Confusion with C version of sieve problem

pkw · November 30, 2025, 5:18am

exercism/c/blob/main/exercises/practice/sieve/test_sieve.c#L74


      
          
             TEST_ASSERT_EQUAL(expected_prime_count, result_prime_count);
             TEST_ASSERT_EQUAL_UINT_ARRAY(expected_prime_array, result_array,
                                          expected_prime_count);
          }
          
          static void test_limit_is_prime_and_small_max_primes(void)
          {
             TEST_IGNORE();
             const uint32_t limit = 13;
             const uint32_t expected_prime_array[] = { 2, 3, 5, 7, 11, 13 };
             const uint32_t expected_prime_count = 4;
          
             const uint32_t result_prime_count = sieve(limit, result_array, 4);
          
             TEST_ASSERT_EQUAL(expected_prime_count, result_prime_count);
             TEST_ASSERT_EQUAL_UINT_ARRAY(expected_prime_array, result_array,
                                          expected_prime_count);
          }
          
          static void test_find_primes_up_to_1000(void)

Can someone explain why this array has six elements, but the expected count is 4 ?

I spot checked the sieve tests for the different languages and they
don’t have that.

IsaacG · November 30, 2025, 5:42am

The header shows,

/// Calculate at most `max_primes` prime numbers in the interval [2,limit]
/// using the Sieve of Eratosthenes and store the prime numbers in `primes`
/// in increasing order.
/// The function returns the number of calculated primes.
uint32_t sieve(uint32_t limit, uint32_t *primes, size_t max_primes);

The function is supposed to compute 4 primes within the limit of 13. I’m not sure why the array is populated until 13 but I believe the test ignores everything after the first four elements.

pkw · November 30, 2025, 5:30pm

Thanks, I got my tests to pass, by just ignoring the length of the array.
And I assume it doesn’t get used, because mine doesn’t match the test
code.

I still think it might be wrong, but i’m speedrunning the C track and already spent too long on sieve. :) thanks

IsaacG · November 30, 2025, 5:31pm

That’s what the code review button is there for ;)