terminate - My C code doesn't output anything, or end running -

i'm trying answer question:

the prime factors of 13195 5, 7, 13 , 29.

what largest prime factor of number 600851475143 ?

here code:

#include <stdio.h>  int isprime(long long num) {     (long long k = 1; k < num; k++)     {             if (num%k == 0)                     return 0;     }      return 1; }  long long num = 600851475143; int main(void) {     long long prime_factor = 0;     (long long j = 2; j < num; j++)     {             if (num % j == 0 && isprime(j) == 1)                     prime_factor = j;     }      printf("%lli\n", prime_factor);   } 

but reason doesn't print anything, or end. causing this?

that's terribly inefficient way of finding prime factors of number.

to find prime factors, should:

1. create static list of primes less or equal (number / 2).
2. iterate on each element in list of primes , see if each 1 can evenly divide number.
3. divide original number prime, , check last number again.

why? smallest prime 2. if number isn't first prime can divide number > 2 3.

every number can uniquely identified prime factors. called fundamental theorem of arithmetic. means there 1 way represent e.g. 15 product of primes, in case { 3, 5 }.

however prime can divide number more once. example, 49 product of 2 primes { 7, 7 }. dividing original number 1 of factors makes subsequent divisions quicker. , can logically stop checking when number == 1.