C Program to Display Prime Numbers Between Two Intervals

In this example, you will learn to print all prime numbers between two numbers entered by the user.

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To understand this example, you should have the knowledge of the following C programming topics:

Display Prime Numbers Between Two Intervals

#include 

int main() {
   int low, high, i, flag;
   printf("Enter two numbers(intervals): ");
   scanf("%d %d", &low, &high);
   printf("Prime numbers between %d and %d are: ", low, high);

   // iteration until low is not equal to high
   while (low < high) {
      flag = 0;

      // ignore numbers less than 2
      if (low <= 1) {
         ++low;
         continue;
      }

      // if low is a non-prime number, flag will be 1
      for (i = 2; i <= low / 2; ++i) {

         if (low % i == 0) {
            flag = 1;
            break;
         }
      }

      if (flag == 0)
         printf("%d ", low);

      // to check prime for the next number
      // increase low by 1
      ++low;
   }

   return 0;
}

Output

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

In this program, the while loop is iterated ( high-low-1) times.

In each iteration, whether low is a prime number or not is checked, and the value of low is incremented by 1 until low is equal to high.

Visit this page to learn more about how to check whether a number is prime or not.

If the user enters the larger number first, the above program doesn’t work as intended. You can solve this issue by swapping the numbers.

Display Prime Numbers when Larger Number is Entered first

#include 

int main() {
   int low, high, i, flag, temp;
   printf("Enter two numbers(intervals): ");
   scanf("%d %d", &low, &high);

   // swap numbers if low is greather than high
   if (low > high) {
      temp = low;
      low = high;
      high = temp;
   }

   printf("Prime numbers between %d and %d are: ", low, high);
   while (low < high) {
      flag = 0;

      // ignore numbers less than 2
      if (low <= 1) {
         ++low;
         continue;
      }

      for (i = 2; i <= low / 2; ++i) {
         if (low % i == 0) {
            flag = 1;
            break;
         }
      }
      if (flag == 0)
         printf("%d ", low);
      ++low;
   }

   return 0;
}

Visit this page to learn how you can display all the prime numbers between the two intervals by creating a user-defined function