Recursive sum of digits of a number is prime or not

28 Apr, 2021
Given a number n, we need to find the sum of each digits of the number till the number becomes a single digit. We need to print “yes” if the sum is a prime or “no” if it is not prime. 
Examples: 
Input : 5602
Output: No
Explanation:
Step 1- 5+6+0+2 = 13
Step 2- 1+3 = 4 
4 is not prime

Input : 56
Output : Yes
Explanation:
Step 1- 5+6 = 11
Step 2- 1+1 = 2 hence 2 is prime 
The idea is simple, we can quickly find recursive sum of digits.
int recDigSum(int n)
{
    if (n == 0) 
       return 0;
    return (n % 9 == 0) ? 9 : (n % 9);
}
Once, recursive sum is calculated, check if it is prime or not by simply checking if it is 2, 3, 5 or 7 (These are only single digit primes).
C++
Java
Python
C#
PHP
Javascript
// CPP code to check if// recursive sum of// digits is prime or not.#include<iostream>using namespace std; // Function for recursive// digit sumint recDigSum(int n){    if (n == 0)        return 0;    else    {        if (n % 9 == 0)            return 9;        else            return n % 9;    }}     // function to check if prime// or not the single digitvoid check(int n){    // calls function which    // returns sum till    // single digit    n = recDigSum(n);         // checking prime    if (n == 2 or n == 3 or n == 5 or n == 7)        cout << "Yes";    else        cout << "No";} // Driver codeint main(){   int n = 5602;    check(n);} // This code is contributed by Shreyanshi.
// java code to check if// recursive sum of// digits is prime or not.import java.io.*; class GFG{     // Function for recursive    // digit sum    static int recDigSum(int n)    {        if (n == 0)            return 0;        else        {            if (n % 9 == 0)                return 9;            else                return n % 9;        }    }             // function to check if prime    // or not the single digit    static void check(int n)    {        // calls function which        // returns sum till        // single digit        n = recDigSum(n);                 // checking prime        if (n == 2 || n == 3 || n == 5 || n == 7)            System.out.println ( "Yes");        else            System.out.println("No");    }         // Driver code    public static void main (String[] args)    {        int n = 5602;        check(n);         }} // This code is contributed by vt_m
# Python code to check if recursive sum# of digits is prime or not.def recDigSum(n):         if n == 0:        return 0    else:        if n % 9 == 0:            return 9        else:            return n % 9     # function to check if prime or not# the single digitdef check(n):         # calls function which returns sum    # till single digit    n = recDigSum(n)         # checking prime    if n == 2 or n == 3 or n == 5 or n == 7:        print "Yes"    else:        print "No"      # driver coden = 5602check(n)
// C# code to check if// recursive sum of// digits is prime or not.using System; class GFG{     // Function for recursive    // digit sum    static int recDigSum(int n)    {        if (n == 0)            return 0;        else        {            if (n % 9 == 0)                return 9;            else                return n % 9;        }    }             // function to check if prime    // or not the single digit    static void check(int n)    {        // calls function which        // returns sum till        // single digit        n = recDigSum(n);                 // checking prime        if (n == 2 || n == 3            || n == 5 || n == 7)            Console.WriteLine ( "Yes");        else            Console.WriteLine("No");    }         // Driver code    public static void Main ()    {        int n = 5602;        check(n);         }} // This code is contributed by anuj_67.
<?php// PHP code to check if// recursive sum of// digits is prime or not. // Function for recursive// digit sumfunction recDigSum($n){    if ($n == 0)        return 0;    else    {        if ($n % 9 == 0)            return 9;        else            return $n % 9;    }}     // function to check if prime// or not the single digitfunction check( $n){    // calls function which    // returns sum till    // single digit    $n = recDigSum($n);         // checking prime    if ($n == 2 or $n == 3 or        $n == 5 or $n == 7)        echo "Yes";    else        echo "No";} // Driver code$n = 5602;check($n); // This code is contributed by ajit.?>
<script> // Javascript code to check if// recursive sum of digits is// prime or not. // Function for recursive// digit sumfunction recDigSum(n){    if (n == 0)        return 0;    else    {        if (n % 9 == 0)            return 9;        else            return n % 9;    }}       // Function to check if prime// or not the single digitfunction check(n){         // Calls function which    // returns sum till    // single digit    n = recDigSum(n);           // Checking prime    if (n == 2 || n == 3 ||        n == 5 || n == 7)        document.write("Yes");    else        document.write("No");} // Driver codelet n = 5602; check(n); // This code is contributed by susmitakundugoaldanga </script>
Output: 
No
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