C program to display prime numbers between two numbers using function and without using function

In this example, you will learn about C program to display prime numbers between two numbers interval or in a given range by the user with and without using the function.

What is prime number ?

A prime number is a natural number that is divisible by 1 and itself only.

For example: 2, 3, 5, 7 …

Please go through following articles of C programming to understand the concept of the program.

• C programming if else
• C programming for loop
• C programming function
• C programming break and continue statement

C program to display prime numbers between two integers without using function

#include <stdio.h>

int main ()
{
   int num1, num2, i, j, flag;

   printf("Enter two numbers: ");
   scanf("%d %d", &num1, &num2);

   printf("Prime numbers between %d and %d are:\n", num1, num2);
   // Displaying prime number between num1 and num2
   for (i = num1 + 1; i < num2; ++i)
   {
      flag = 0; //flag is set to 0
      for (j = 2; j <= i/2; ++j)
      {
         if (i % j == 0)
         {
            flag = 1;
            break;
         }
      }
      if (flag == 0) //if flag == 0, then display i
          printf("%d\t", i);
   }
   return 0;
}

Output

C program to display prime numbers between two integers using function

#include <stdio.h>

int displayPrimeNumber (int x); //function prototype
int main ()
{
   int num1, num2, i, flag;

   printf("Enter two numbers: ");
   scanf("%d %d", &num1, &num2);

   printf("Prime numbers between %d and %d are:\n", num1, num2);
   // Displaying prime number between num1 and num2
   for (i = num1 + 1; i < num2; ++i)
   {
      flag = displayPrimeNumber(i);
      if (flag == 0) //if flag == 0, then display i
         printf("%d\t", i);
   }
   return 0;
}

int displayPrimeNumber(int x)
{
   int j, flag = 0;
   for (j = 2; j <= x/2; ++j)
   {
      if (x % j == 0)
      {
         flag = 1;
         break;
     }
   }
   return flag;
}

Output

Explanation
The first example displays all prime numbers without using user defined function.

However, we have used displayPrimeNumber user defined function for same purpose.

The sole purpose of displayPrimeNumber user defined function is to check the number is prime or not and set the value of flag.