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Pascal - Units

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A Pascal program can consist of modules called units. A unit might consist of some code blocks which in turn are made up of variables and type declarations, statements, procedures etc. There are many built-in units in Pascal and Pascal allows programmers to define and write their own units to be used later in various programs.

 

1. Using Built-in Units

Both the built-in units and user defined units are included in a program by the uses clause. We have already used the variants unit in the Pascal - Variants tutorial. This tutorial explains creating and including user-defined units. However, let us first see how to include a built-in unit crt in your program:

program myprog;
uses crt; 
 

The following example illustrates using the crt unit:

Program Calculate_Area(input, output);
uses crt;
var 
   a, b, c, s, area: real;
begin
   textbackground(white);(* gives a white background *)
   clrscr;(*clears the screen *)
   textcolor(green);(* text color is green *)
   gotoxy(30,4);(* takes the pointer to the 4th line and30th column *) 
   writeln('This program calculates area of a triangle:');
   writeln('Area = area = sqrt(s(s-a)(s-b)(s-c))');
   writeln('S stands for semi-perimeter');
   writeln('a, b, c are sides of the triangle');
   writeln('Press any key when you are ready');
   readkey;
   clrscr;
   gotoxy(20,3);
   write('Enter a: ');
   readln(a);
   gotoxy(20,5);
   write('Enter b:');
   readln(b);
   gotoxy(20,7);
   write('Enter c: ');
   readln(c);
   s :=(a + b + c)/2.0;
   area := sqrt(s *(s - a)*(s-b)*(s-c));
   gotoxy(20,9);
   writeln('Area: ',area:10:3);
   readkey;
end.
 

It is the same program we used right at the beginning of the Pascal tutorial, compile and run it to find the effects of the change.

 

2. Creating and Using a Pascal Unit

To create a unit, you need to write the modules, or subprograms you want to store in it and save it in a file with .pas extension. The first line of this file should start with the keyword unit followed by the name of the unit. For example:

unit calculateArea;
 

Following are three important steps in creating a Pascal unit:

  • The name of the file and the name of the unit should be exactly same. So our unit calculateArea will be saved in a file named calculateArea.pas
  • The next line should consist of a single keyword interface. After this line, you will write the declarations for all the functions and procedures that will come in this unit.
  • Right after the function declarations, write the word implementation, which is again a keyword. After the line containing the keyword implementation, provide definition of all the subprograms.

The following program creates the unit named calculateArea:

unit CalculateArea;
 
 interface
 
function RectangleArea( length, width: real): real;
function CircleArea(radius: real): real;
function TriangleArea( side1, side2, side3: real): real;
 
 implementation
 
function RectangleArea( length, width: real): real;
begin
   RectangleArea:= length * width;
end;
function CircleArea(radius: real): real;
const
   PI =3.14159;
begin
   CircleArea:= PI * radius * radius;
end;
function TriangleArea( side1, side2, side3: real): real;
var
   s, area: real;
begin
   s :=(side1 + side2 + side3)/2.0;
   area := sqrt(s *(s - side1)*(s-side2)*(s-side3));
   TriangleArea:= area;
end;
end.
 

Next, let us write a simple program that would use the unit we defined above:

program AreaCalculation;
uses CalculateArea, crt;
var
   l, w, r, a, b, c, area: real;
begin
   clrscr;
   l :=5.4;
   w :=4.7;
   area :=RectangleArea(l, w);
   writeln('Area of Rectangle 5.4 x 4.7 is: ', area:7:3);
   r:=7.0;
   area:=CircleArea(r);
   writeln('Area of Circle with radius 7.0 is: ', area:7:3);
   a :=3.0;
   b:=4.0;
   c:=5.0;
   area:=TriangleArea(a, b, c);
   writeln('Area of Triangle 3.0 by 4.0 by 5.0 is: ', area:7:3);
 
   readln; // to see the output
end.
 

When the above code is compiled and executed, it produces following result:

Area of Rectangle 5.4 x 4.7 is: 25.380
Area of Circle with radius 7.0 is: 153.938
Area of Triangle 3.0 by 4.0 by 5.0 is: 6.000