4.Prime numbers.

/* C program to display all prime numbers between Two interval entered by user. */
#include <stdio.h>
 main()
{
  int n1, n2, i, j, flag;
  printf("Enter two numbers(intevals): ");
  scanf("%d %d", &n1, &n2);
  printf("Prime numbers between %d and %d are: ", n1, n2);
  for(i=n1+1; i<n2; ++i)
  {
      flag=0;
      for(j=2; j<=i/2; ++j)
      {
        if(i%j==0)
        {
          flag=1;
          break;
        }
      }
      if(flag==0)
        printf("%d ",i);
  }
  }

Output

Enter two numbers(intervals): 20
50
Prime numbers between 20 and 50 are: 23 29 31 37 41 43 47