Pascal - Program Structures

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 block
• Comments

Every pascal program generally has 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. 

This will produce following result −

Hello, World!

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 statement. 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 2.6.0 [2011/12/23] for x86_64
Copyright (c) 1993-2011 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.