A constant is a literal value or a named value defined using the const or #define keyword.
Unlike variables, the value of a constant cannot be modified once it is defined.

Example:

const int age = 18;
age = 20;   // ❌ Error: assignment of read-only variable

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Types of Constants in C

C supports the following kinds of constants:

1. Integer Constants
2. Real (Floating-Point) Constants
3. Character Constants
4. String Constants
5. Enumeration Constants
6. Constants Using const Keyword
7. Constants Using #define

Let’s understand them in detail.

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1. Integer Constants

These represent whole numbers without any fractional part.

Valid Examples:

10  
-50  
0  
2000  

Invalid:

10.5     // Contains decimal  
1,000    // Comma not allowed  

Integer Constants Types:

• Decimal (base 10): 10, 245
• Octal (base 8): Must start with 0 → 015
• Hexadecimal (base 16): Must start with 0x → 0xAF

int a = 15;     // decimal
int b = 015;    // octal (13 in decimal)
int c = 0xAF;   // hexadecimal (175 in decimal)

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2. Real (Floating-Point) Constants

These represent numbers with decimal points or scientific notation.

Examples:

10.5  
0.001  
-25.75  
6.02e23  

float price = 99.99;
double gravity = 9.81;

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3. Character Constants

A character constant is a single character enclosed in single quotes (‘ ‘).

Examples:

'A'
'9'
'$'
'a'

Every character constant has an ASCII value.

printf("%d", 'A');  // Output: 65

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4. String Constants

A sequence of characters enclosed in double quotes (” “).

Examples:

"Hello"
"C Programming"
"1234"
"@#&$"

Strings end with a special null character \0 automatically.

char message[] = "Welcome to C";

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5. Escape Sequence Constants

These are special characters that represent actions such as newline or tab, starting with backslash \.

Escape Sequence	Meaning
\n	New line
\t	Horizontal tab
\\	Backslash
\'	Single quote
\"	Double quote
\0	Null character

printf("Hello\nWorld");

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6. Enumeration Constants (enum)

Used to define a set of named integer constants.

enum week { Mon, Tue, Wed, Thu, Fri, Sat, Sun };

Here, Mon = 0, Tue = 1, etc., unless manually assigned.

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7. Constants Using const Keyword

The const keyword makes variables read-only.

const float PI = 3.14159;
const int MAX = 100;

These constants have fixed values that cannot change.

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8. Constants Using #define Macro

The #define preprocessor directive creates symbolic constants.

#define PI 3.14159
#define MAX 100

Unlike const, this is replaced during preprocessing (before compilation).

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const vs #define (Comparison)

Feature	const	#define
Checked by compiler	Yes	No
Has data type	Yes	No
Stored in memory	Yes	No (text replacement)
Debugging support	Strong	Weak
Example	const int a=10;	#define a 10

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Advantages of Using Constants

• Prevent accidental value changes
• Improve readability
• Easier to modify (change value once → updates everywhere)
• Improve maintainability
• Reduce hard-coded values in code

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Example Program Using Different Constants

#include <stdio.h>
#define MIN_AGE 18

int main() {
    const float PI = 3.14;
    char grade = 'A';
    char name[] = "Sagar";

    printf("PI: %.2f\n", PI);
    printf("Name: %s\n", name);
    printf("Grade: %c\n", grade);
    printf("Minimum Age: %d\n", MIN_AGE);

    return 0;
}

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Conclusion

Constants are critical in C programming because they ensure that certain values remain fixed throughout the program. From numbers to strings and characters to macros, constants help create stable and maintainable applications.

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