C中存在高迭代循环的程序没有给我输出..为什么呢？

Question

I made a program which print the sum of prime numbers which is below 4 million, it doesn't give me an output. When I count prime numbers below 1000 for instance it works well. So what's the problem with high iterations? Here is the function calculating the sum of the primes under a certain number.

long long is_prime(){

long long i,j,c=0,sum=0;

for(i=2;i<=4000000;i++){

    c=0;

    for(j=1;j<i;j++){

         if(i%j==0) c++;

    }

    if(c==1) sum+=i ;
}

return sum;
}

Can anyone help me?

Answer 1

It doesn't make sense 'doesn't give me output', whether it produces error or it just executing for long time. Your code's complexity is in O(n*n) that will take time for 4 million* 4 million iterations in worst case, this could be reason, wait for result.