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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:

• C if…else Statement
• C for Loop
• C break and continue

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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

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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.

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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