C program can not display 3000 numbers

Question

This program is supposed to display the prime numbers in the range of 1-3000, but it only displays the prime numbers from 743-3000. How do I get it to print all prime numbers in the range?

Here is the program:

#include <stdio.h>
#include <math.h>
#include <stdbool.h>

main()
{

      unsigned long long num, divisible_num;

      printf("The prime numbers in the range of 1 - 3000 are: \n");
      for(num=1;num<=3000;num++)
       {
             for(divisible_num=2;divisible_num<=sqrt(num);divisible_num++)
             {     
                 if(num%divisible_num==0)
                    break;
             }
                 if(num%divisible_num!=0)
                 {
                 printf("%lu\n", num);
                 }
       }
      getchar();
}

Answer 1

The foremost thing to do is to get the sqrt call out of that for loop. You may compute it once. Better still, remove it all together with

for(divisible_num=2;divisible_num * divisible_num<=num;divisible_num++)

A number is prime if it is not divisible by any of the prime numbers less than or equal to the sqrt(num).

int prime = 1;
for(divisible_num=2;divisible_num * divisible_num<=num;divisible_num++) {
    if (num % divisible_num == 0) {
       prime = 0;
       break; 
    }

}

if (prime) {
    //print the num
} 

Your outer loop must also start from 2.

Answer 2

When I ran your code it ranged from 1-3000. I'm not sure what your problem is. The get char() seems a little out of place.

Also once checking two, you don't have to divide by any more even numbers. This code works from 3 - 3000

for(num=3;num<=3000;num++)
   {
         int prime = (num%2 != 0);
         long root = sqrt(num);
         for(divisible_num=3;prime && divisible_num <= root;divisible_num+=2)
         {     
             if(num%divisible_num==0){
                prime = 0;
             }
         }
             if(prime)
             {
             printf("%lu\n", num);
             }
   }