Palindromic Primes

A palindromic prime (sometimes called a palprime) is a prime number that is also a palindromic number.

Given a number n, print all palindromic primes smaller than or equal to n. For example, If n is 10, the output should be “2, 3, 5, 7′. And if n is 20, the output should be “2, 3, 5, 7, 11′.

Idea is to generate all prime numbers smaller than or equal to given number n and checking every prime number whether it is palindromic or not.



Methods used

Below is the implementation of above algorithm:

C++

// C++ Program to print all palindromic primes
// smaller than or equal to n.
#include<bits/stdc++.h>
using namespace std;
  
// A function that reurns true only if num
// contains one digit
int oneDigit(int num)
{
    // comparison operation is faster than
    // division operation. So using following
    // instead of "return num / 10 == 0;"
    return (num >= 0 && num < 10);
}
  
// A recursive function to find out whether
// num is palindrome or not. Initially, dupNum
// contains address of a copy of num.
bool isPalUtil(int num, int* dupNum)
{
    // Base case (needed for recursion termination):
    // This statement/ mainly compares the first
    // digit with the last digit
    if (oneDigit(num))
        return (num == (*dupNum) % 10);
  
    // This is the key line in this method. Note
    // that all recursive/ calls have a separate
    // copy of num, but they all share same copy
    // of *dupNum. We divide num while moving up
    // the recursion tree
    if (!isPalUtil(num/10, dupNum))
        return false;
  
    // The following statements are executed when
    // we move up the recursion call tree
    *dupNum /= 10;
  
    // At this point, if num%10 contains i'th
    // digit from beiginning, then (*dupNum)%10
    // contains i'th digit from end
    return (num % 10 == (*dupNum) % 10);
}
  
// The main function that uses recursive function
// isPalUtil() to find out whether num is palindrome
// or not
int isPal(int num)
{
    // If num is negative, make it positive
    if (num < 0)
       num = -num;
  
    // Create a separate copy of num, so that
    // modifications made to address dupNum don't
    // change the input number.
    int *dupNum = new int(num); // *dupNum = num
  
    return isPalUtil(num, dupNum);
}
  
// Function to generate all primes and checking
// whether number is palindromic or not
void printPalPrimesLessThanN(int n)
{
    // Create a boolean array "prime[0..n]" and
    // initialize all entries it as true. A value
    // in prime[i] will finally be false if i is
    // Not a prime, else true.
    bool prime[n+1];
    memset(prime, true, sizeof(prime));
  
    for (int p=2; p*p<=n; p++)
    {
        // If prime[p] is not changed, then it is
        // a prime
        if (prime[p] == true)
        {
            // Update all multiples of p
            for (int i=p*2; i<=n; i += p)
                prime[i] = false;
        }
    }
  
    // Print all palindromic prime numbers
    for (int p=2; p<=n; p++)
  
       // checking whether the given number is
       // prime palindromic or not
       if (prime[p] && isPal(p))
          cout << p << " ";
}
  
// Driver Program
int main()
{
    int n = 100;
    printf("Palindromic primes smaller than or "
           "equal to %d are :\n", n);
    printPalPrimesLessThanN(n);
}

Java

// Java Program to print all palindromic primes
// smaller than or equal to n.
import java.util.*;
  
class GFG {
      
    // A function that reurns true only if num
    // contains one digit
    static boolean oneDigit(int num)
    {
        // comparison operation is faster than
        // division operation. So using following
        // instead of "return num / 10 == 0;"
        return (num >= 0 && num < 10);
    }
       
    // A recursive function to find out whether
    // num is palindrome or not. Initially, dupNum
    // contains address of a copy of num.
    static boolean isPalUtil(int num, int dupNum)
    {
        // Base case (needed for recursion termination):
        // This statement/ mainly compares the first
        // digit with the last digit
        if (oneDigit(num))
            return (num == (dupNum) % 10);
       
        // This is the key line in this method. Note
        // that all recursive/ calls have a separate
        // copy of num, but they all share same copy
        // of dupNum. We divide num while moving up
        // the recursion tree
        if (!isPalUtil(num/10, dupNum))
            return false;
       
        // The following statements are executed when
        // we move up the recursion call tree
        dupNum /= 10;
       
        // At this point, if num%10 contains ith
        // digit from beginning, then (dupNum)%10
        // contains ith digit from end
        return (num % 10 == (dupNum) % 10);
    }
       
    // The main function that uses recursive function
    // isPalUtil() to find out whether num is palindrome
    // or not
    static boolean isPal(int num)
    {
        // If num is negative, make it positive
        if (num < 0)
           num = -num;
       
        // Create a separate copy of num, so that
        // modifications made to address dupNum don't
        // change the input number.
        int dupNum = num; // dupNum = num
       
        return isPalUtil(num, dupNum);
    }
       
    // Function to generate all primes and checking
    // whether number is palindromic or not
    static void printPalPrimesLessThanN(int n)
    {
        // Create a boolean array "prime[0..n]" and
        // initialize all entries it as true. A value
        // in prime[i] will finally be false if i is
        // Not a prime, else true.
        boolean prime[] = new boolean[n+1];
       
        Arrays.fill(prime, true);
          
        for (int p = 2; p*p <= n; p++)
        {
            // If prime[p] is not changed, then it is
            // a prime
            if (prime[p])
            {
                // Update all multiples of p
                for (int i = p*2; i <= n; i += p){
                    prime[i] = false;}
            }
        }
       
        // Print all palindromic prime numbers
        for (int p = 2; p <= n; p++){
       
           // checking whether the given number is
           // prime palindromic or not
           if (prime[p] && isPal(p)){
              System.out.print(p + " ");
              }
           }
    }
      
    // Driver function
    public static void main(String[] args) 
    {
         int n = 100;
            System.out.printf("Palindromic primes smaller than or "
                   +"equal to %d are :\n", n);
            printPalPrimesLessThanN(n);
        }
    }
          
// This code is contributed by Arnav Kr. Mandal.

C#

// C# Program to print all palindromic 
// primes smaller than or equal to n.
using System; 
  
class GFG {
      
    // A function that reurns true only
    // if num contains one digit
    static bool oneDigit(int num)
    {
        // comparison operation is faster than
        // division operation. So using following
        // instead of "return num / 10 == 0;"
        return (num >= 0 && num < 10);
    }
      
    // A recursive function to find out whether
    // num is palindrome or not. Initially, dupNum
    // contains address of a copy of num.
    static bool isPalUtil(int num, int dupNum)
    {
        // Base case (needed for recursion termination):
        // This statement/ mainly compares the first
        // digit with the last digit
        if (oneDigit(num))
            return (num == (dupNum) % 10);
      
        // This is the key line in this method. Note
        // that all recursive/ calls have a separate
        // copy of num, but they all share same copy
        // of dupNum. We divide num while moving up
        // the recursion tree
        if (!isPalUtil(num/10, dupNum))
            return false;
      
        // The following statements are executed when
        // we move up the recursion call tree
        dupNum /= 10;
      
        // At this point, if num%10 contains ith
        // digit from beginning, then (dupNum)%10
        // contains ith digit from end
        return (num % 10 == (dupNum) % 10);
    }
      
    // The main function that uses recursive 
    // function isPalUtil() to find out 
    // whether num is palindrome or not
    static bool isPal(int num)
    {
        // If num is negative, make it positive
        if (num < 0)
        num = -num;
      
        // Create a separate copy of num, so that
        // modifications made to address dupNum don't
        // change the input number.
        int dupNum = num; // dupNum = num
      
        return isPalUtil(num, dupNum);
    }
      
    // Function to generate all primes and checking
    // whether number is palindromic or not
    static void printPalPrimesLessThanN(int n)
    {
        // Create a boolean array "prime[0..n]" and
        // initialize all entries it as true. A value
        // in prime[i] will finally be false if i is
        // Not a prime, else true.
        bool []prime = new bool[n+1];
          
    for (int i=0;i<n+1;i++)
    prime[i]=true;
          
        for (int p = 2; p*p <= n; p++)
        {
            // If prime[p] is not changed,
            // then it is a prime
            if (prime[p])
            {
                // Update all multiples of p
                for (int i = p*2; i <= n; i += p){
                    prime[i] = false;}
            }
        }
      
        // Print all palindromic prime numbers
        for (int p = 2; p <= n; p++){
      
        // checking whether the given number is
        // prime palindromic or not
        if (prime[p] && isPal(p)){
            Console.Write(p + " ");
            }
        }
    }
      
    // Driver function
    public static void Main() 
    {
        int n = 100;
        Console.Write("Palindromic primes smaller than or "
                      "equal to are :\n", n);
        printPalPrimesLessThanN(n);
    }
}
          
// This code is contributed by nitin mittal.


Output:

Palindromic primes smaller than or equal to 100 are :
2 3 5 7 11 

This article is contributed by Rahul Agrawal .If you like GeeksforGeeks and would like to contribute, you can also write an article using contribute.geeksforgeeks.org or mail your article to contribute@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.



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Improved By : nitin mittal