Constants in C

In C programming, constants are read-only values that cannot be modified during the execution of a program. These constants can be of various types, such as integer, floating-point, string, or character constants. They are initialized with the declaration and remain same till the end of the program.

Let's take a look at an example:

#include <stdio.h>

int main() {
 	
  	// Defining constant variable
  	const int a = 10;
 	printf("%d", a);
    return 0;
}

10

Explanation: In the above program, we created a constant variable a with value 10. This value cannot be modified later in the program.

Syntax

We define a constant in C using the const keyword. Also known as a const type qualifier, the const keyword is placed at the start of the variable declaration to declare that variable as a constant.

const data_type var_name = value;

Properties of Constant

The important properties of constant variables in C defined using the const keyword are as follows:

1. Initialization with Declaration

We can only initialize the constant variable in C at the time of its declaration. If we do not initialize it at the time of declaration, it will store the garbage value that was previously stored in the same memory.

#include <stdio.h>

int main() {
  
  	// Not initializing a constant variable
	const int a;
  
  	// printing value
  	printf("%d", a);
    return 0;
}

0

The garbage value here is 0 but i can by anything depending on your environment.

2. Immutability

The constant variables in c are immutable after its definition, i.e., they can be initialized only once in the whole program. After that, we cannot modify the value stored inside that variable.

#include <stdio.h>
int main() {
  
    // Declaring a constant variable
    const int a;
  
    // Initializing constant variable var after declaration
    a = 20;
    printf("%d", a);
    return 0;
}

Output

In function 'main':
10:9: error: assignment of read-only variable 'a'
10 |     a = 20;
     |      ^

Constants Using #define

In C, the #define directive can also be used to define symbolic constants that do not require a data type. They are called macros and are replaced by their values at compile time.

Syntax:

#define CONSTANT_NAME value

Example:

#include <stdio.h>
#define PI 3.14

int main() {
    printf("%.2f", PI);  
    return 0;
}

3.14

Explanation: In this example, the value 3.14 is replaced by PI during the compilation.

Difference Between Constants and Literals

The constant and literals are often confused as the same. But in C language, they are different entities and have different semantics. The following table lists the differences between the constants and literals in C:

const vs #define

The following table list the differences between the constants defined using const qualifier and #define in C: