Why We Need #define ?
#define is used to define macros in C. In very simple terms, consider a situation where a particular value is repeated very often in the program, and the value does not change. In most cases, we make a constant variable with the help of the const keyword and put that value in the declared variable. Now we can use this variable everywhere in our program.
The #define preprocessor directive is very similar to the const. However, not only can we define a constant, we can define an expression too. Macro definitions are not variables and cannot be changed by your program code like variables.
Let us first see the syntax of #define, and then we shall see a few examples that will help us understand more about #define.
Syntax
For Constants
#define CONST_NAME value
For an expression
#define CONST_NAME (expression within brackets)
CONST_NAME: This represents the name of the constant. By convention, it is all in capital letters. Wherever in the program the #define directive (CONSt_NAME) will be encountered, it will get replaced with the value assigned to it.
Value: It represents the value assigned to the constants defined using #define.
Expression: It is similar to any expression in C. It must always be enclosed within brackets.
It is important to note that the entire #define statement does not use the semicolon(;) symbol to end the statement.
Examples
Example 1: using #define to define a constant
C
// C program to demonstrate #deine
#include<stdio.h>
// The #define preprocessor directive
// This defines a constant NAME with value as Coding Ninjas
// We do not write '=' after NAME to assign value to it.
// We do not use ';' at the end of the statement
#define NAME "Coding Ninjas"
int main()
{
// We use the value of NAME
// It is not declared in the main() function, still we can access it
printf("I took a course from %s", NAME);
return 0;
}
Output:
I took a course from Coding Ninjas
Key Points
- We do not write '=' after NAME(the constant) to assign value to it
- We do not use ';' at the end of the statement as we do in another part of the code.
- We do not specify the data type while using #define to define the constants.
Example 2: using #define to define an expression
C
// C program to demonstrate #deine for expressions
#include<stdio.h>
// The #define preprocessor directive
// This defines a expression AREA which takes two values as parameter
// and returns the area
// We do not write '=' after AREA to assign value to it.
// We do not use ';' at the end of the statement
#define AREA(l,b) (l*b)
int main()
{
// we can simply use the AREA(l,b) as a normal function
// AREA(5,8) returns 40
int area1 = AREA(5,8);
// AREA(40,7) returns 280
int area2 = AREA(40,7);
printf("The first area is = %d", area1);
printf("\nThe second area is = %d", area2);
return 0;
}
Output:
The first area is = 40
The second area is = 280
Key Points
- We do not write '=' after AREA to assign value to it.
- We do not use ';' at the end of the statement.
Also see, Tribonacci Series and Short int in C Programming
Frequently Asked Questions
What is #define in C?
#define is a preprocessor directive used in C programs to define macros. The #define directive is used to declare constant values or a named expression that can be used throughout our C program.
Why is #define used?
The #define command is part of the C preprocessor languages. When used in code, the compiler simply replaces the #define statement with whatever you specify. It can be useful in doing tasks that are repeated in the code.
For e.g , by using #define fori(x) for (int i=0; i<x; i + +), we can simply write fori(10) to iterate from i=0 to 9, instead of typing the entire code for the for loop.
How can we define a macro in the C programming language?
In C, we can define a macro using the #define preprocessor directive.
For e.g., #define PI 3.14
Can we change the value of #define in our code?
No, we cannot change the value of macro defined using #define in our code as it is not like a normal variable. It can only be changed at the place of definition.
What are the 4 conditional statements in C?
In C conditional statements are used for decision making. Some of the most used conditional statements in C are if statement, if-else statement, else-if statement, and switch statement.
Conclusion
In this article, we have extensively discussed #define in C programming language along with the code examples to get a better understanding of #define in C. Learn more about functions in C here. You can explore all the blogs related to C here.
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