name can be any name you choose for the procedure. Later, in the main program, you use the procedure by writing this name, followed by a semicolon, as shown below. Note that a procedure definition can consist of just the begin..end block, as the sections for constants and variables may be omitted if empty.

Proc_Name;

Parameters

Variables from the main program flow (which are otherwise not accessible by the subprogram because they are outside its scope) may be used in a subprogram by passing them as parameters. This is done by listing the variables used within brackets after the name of the subprogram. Multiple variables of the same type are separated by commas, and different variable types are separated by semicolons. The following is a sample program using a procedure and a couple of parameters.

program dlg;

uses crt;

procedure talk(speaker:integer; dialogue:string);
begin;
  textcolor(speaker);
  if (speaker = 1) then
      write('Shopkeeper')
  else if (speaker = 2) then
      write('Joe Black ');
  write(': ');
  textcolor(7);
  writeln(dialogue);
  writeln;
end;

begin;
  clrscr;
  writeln('Fictional dialogue.');
  writeln;
  writeln('-----');
  writeln;
  talk(1,'Good morning, sir. May I help you?');
  talk(2,'I''m looking for a girl.');
  talk(1,'Aren''t we all?');
  talk(2,'No, I mean she works here. Do you know Sandra?');
  talk(1,'Of course. Let me call her.');
  readln;
end.

Functions

Functions are like procedures, but they return a value that can be used within the main program. This means that they are not just used to carry out instructions, as procedures are, but they are used to manipulate data and return it for use within the main program flow. Like procedures, they can take constants, variables and parameters. The only difference in definition is that the function name (or the parameters, if any) must be followed by the type of data that the function will return.

The following is an example of the classic function to multiply two numbers. Of course, here the use of the function could easily be replaced by just using a*b within the writeln, but this is just to show you how it works.

program multiply_test;

var a,b:integer;

function multiply(x,y:integer):integer;
begin;
  multiply := x*y;
end;

begin;
  writeln('Enter the numbers you want to multiply.');
  readln(a,b);
  writeln(a,' x ',b,' = ',multiply(a,b));
  readln;
end.

As you can see, the function is assigned a value which is later used in the main program. Since the function consists of a value, calling it on its own like a procedure would do nothing (even though code can be added to the function to write the value, or whatever is needed). Instead, this value is used with another function (in this case writeln) to show the data it represents.

Subprogram techniques

Following are a few things you should know if you want to use subprograms properly in your programs.

Let's say you have two procedures, defined one after another. If you wanted both procedures to call each other, this would not be possible. The first procedure cannot call the second procedure, which is defined later. The solution is to declare your subprograms before they are defined, like this:

procedure First_Procedure; forward;
procedure Second_Procedure; forward;

As you can see, declaration involves writing the first line of your definition (including the procedure keyword, subprogram name and any parameters; followed by the forward keyword. This is called forward referencing.

Note that apart from calling other subprograms, subprograms may also call themselves. However, if this is done, it should be restricted by the condition of a loop. Otherwise, the subprogram would call itself infinitely, causing an infinite loop.

Data types

BGI Graphics

Pascal Reference

Index of Pascal Reserved Words

There is a set of reserved words in Pascal that cannot be used as identifiers, i.e. you cannot name variables, constants, functions, etc. with the name of these reserved words. You will notice if a word is reserved because it will display in white rather than the normal yellow.

Reserved WordUse
andBoolean Operator
asm
array
beginBeginning of a code block
caseConditional Statement
constDeclaration of Constants
constructor
destructor
divArithmetic Operator
doPerforms the commands in Loops
downtoCounts down in for..do loops
else"Otherwise" command in Conditional Statements
endEnd of a code block
exports
file
forLoop
function
goto
ifConditional Statement
implementation
in
inherited
inline
interface
label
library
modArithmetic Operator
nil
notBoolean Operator
object
ofSpecifies each case of a case statement
orBoolean Operator
packed
procedure
programSpecifies the program's name (see Basic Input and Output)
record
repeatLoop
set
shl
shr
stringData type (see Variables)
thenPerforms the commands in If statements
toCounts up in for..do loops
type
unit
untilSpecifies the break condition of the repeat..until loop
usesIncludes units (see The CRT Unit)
varDeclaration of Variables
whileLoop
with
xorBoolean Operator

Index of CRT Colours

There are 16 CRT colours in Pascal. They are the same 8 colours, but 0-7 are low-intensity and 8-15 are high-intensity. If you exceed 15, the colours start over (e.g. 0, 16 and 32 are all black).

NumberSampleName
0 black
1 blue
2 green
3 cyan
4 red
5 magenta
6 brown
7 lightgray
8 darkgray
9 lightblue
10 lightgreen
11 lightcyan
12 lightred
13 lightmagenta
14 yellow
15 white

You can use the following program to list the colours in Pascal.

program crt_colour_list;

uses crt;

var counter:integer;

begin;
  clrscr;
  for counter:=0 to 15 do
    begin;
      textcolor(counter);
      writeln(counter);
    end;
  readln;
end.

Index of Acronyms

AcronymFull TermDescription
ALGOLAlgorithmic LanguageHigh-level language developed in the late 1950s by a committee led by Peter Naur (see Introduction to Pascal)
ASCIIAmerican Standard for Character Information InterchangeThe ASCII Codes are numbers corresponding to a character. See Table of ASCII Codes for a list or Ordinal Functions for the chr() function which displays ASCII codes
BGIBorland Graphical InterfaceThe Pascal graphics library provided by Borland.
CRTCathode Ray TubeThe Cathode Ray Tube is what makes a monitor or screen. Look it up in a Physics book to see how it works.

Table of ASCII Codes

Please refer to www.asciitable.com/ until I put up this section.

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