#define in C

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 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; } 

Area 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 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; } 

Area 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 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; } 

Area 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.