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.

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 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 and 30th 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.

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);
end.

When the above code is compiled and executed, it produces the 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