Code

 Header...
//
// Find the lowest factor of the integers 2 to 1000, or say it is prime.
// The basic algoritm is to try every possible factor up to the square
// root of the number being looked at (the target)
//

 int main(){
   ... 

»  for ( int target=2;target<=1000; ++target) {
»    int factor, maxfactor;

»    maxfactor=sqrt(target);// Largest factor we need to try

»    factor=2;
»    while (target%factor!=0&&factor<=maxfactor) 
      ++factor;
    
»    if (factor>maxfactor) 
      printf("%d is prime\n", target);
    else 
      printf("\t%d is not prime, its lowest factor is %d\n", 
       target, factor);
  }
»  return 0;
}

Data