Constants in C

C Constants – Complete Guide

1. What is a Constant?

A constant in C is a value that cannot be changed during program execution. Constants make programs more readable, maintainable, and safe.

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2. Types of Constants

Constants are broadly classified into:

1. Primary Constants – Basic, fundamental constants.
2. Secondary Constants – Derived from primary constants.

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3. Primary vs. Secondary Constants – Quick Table

Category	Type	Example
Primary Constants	Integer Constant	10, -3, 0xA
	Real Constant (Floating Point)	3.14, -5.6, 2.5e3
	Character Constant	'A', '9', '%'
Secondary Constants	String Constant	"Hello"
	Array Constant	{1, 2, 3}
	Pointer Constant	int *const p;
	Enumeration Constant (enum)	enum Colors { RED, GREEN };
	Structure Constant (struct)	struct Point { const int x; }
	Macro Constant (#define)	#define MAX 100

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4. Primary Constants

4.1 Integer Constants

• Whole numbers without a decimal point.
• Can be positive, negative, or zero.

• Number systems:

  • Decimal (Base 10) → 1, -43, 500
  • Octal (Base 8) → Starts with 0 → 077 (63 in decimal)
  • Hexadecimal (Base 16) → Starts with 0x → 0x1A (26 in decimal)

Example:

#include <stdio.h>
int main() {
    const int age = 25;
    printf("Age: %d\n", age);
    return 0;
}

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4.2 Real Constants (Floating-Point)

• Contain digits and a decimal point.

• Can be in:

  • Fractional form → 3.14, -54.5
  • Exponential form → 2.5e3 (2500), 3.2E-4 (0.00032)

Example:

#include <stdio.h>
int main() {
    const float PI = 3.1416;
    printf("Value of PI: %.4f\n", PI);
    return 0;
}

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4.3 Character Constants

• A single character in single quotes (' ').
• Stored internally as an ASCII integer value.

Example:

#include <stdio.h>
int main() {
    const char grade = 'A';
    printf("Grade: %c\n", grade);
    return 0;
}

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5. Secondary Constants

5.1 String Constants

• A sequence of characters in double quotes (" ").
• Automatically ends with \0 (null character).

Example:

#include <stdio.h>
int main() {
    const char message[] = "Hello, World!";
    printf("%s\n", message);
    return 0;
}

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5.2 Array Constants

• A fixed collection of constant values.

Example:

#include <stdio.h>
int main() {
    const int numbers[] = {10, 20, 30};
    printf("First element: %d\n", numbers[0]);
    return 0;
}

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5.3 Pointer Constants

• A pointer whose address cannot change after initialization.

Example:

#include <stdio.h>
int main() {
    int value = 50;
    int *const ptr = &value;
    *ptr = 100; // Allowed (change value)
    printf("Value: %d\n", value);
    return 0;
}

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5.4 Enumeration Constants (enum)

• Named integer constants for better readability.

Example:

#include <stdio.h>
enum Days { MON = 1, TUE, WED, THU, FRI, SAT, SUN };
int main() {
    enum Days today = WED;
    printf("Day number: %d\n", today);
    return 0;
}

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5.5 Structure Constants (struct)

• Immutable structure members using const.

Example:

#include <stdio.h>
struct Point {
    const int x;
    const int y;
};
int main() {
    struct Point p = {10, 20};
    printf("Point: (%d, %d)\n", p.x, p.y);
    return 0;
}

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5.6 Macro Constants (#define)

• Defined before compilation using the preprocessor.

Example:

#include <stdio.h>
#define PI 3.1416
int main() {
    printf("PI: %.4f\n", PI);
    return 0;
}