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 – Variantstutorial. 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 −
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