Learn Pascal in a Three Days (2nd Ed.)
(Publisher: Wordware Publishing, Inc.)
Author(s):
ISBN: 1556225679
Publication Date: 07/01/97

The Assignment Statement

To store a value in a variable you can use the assignment operator (:=) as in the following examples:

    a:= 55;
    x:= 1.5;
    y:= 2.3E+02;

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CAUTION:  Do not use a real number like this:.1234
A legal real number in Pascal must have a digit to the left of the decimal point, like this: 0.1234
Also, the number: 123.
may be rejected by some compilers. It would be better to use the legal form: 123.0

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In the following program, two integers “a” and “b” are declared in the declaration part, then assigned integer values in the program block. The WRITELN statement is then used to evaluate and display the results of different arithmetic operations performed on those variables.

{ --------------------------- figure 1-8 --------------------------- }
PROGRAM Arithmetic(OUTPUT);
{ Variable Declarations }
VAR
 a, b :INTEGER;
{ Program block }
BEGIN
 a:= 25;
 b:= 2;
 WRITELN('a=',a);
 WRITELN('b=',b);
 WRITELN('a+b=',a+b);
 WRITELN('a-b=',a-b);
 WRITELN('a*b=',a*b);
 WRITELN('a/b=',a/b);
 WRITELN('a div b=',a DIV b); { used with integers only }
 WRITELN('a mod b=',a MOD b) { used with integers only }
END.

The output of the program is:

a=25
b=2
a+b=27
a-b=23
a*b=50
a/b= 1.2500000000E+01             ----> Real division
a div b=12                ----> Integer division
a mod b=1                 ----> Remainder of integer division

You may assign one variable to another thus:

    x:= y;

In this case, the contents of the variable “y” are copied to the variable “x.” You may also assign an arithmetic expression to a variable, like this:

    z:= a + b - 2;
    GrossPay:= PayRate * HoursWorked;

In these statements the value of the expression to the right of the assignment operator is calculated and stored in the variable to the left of the assignment operator (“z” or “GrossPay”).

1-5 Named Constants

Data values (in many languages including Pascal) are called constants, as they never change during the program execution. In Pascal there are two types of constants:

•  Literal constants

•  Named constants

Literal constants are data values such as explicit numbers and text strings, while a named constant is a “constant variable.” The difference between a named constant and a variable is that the value of the named constant does not change during the program. Like variables, a named constant is given a name and has to be declared in the declaration part. Actually, the declaration part is divided into two sections, CONST and VAR; the CONST section comes before the VAR section. Suppose that you would like to use the value 3.14159 (a numerical constant known as “Pi”) many times in your calculations. It would be more convenient to give it a name and use the name in your code. You can declare named constants as in the following example:

    CONST
     Pi = 3.14159;
     ThisYear = 1997;
     Department= 'OtoRhinoLaryngology';

Some constants are predefined in Pascal as standard identifiers. One useful predefined named constant is MAXINT, which gives the maximum value an integer can possess. The value depends on the computer used. If you want to know the value of MAXINT in your computer, use the statement:

    WRITELN(MAXINT);

A typical value is 32767 (two bytes).

In the following program, the perimeter of a circle is calculated using the named constant Pi.

{ ---------------------------  figure 1-9 --------------------------- }
PROGRAM Constants(OUTPUT);
{ Constant Declarations }
CONST
 Pi = 3.14159;
{ Variable Declarations }
VAR
 Radius, Perimeter :REAL;
{ Program block }
BEGIN
 Radius:= 4.9;
 Perimeter:= 2 * Pi * Radius;
 WRITELN('Perimeter=', Perimeter)
END.

The output of this program is:

Perimeter= 3.0787582000E+01

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NOTE:  If you are using Turbo Pascal, you do not need to redefine the constant Pi, as it is predefined as a standard identifier.

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1-6 Type Conversion: ROUND, TRUNC

You can assign an integer to a variable of the type REAL, but the opposite is not permitted. The reason for this is because the storage size allocated for an integer is smaller than that allocated for a real number. If this were permitted, data could be lost or corrupted when a large number was moved to a smaller location in which it did not fit. You can, however, perform the conversion with one of the two functions:

where:

Consider these examples:

    ROUND(8.4)     returns 8
    ROUND(8.5)     returns 9
    TRUNC(8.4)     returns 8
    TRUNC(8.5)     returns 8

As you can see in the examples, the two functions may or may not return the same integer value for the same argument.

In the following program the two functions are used to get the rounded and the truncated integer values of the real variable “Perimeter.”

{ --------------------------- figure 1-10 --------------------------- }
PROGRAM Functions1(OUTPUT);
{ Constant Declarations }
CONST
 Pi = 3.14159;
{ Variable Declarations }
VAR
 Perimeter, Radius :REAL;
 RoundedPerimeter, TruncatedPerimeter :INTEGER;
{ Program block }
BEGIN
 Radius:= 4.9;
 Perimeter:= 2*Pi*Radius;
 RoundedPerimeter:= ROUND(Perimeter);
 TruncatedPerimeter:= TRUNC(Perimeter);
 WRITELN('Perimeter=', Perimeter);
 WRITELN('Perimeter (rounded)=', RoundedPerimeter);
 WRITELN('Perimeter (truncated)=', TruncatedPerimeter)
END.

The output is:

Perimeter= 3.0772000000E+01        ----> The actual result
Perimeter (rounded)=31             ----> Rounded result
Perimeter (truncated)=30           ----> Truncated result