Prime Generator Algorithm

Question

I've been trying to solve the SPOJ problem of Prime number Generator Algorithm.

Here is the question

Peter wants to generate some prime numbers for his cryptosystem. Help him! Your task is to generate all prime numbers between two given numbers!

Input

The input begins with the number t of test cases in a single line (t<=10). In each of the next t lines there are two numbers m and n (1 <= m <= n <= 1000000000, n-m<=100000) separated by a space.

Output

For every test case print all prime numbers p such that m <= p <= n, one number per line, test cases separated by an empty line.

It is very easy, but the online judge is showing error, I didn't get what the problem meant by 'test cases' and why that 1000000 range is necessary to use.

Here is my code.

#include<stdio.h>

main()
{
int i, num1, num2, j;
int div = 0;
scanf("%d %d", &num1, &num2);
for(i=num1; i<=num2; i++)
  {
    for(j=1; j<=i; j++)
    {
      if(i%j == 0)
      {
        div++;
      }
    }
    if(div == 2)
    {
     printf("%d\n", i);
    }
    div = 0;
  }
  return 0;
}

Answer 1

I can't comment on the alogirthm and whether the 100000 number range allows optimisations but the reason that your code is invalid is because it doesn't seem to be parsing the input properly. The input will be something like:

2
123123123 123173123 
987654321 987653321

That is the first line will give the number of sets of input you will get with each line then being a set of inputs. Your program, at a glance, looks like it is just reading the first line looking for two numbers.

I assume the online judge is just looking for the correct output (and possibly reasonable running time?) so if you correct for the right input it should work no matter what inefficiencies are in your algorithm (as others have started commenting on).