In C programming, #define is a preprocessor directive that is used to define macros. The macros are the identifiers defined by #define which are replaced by their value before compilation. We can define constants and functions like macros using #define. The generics in C are also implemented using the #define preprocessor directive along with _Generic.
Syntax of C #define
The syntax of #define preprocessor directive in C is:
For Defining Constants
#define MACRO_NAME value
For Defining Expressions
#define MACRO_NAME (expression within brackets)
For Defining Expression with Parameters
Arguments passed in the macros can be used in the expression.
#define MACRO_NAME(ARG1, ARG2,..) (expression within brackets)
There are a few more ways using which we can define macros. To know more, refer to this article - Macros and its types in C
Examples of C #define
Example 1:
In the below example, we have defined a macro 'PI' and assigned it a constant value which we can use later in the program to calculate the area of a circle.
C
// C Program to illustrate how to use #define to declare
// constants
#include <stdio.h>
// Defining macros with constant value
#define PI 3.14159265359
int main()
{
int radius = 21;
int area;
// Using macros to calculate area of circle
area = PI * radius * radius;
printf("Area of Circle of radius %d: %d", radius, area);
return 0;
}
OutputArea of Circle of radius 21: 1385
Example 2:
In the below example, we have defined a macro 'PI' and assigned it an expression, and that value of the expression is used in the program using 'PI'.
C
// C Program to illustrate the defining of expression using
// #define
#include <stdio.h>
// Defining macros with expression
#define PI (22 / 7)
int main()
{
int radius = 7;
int area;
// Using macros to calculate area of circle
area = PI * radius * radius;
printf("Area of Circle of radius %d: %d", radius, area);
return 0;
}
OutputArea of Circle of radius 7: 147
Example 3:
In the below example, we have defined two macros CIRCLE_AREA and SQUARE_AREA with a parameter and that parameter is used in the expression to calculate the area of circle and square respectively.
C
// C Program to define the function like macros using
// #define
#include <stdio.h>
// Defining parameterized macros with expression
#define CIRCLE_AREA(r) (3.14 * r * r)
#define SQUARE_AREA(s) (s * s)
int main()
{
int radius = 21;
int side = 5;
int area;
// Using macros to calculate areas by
// passing argument
area = CIRCLE_AREA(radius);
printf("Area of Circle of radius %d: %d \n", radius,
area);
area = SQUARE_AREA(side);
printf("Area of square of side %d: %d", side, area);
return 0;
}
OutputArea of Circle of radius 21: 1384
Area of square of side 5: 25
Important Points
- Macros declared using #define are used to store constants and cannot be changed. we cannot assign variables to the macros.
- We cannot use the '=' operator to assign value to the macros (eg. #define PI 3.14).
- We do not use the semicolon ';' at the end of the statement in #define.
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