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APC-PRAC-041.c

Problem Statement

Write a C program to find and print all twin prime pairs between 1 and n using nested loops. (Twin primes are prime numbers having a difference of 2, like 11 and 13)

Metadata

Property Detail
Author Amit Dutta amitdutta4255@gmail.com
Date 12 Dec 2025
License MIT License (See the LICENSE file for details)
Difficulty Beginner (index: 3 / 10)

Concepts

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  • Recursion
  • Sorting (possible)
  • Iteration

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Source Code

#include <stdio.h>
#include <math.h>

int isPrime(int n)
{
    if (n < 2)
        return 0;
    if (n == 2)
        return 1;
    if (n % 2 == 0)
        return 0;
    int i, temp = (int)sqrt(n);
    for (i = 3; i <= temp; i += 2)
        if (n % i == 0)
            return 0;
    return 1;
}

int main()
{
    int n, i, count = 0;
    printf("enter the n: ");
    scanf("%d", &n);
    printf("\nAll the twin numbers:  ");
    for (i = 1; i <= n - 2; i++)
    {
        if (isPrime(i))
        {
            if (isPrime(i + 2))
            {
                printf("(%d, %d)  ", i, i + 2);
                count++;
            }
        }
    }
    printf("\nCount; %d", count);
    return 0;
}

Explanation

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    You are explaining a C programming code to a beginner.

    STRICT RULES:

    - Only use the given code. Do NOT assume anything not present.

    - Do NOT add extra examples.

    - Keep explanation clear and short.

    - If something is unclear, say "Not clear from code".

    - Follow the exact format below. Do NOT change headings.

    FORMAT:

    [START]

    ## What it does

    (Explain the overall purpose in 1-2 sentences)

    ## Step-by-step

    (Explain how the code works in steps, simple language)

    ## Key Concepts

    (List concepts like loop, condition, function, etc.)

    ## Notes

    (Mention any limitations, errors, or assumptions)

    [END]

    CODE (APC-PRAC-041.c):

    #include <stdio.h>
    #include <math.h>

    int isPrime(int n)
    {
        if (n < 2)
            return 0;
        if (n == 2)
            return 1;
        if (n % 2 == 0)
            return 0;
        int i, temp = (int)sqrt(n);
        for (i = 3; i <= temp; i += 2)
            if (n % i == 0)
                return 0;
        return 1;
    }

    int main()
    {
        int n, i, count = 0;
        printf("enter the n: ");
        scanf("%d", &n);
        printf("\nAll the twin numbers:  ");
        for (i = 1; i <= n - 2; i++)
        {
            if (isPrime(i))
            {
                if (isPrime(i + 2))
                {
                    printf("(%d, %d)  ", i, i + 2);
                    count++;
                }
            }
        }
        printf("\nCount; %d", count);
        return 0;
    }