GMP Factorial

gmp_factorial.c

#include <gmp.h>
#include <stdio.h>
#include <stdlib.h>

/**
 * Compute the factorial of `n` and store it in `f`.
 */
void factorial(mpz_t f, mpz_t n) {
    // f ← 1
    mpz_set_ui(f, 1);

    // for i ∈ [2, n]:
    //     f ← f × i
    mpz_t i;
    mpz_init(i);
    for (mpz_set_ui(i, 2); mpz_cmp(i, n) <= 0; mpz_add_ui(i, i, 1)) {
        mpz_mul(f, f, i);
    }

    mpz_clear(i);
    return;
}

/**
 * Prompt the user for `n` and print the factorial of `n`.
 */
int main(void) {
    // What is n?
    mpz_t n;
    mpz_init(n);
    printf("n = ");
    gmp_scanf("%Zd", n);

    // Calculate factorial(n).
    mpz_t f;
    mpz_init(f);
    factorial(f, n);

    // Print factorial(n).
    gmp_printf("%Zd! = %Zd\n", n, f);

    // Free memory.
    mpz_clear(n);
    mpz_clear(f);
    return EXIT_SUCCESS;
}