 Software Engineering

# How to Find the Divisors in C

## The challenge#

Create a function named `divisors`/`Divisors` that takes an integer `n > 1` and returns an array with all of the integer’s divisors(except for 1 and the number itself), from smallest to largest. If the number is prime return the string ‘(integer) is prime’ (`null` in C#) (use `Either String a` in Haskell and `Result<Vec<u32>, String>` in Rust).

#### Example:#

``````divisors(12); // results in {2, 3, 4, 6}
divisors(25); // results in {5}
divisors(13); // results in NULL
``````

## The solution in C#

Option 1:

``````#include <stddef.h>

void divisors(unsigned n, size_t *z, unsigned *array) {
*z = 0;

for (int i = 2; i <= (n / 2); i++) {
if (n % i == 0)
array[(*z)++] = i;
}
}
``````

Option 2:

``````#include <stddef.h>

void divisors(unsigned n, size_t *length, unsigned array[]) {
int i;
int j = 0;
for (i = 2; i < (int) n; i++) {
if (n % i == 0) {
array[j] = i;
j++;
}
}
*length = (size_t) j;
}
``````

Option 3:

``````#include <stddef.h>

void divisors(unsigned n, size_t *z, unsigned *array) {
int len = 0;
for (unsigned i = 2; i < n; i++) {
if (n % i == 0) array[len++] = i;
}
*z = len;
}
``````

## Test cases to validate our solution#

``````#include <criterion/criterion.h>
#include <stddef.h>

extern void tester(unsigned n, size_t length, const unsigned expected[length]);

Test(Sample_Tests, should_pass_all_tests)
{
{ unsigned n =  15; const unsigned expected = {3, 5};              tester(n, 2, expected); }
{ unsigned n = 253; const unsigned expected = {11, 23};            tester(n, 2, expected); }
{ unsigned n =  24; const unsigned expected = {2, 3, 4, 6, 8, 12}; tester(n, 6, expected); }
{ unsigned n =  25; const unsigned expected = {5};                 tester(n, 1, expected); }
{ unsigned n =  13; const unsigned *expected = NULL;                  tester(n, 0, expected); }
{ unsigned n =   3; const unsigned *expected = NULL;                  tester(n, 0, expected); }
{ unsigned n =  29; const unsigned *expected = NULL;                  tester(n, 0, expected); }
}
``````