int iscompute(int n)
{
for (int i=2;i<=sqrt(n); i++)
if(n%i) == 0)
{
printf("not computed");
return 0;
}
return 1;
}

For loop will run sqrt(n) times at worst when $n$ is a prime number. if n is any even number then if condition becomes true in 1st iteration only. So it will terminate after that.This will result Constant best case complexity.