Constants in C

Constants are fixed values that do not change during the execution of a program. They are used to define values that remain constant throughout the program, ensuring better readability, maintainability, and security of the code.

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Types of Constants

Constants in C are broadly classified into two categories:

1. Primary Constants
2. Secondary Constants

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Comparison Table: Primary vs. Secondary Constants

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

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1. Primary Constants

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Primary constants are fundamental values that remain unchanged. These include:

1.1 Integer Constants

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• An integer constant contains only digits and does not include any decimal point.
• It can be positive, negative, or zero.
• It can be represented in different number systems:
  • Decimal (Base 10): Example: 1, -43, 500
  • Octal (Base 8): Starts with 0. Example: 077 (equals 63 in decimal)
  • Hexadecimal (Base 16): Starts with 0x. Example: 0x1A (equals 26 in decimal)

Example in C:

#include <stdio.h>

int main() {
    const int age = 25;
    printf("Age: %d\n", age);
    return 0;
}

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1.2 Real Constants (Floating Point Constants)

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• A real constant (or floating-point constant) contains digits and must have a decimal point.
• It can be expressed in:
  • Fractional form: Example: 3.1416, -54.5
  • Exponential form: Example: 2.5e3 (equals 2500), 3.2E-4 (equals 0.00032)

Example in C:

#include <stdio.h>

int main() {
    const float PI = 3.1416;
    printf("Value of PI: %f\n", PI);
    return 0;
}

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1.3 Character Constants

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• A character constant is a single character enclosed in single quotes (' ').
• Examples: 'A', '1', '%', 'z'
• Each character constant is internally stored as an integer (ASCII value).

Example in C:

#include <stdio.h>

int main() {
    const char grade = 'A';
    printf("Grade: %c\n", grade);
    return 0;
}

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2. Secondary Constants

Secondary constants are derived from primary constants. These include:

2.1 String Constants

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• A string constant is a sequence of characters enclosed in double quotes (" ").
• It automatically ends with a null character (\0).

Example in C:

#include <stdio.h>

int main() {
    const char message[] = "Hello, World!";
    printf("%s\n", message);
    return 0;
}

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2.2 Array Constants

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• An array constant is a fixed collection of values stored in contiguous memory locations.

Example in C:

#include <stdio.h>

int main() {
    const int numbers[] = {10, 20, 30};
    printf("First Element: %d\n", numbers[0]);
    return 0;
}

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2.3 Pointer Constants

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• A pointer constant is a pointer whose memory address cannot be changed.

Example in C:

#include <stdio.h>

int main() {
    int value = 50;
    int *const ptr = &value;
    *ptr = 100;  // Allowed
    // ptr = &new_value;  // Not Allowed
    printf("Value: %d\n", value);
    return 0;
}

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2.4 Enumeration Constants (enum)

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• An enumeration is a set of named integer constants.

Example in C:

#include <stdio.h>

enum Days { MON = 1, TUE, WED, THU, FRI, SAT, SUN };

int main() {
    enum Days today = WED;
    printf("Today is day number: %d\n", today);
    return 0;
}

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2.5 Structure Constants (struct)

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• A structure constant allows defining immutable fields inside a struct.

Example in C:

#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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2.6 Macro Constants (#define)

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• A preprocessor directive to define constants before compilation.

Example in C:

#include <stdio.h>

#define PI 3.1416

int main() {
    printf("PI: %f\n", PI);
    return 0;
}

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