Variables and Constants in C

Introduction

A variable is a named memory location that stores a value. A constant is a value that does not change during program execution. Understanding both is essential before writing any C program.

Declaring Variables

data_type variable_name;
data_type variable_name = initial_value;

Examples:

int count;
int total = 0;
float price = 19.99f;
char initial = 'T';

Assigning and Updating Values

#include <stdio.h>

int main(void) {
    int x = 10;
    x = x + 5;          /* x is now 15 */
    x += 3;             /* same as x = x + 3 */
    printf("x = %d\n", x);
    return 0;
}

Sample Output

x = 18

Constants with const

The const keyword prevents modification after initialization:

const float PI = 3.14159f;
const int MAX_STUDENTS = 100;

/* PI = 3.14;  ERROR — cannot assign to const variable */

Constants with #define

Preprocessor macros define symbolic constants before compilation:

#include <stdio.h>

#define PI 3.14159
#define MAX_SIZE 100

int main(void) {
    printf("PI = %.5f, MAX = %d\n", PI, MAX_SIZE);
    return 0;
}

See C Preprocessor Directives for more on #define and macros.

Multiple Variables in One Line

int a, b, c;
int x = 1, y = 2, z = 3;

Naming Rules

Variable names must follow C identifier rules — see C Identifiers. Use meaningful names like studentCount instead of x.

Best Practices

• Initialize every variable before reading its value.
• Use const for values that should never change (more type-safe than #define).
• Use UPPER_CASE for macro constants (#define MAX 100).
• Declare variables close to where they are first used.

Common Mistakes

• Using a variable before initialization — leads to unpredictable output.
• Confusing = (assignment) with == (comparison).
• Redefining a macro or const name elsewhere in the program.

Related Pages

Continue learning with these related tutorials and programs:

• C Tutorials — Browse all C Tutorials.
• C Introduction — Start here — what C is and why it matters.
• Command Line Arguments in C (argc and argv) — More in c basics.
• C History — More in c basics.