Possible Solutions

With for-loop

#include <stddef.h>
#include <stdio.h>

#define N 6

size_t factorial(size_t n)
{
    size_t res = 1;

    for (size_t i=2; i<=n; ++i) {
        res *= i;
    }
    return res;
}

int main()
{
    printf("result = %zu\n", factorial(N));

    return 0;
}

With while-loop

session03/factorial_while.c

#include <stddef.h>
#include <stdio.h>

#define N 6

size_t factorial(size_t n)
{
    size_t res = 1;

    size_t i=2;
    while (i<=n) {
        res *= i;
        ++i;
    }
    return res;
}

int main()
{
    printf("result = %zu\n", factorial(N));

    return 0;
}

With goto and if statements

session03/factorial_goto.c

#include <stddef.h>
#include <stdio.h>

#define N 6

size_t factorial(size_t n)
{
    size_t res = 1;

    size_t i=2;
    goto while_check;
    while_loop:
        res *= i;
        ++i;
    while_check:
        if (i<=n) {
            goto while_loop;
        }

    return res;
}

int main()
{
    printf("result = %zu\n", factorial(N));

    return 0;
}