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Wiki ## 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');
clrscr;
gotoxy(20,3);
write('Enter a: ');
gotoxy(20,5);
write('Enter b:');
gotoxy(20,7);
write('Enter c: ');
s :=(a + b + c)/2.0;
area := sqrt(s *(s - a)*(s-b)*(s-c));
gotoxy(20,9);
writeln('Area: ',area:10:3);
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 TriangleArea( side1, side2, side3: real): real;

implementation

function RectangleArea( length, width: real): real;
begin
RectangleArea:= length * width;
end;
const
PI =3.14159;
begin
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
```