Pascal - Program Structure

Before we study basic building blocks of the Pascal programming language, let us look a bare minimum Pascal program structure so that we can take it as a reference in upcoming chapters.

 

Pascal Program Structure

A Pascal program basically consists of the following parts:

  • Program name
  • Uses command
  • Type declarations
  • Constant declarations
  • Variables declarations
  • Functions declarations
  • Procedures declarations
  • Main program block
  • Statements and Expressions within each blocks
  • Comments

 

Every pascal program generally have a heading statement, a declaration and an execution part strictly in that order. Following format shows the basic syntax for a Pascal program:

program        //name of the program
uses          // comma delimited names of libraries you use
const         // global constant declaration block
var           //global variable declaration block
function      //function declarations,if any
 // local variables 
begin
  ...
end;
procedure  // procedure declarations,if any
//local variables 
begin
  ...
end;
begin     // main program block starts
  ...
end.     // the end of main program block 
 

 

Pascal Hello World Example

Following is a simple pascal code that would print the words "Hello, World!":

program HelloWorld;
uses crt;  //Here the main program block starts 
begin
   writeln('Hello, World!');
   readkey;
end.
 

Let us look various parts of the above program:

  • The first line of the program program HelloWorld; indicates the name of the program.
  • The second line of the program uses crt; is a preprocessor command which tells the compiler to include the crt unit before going to actual compilation.
  • The next lines enclosed within begin and end statements are the main program block. Every block in Pascal is enclosed within a begin statement and an end statements. However, the end statement indicating the end of the main program is followed by a full stop (.) instead of semicolon (;).
  • The begin statement of the main program block is where the program execution begins.
  • The lines within (*...*) will be ignored by the compiler and it has been put to add a comment in the program.
  • The statement writeln('Hello, World!'); uses the writeln function available in Pascal which causes the message "Hello, World!" to be displayed on the screen.
  • The statement readkey; allows the display to pause until the user presses a key. It is part of the crt unit. A unit is like a library in Pascal.
  • The last statement end. ends your program.

 

Compile and Execute Pascal Program:

  • Open a text editor and add the above mentioned code.
  • Save the file as hello.pas
  • Open a command prompt and go to the directory where you saved the file.
  • Type fpc hello.pas at command prompt and press enter to compile your code.
  • If there are no errors in your code the command prompt will take you to the next line and would generate hello executable file and hello.o object file.
  • Now type hello at command prompt to execute your program.
  • You will be able to see "Hello World" printed on the screen and program waits till you press any key.

 

$ fpc hello.pas
Free Pascal Compiler version 2.7.1 [2013/12/23] for x86_64
Copyright (c) 1993-2013 by Florian Klaempfl and others
Target OS: Linux for x86-64
Compiling hello.pas
Linking hello
8 lines compiled, 0.1 sec
$ ./hello
Hello, World!

Make sure that free pascal compiler fpc is in your path and that you are running it in the directory containing source file hello.pas.