Q.The textbook explains that a simple macro is used to define a constant. It gives the general form #define identifier string and the following examples:

#define    COUNT     200  
#define    FALSE     0  
#define    CAPITAL   "BANGALORE"  
#define    PI        3.1415926  

#define is a preprocessor directive that performs text substitution before compilation, so wherever the identifier appears in the source code the preprocessor replaces it with the given string.

[!ANSWER]

Each line defines a symbolic constant: COUNT becomes 200, FALSE becomes 0, CAPITAL becomes the string "BANGALORE" and PI becomes 3.1415926 throughout the program.

[!TLDR]

#define creates a symbolic constant by textual substitution done by the preprocessor before the compiler ever sees the code.

The #define directive is a preprocessor compiler directive (not a statement); it is placed at the beginning of the program, before main(), begins with the # symbol in column one, and does not end with a semicolon. Its general form is:

#define identifier string

When such a line is included, the preprocessor replaces every occurrence of the identifier in the source code with the string, before compilation. The four examples each define a constant — COUNT is 200, FALSE is 0, CAPITAL is the text "BANGALORE", and PI is 3.1415926.

A short complete program shows the substitution in action:

#include <stdio.h>
#define PI 3.1415926

int main(void)
{
    float r = 2.0, area;
    area = PI * r * r;
    printf("area = %f\n", area);
    return 0;
}

Before compilation the preprocessor rewrites PI * r * r as 3.1415926 * r * r, so the program computes the area of a circle of radius 2.

[!ANSWER]

#define performs preprocessor text substitution: COUNT to 200, FALSE to 0, CAPITAL to "BANGALORE" and PI to 3.1415926. For the demo program above the output is area = 12.566370.