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C Program to print Twin prime numbers between two ranges

Last updated on September 23, 2020

What are Twin Primes Numbers? #

Pair of prime numbers which differ by 2 is called Twin Prime. For example, the first 4 twin primes are: 

(3, 5), (11, 13), (17, 19), (29, 31)

The following is a C program to print Twin prime numbers between two ranges:

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/*******************************************************
 Program to print Twin prime numbers between two ranges
 *******************************************************/

#include<stdio.h> // include stdio.h library
int check_prime(int n);

int main(void)
{   

    int start, end;

    printf("Enter start: ");
    scanf("%d", &start);

    printf("Enter end: ");
    scanf("%d", &end);

    for(int i = start; i < end; i++)
    {
        if(check_prime(i) && check_prime(i + 2))
        {            
            printf("{%d, %d}\n", i, i + 2);
            i = i + 1; 
        }                
    }    

    return 0; // return 0 to operating system
}

int check_prime(int n)
{
    if(n == 1)
    {
        return 0;
    }

    for(int i = 2; i < n; i++)
    {
        if(n % i == 0)
        {
            // number is not prime
            return 0;
        }
    }

    // number is prime
    return 1;
}

Try it now

Expected Output: 1st run:

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Enter start: 1
Enter end: 50
{3, 5}
{5, 7}
{11, 13}
{17, 19}
{29, 31}
{41, 43}

2nd run:

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Enter start: 500
Enter end: 600
{521, 523}
{569, 571}
{599, 601}

How it works #

In line 19, we have a for loop that iterates over the numbers between the specified range.

In line 21, we call check_prime() number two times. Once with the number i and once with the number i + 2.

If the condition satisfies then i and i + 2 are twin primes. In line 23, we print the twin prime numbers and in line 24, we increment the counter i by 1, to check for the next number.

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