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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.
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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.
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Save the file as hello.pas
-
Open a command prompt and go to the directory, where you saved the file.
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Type fpc hello.pas at command prompt and press enter to compile your code.
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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.
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Now, type hello at command prompt to execute your program.
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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.
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