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Table of contents
1.
Introduction
2.
What is #define in c?
3.
Why We Need #define ?
4.
Syntax
5.
Examples 
5.1.
C
5.2.
C
6.
Frequently Asked Questions
6.1.
What is #define in C?
6.2.
Why is #define used?
6.3.
How can we define a macro in the C programming language?
6.4.
Can we change the value of #define in our code?
6.5.
What are the 4 conditional statements in C?
7.
Conclusion
Last Updated: Apr 15, 2024
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C #define

Author ANKIT KUMAR
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Introduction

In the C programming language, all the statements starting with the # (hash) symbol are known as preprocessor directives/commands. The #define is also a preprocessor directive. Preprocessor directives are executed before any other command in our program. 

C #define

In a C Program, we generally write all the preprocessor directives outside the main() function at the top of our C program. #define directive is used to define constants or an expression in our C program.

Before we move on to learn the syntax of the #define preprocessor directive, let us first see why we even need it.

Recommended Topics, Sum of Digits in C

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. The preprocessor command #define is used in C programming to define symbolic constants and macros. By giving a constant variable or a section of code a name, it can improve the readability, flexibility, and maintainability of the code.

There are multiple use cases of #define in C:

Defining constants 

#define PI 3.14159

 
Defining macros 

#define SQUARE(x) ((x) * (x))
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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

// 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

// 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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