const Keyword in C with Example: 7 Practical Programs

const keyword: quick recap

Example 1: Basic const variable

#include <stdio.h>
int main() {
    const int MAX_MARKS = 100;
    printf("Maximum marks: %d\n", MAX_MARKS);
    return 0;
}

Example 2: Compiler error on modification

int main() {
    const int LIMIT = 50;
    LIMIT = 60;      // ERROR: assignment of read-only variable 'LIMIT'
    return 0;
}

This program does not compile. The error itself is the feature — const catches the bug before the program ever runs.

Example 3: const in a calculation

#include <stdio.h>
int main() {
    const float PI = 3.14159f;
    float r = 5.0f;
    printf("Circumference = %.2f\n", 2 * PI * r);
    return 0;
}

Example 4: const function parameter

#include <stdio.h>

void printName(const char *name) {
    // name[0] = 'X';   // ERROR: cannot modify through const pointer
    printf("Student: %s\n", name);
}

int main() {
    printName("Gagan");
    return 0;
}

This is the most important professional use of const: the function promises not to modify the caller's data. Every standard library function like strlen(const char *s) uses this pattern.

Example 5: const array

#include <stdio.h>
int main() {
    const int weekdays[7] = {1, 2, 3, 4, 5, 6, 7};
    // weekdays[0] = 99;   // ERROR: array elements are read-only
    printf("First day code: %d\n", weekdays[0]);
    return 0;
}

Example 6: const with a pointer

#include <stdio.h>
int main() {
    int marks = 85;
    const int *p = &marks;   // pointer to const int
    printf("Marks: %d\n", *p);
    // *p = 90;              // ERROR: cannot change value through p
    marks = 90;              // OK: variable itself is not const
    printf("Marks: %d\n", *p);
    return 0;
}

Example 7: const in a real mini program

#include <stdio.h>

int main() {
    const float GST_RATE = 0.18f;      /* fixed by law   */
    const int   FREE_LIMIT = 500;      /* business rule  */
    float bill = 1200.0f;

    float gst = (bill > FREE_LIMIT) ? bill * GST_RATE : 0;
    printf("Bill: %.2f, GST: %.2f, Total: %.2f\n", bill, gst, bill + gst);
    return 0;
}

Business rules like tax rates are perfect const candidates — if a teammate accidentally writes GST_RATE = 0; somewhere, the compiler catches it instantly.