Sieve of Eratosthenes in C: solved exercise

Sieve of Eratosthenes in C: solved exercise

If you are looking for sieve of eratosthenes in c: solved exercise, here is a practical, compilable example focused on the reusable idea behind the exercise.

Problem statement

List all prime numbers less than or equal to 20.

C solution

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#include <stdio.h>

int main(void) {
    int limite = 20;
    int primo[21];

    for (int i = 0; i <= limite; i++) {
        primo[i] = 1;
    }
    primo[0] = 0;
    primo[1] = 0;

    for (int i = 2; i * i <= limite; i++) {
        if (primo[i]) {
            for (int j = i * i; j <= limite; j += i) {
                primo[j] = 0;
            }
        }
    }

    int primero = 1;
    for (int i = 2; i <= limite; i++) {
        if (primo[i]) {
            if (!primero) {
                printf(" ");
            }
            printf("%d", i);
            primero = 0;
        }
    }
    printf("\n");
    return 0;
}

Expected output

1

2 3 5 7 11 13 17 19

Common mistakes

  • Not testing edge cases such as small or empty inputs.
  • Not validating indices, pointers, or limits carefully enough.
  • Copying the mechanics without understanding the general pattern.

Practical use

The key idea is to identify a reusable pattern instead of stopping at “it works once”.

Guided practice and full book

If you want a complete path with progressive difficulty:

FAQ

Is this exercise useful in practice?

Yes. It is designed to teach a reusable C pattern rather than a one-off toy example.

How should I practice it better?

Change the input data, add edge cases, and rewrite it from scratch without looking at the solution.

How should I practice this exercise type to improve faster?

Start with small inputs, run edge cases (empty, one item, max capacity), then rewrite the solution from scratch without copying.