50 Important C Programs

Why practice programs?

You cannot learn to swim by reading about water, and you cannot learn C without writing programs. This is a curated list of 50 important C programs grouped by topic, with several fully worked examples. Try each one yourself first, then check your logic.

Every program here uses only the basics covered elsewhere on the site, so each links naturally to a lesson when you want more depth.

Number programs

1. Check even or odd
2. Check prime number
3. Factorial of a number
4. Fibonacci series
5. Reverse a number
6. Sum of digits
7. Check palindrome number
8. Check Armstrong number
9. Largest of three numbers
10. Multiplication table
11. Convert decimal to binary
12. GCD and LCM of two numbers

Pattern programs

13. Right-angled star triangle
14. Inverted star triangle
15. Pyramid of stars
16. Number triangle (1, 12, 123...)
17. Floyd's triangle
18. Pascal's triangle
19. Diamond pattern
20. Hollow square pattern

Array and string programs

21. Largest and smallest in an array
22. Sum and average of an array
23. Reverse an array
24. Sort an array (bubble/selection/insertion)
25. Search an element (linear/binary)
26. Matrix multiplication
27. Transpose of a matrix
28. String length without strlen
29. Reverse a string
30. Check string palindrome
31. Count vowels and consonants
32. Convert case (upper/lower)

...and more covering linked lists, stacks, queues, file handling and small mini projects — 50 in total.

Example: prime number

#include <stdio.h>
int main() {
    int n, isPrime = 1;
    printf("Enter a number: ");
    scanf("%d", &n);
    if (n < 2) isPrime = 0;
    for (int i = 2; i * i <= n; i++)
        if (n % i == 0) { isPrime = 0; break; }
    printf(isPrime ? "Prime\n" : "Not prime\n");
    return 0;
}

Example: reverse a number

#include <stdio.h>
int main() {
    int n, rev = 0;
    scanf("%d", &n);
    while (n != 0) {
        rev = rev * 10 + n % 10;   // pull off last digit
        n /= 10;
    }
    printf("Reversed: %d\n", rev);
    return 0;
}

The trick rev * 10 + n % 10 peels the last digit off n and appends it to rev each loop.

Example: a star pattern

#include <stdio.h>
int main() {
    int rows = 4;
    for (int i = 1; i <= rows; i++) {
        for (int j = 1; j <= i; j++)
            printf("* ");
        printf("\n");
    }
    return 0;
}

The outer loop picks the row; the inner loop prints as many stars as the row number. Almost every pattern is a variation of this nested-loop idea.

How to practise well

• Write first, look later — attempt each program before reading any solution.
• Trace by hand — run a small input on paper to see the logic.
• Fix your own bugs — the errors you solve yourself stick best.
• Vary it — change a program slightly (descending pattern, different range) to test understanding.

Summary

• Practice is the only way to get fluent in C — reading is not enough.
• The 50 programs span numbers, patterns, arrays, strings and small projects.
• Classic programs (prime, reverse, palindrome, patterns) also appear in interviews.
• Always try writing a program yourself before checking a solution.
• Trace by hand and modify programs to deepen understanding.