C Programming

Variables and Constants in C

Variables are storage locations with an associated name that can contain data. Constants are fixed values that cannot be changed during program execution.

Variables

A variable is a name given to a memory location where data is stored. Variables must be declared before use.

#include <stdio.h>

int main() {
    // Variable declaration
    int age;
    float salary;
    char grade;
    
    // Variable initialization
    age = 25;
    salary = 50000.50;
    grade = 'A';
    
    // Declaration and initialization together
    int count = 10;
    double pi = 3.14159;
    
    printf("Age: %d\n", age);
    printf("Salary: %.2f\n", salary);
    printf("Grade: %c\n", grade);
    
    return 0;
}

Variable Naming Rules

Constants

Constants are fixed values that cannot be modified during program execution. There are several ways to define constants in C:

#include <stdio.h>
#define PI 3.14159  // Preprocessor constant

int main() {
    // const keyword
    const int MAX_SIZE = 100;
    const float GRAVITY = 9.8;
    
    // Literal constants
    int number = 42;        // Integer literal
    float price = 19.99;    // Float literal
    char letter = 'X';      // Character literal
    char name[] = "John";   // String literal
    
    printf("PI: %.5f\n", PI);
    printf("Max Size: %d\n", MAX_SIZE);
    printf("Gravity: %.1f\n", GRAVITY);
    
    return 0;
}

Explanation

• Variables: Named memory locations with a specific type; declare before use and initialize to avoid undefined values.
• Constants: Values that do not change; use const for typed, scoped constants and #define for macros.
• Why it matters: Clear variable names and appropriate constants make code safer and easier to maintain.
• Common pitfalls: Modifying a const value, mixing #define macros with typed constants, or using vague names.