Pascal - Program Structure
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- Category: Chapter 1
- Published: Sunday, 14 April 2013 14:10
- Written by Sternas Stefanos
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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.
1. 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
2. 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.
3. 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.