To check divisibility of any large number by 999

You are given an n-digit large number, you have to check whether it is divisible by 999 without dividing or finding modulo of number by 999.

Examples:

Input : 235764 
Output : Yes

Input : 23576 
Output : No

Since input number may be very large, we cannot use n % 999 to check if a number is divisible by 999 or not, especially in languages like C/C++. The idea is based on following fact.

The solutions is based on below fact.

A number is divisible by 999 if sum of its 3-digit-groups (if required groups are formed by appending a 0s at the beginning) is divisible by 999.

Illustration:

Input : 235764 
Output : Yes
Explanation : Step I - read input : 235, 764
              Step II - 235 + 764 = 999
              As result is 999 then we can 
              conclude that it is divisible by 999.

Input : 1244633121
Output : Yes
Explanation : Step I - read input : 1, 244, 633, 121
              Step II - 001 + 244 + 633 + 121 = 999
              As result is 999 then we can conclude 
              that it is divisible by 999.

Input : 999999999
Output : Yes
Explanation : Step I - read input : 999, 999, 999
              Step II - 999 + 999 + 999 = 2997
              Step III - 997 + 002 = 999
              As result is 999 then we can conclude  
              that it is divisible by 999.

How does this work?

Let us consider 235764, we can write it as
235764 = 2*10⁵ + 3*10⁴ + 5*10³ + 
         7*10² + 6*10 + 4

The idea is based on below observation:
Remainder of 10³ divided by 999 is 1
For i > 3, 10ⁱ % 999 = 10^i-3 % 999 

Let us see how we use above fact.
Remainder of 2*10⁵ + 3*10⁴ + 5*10³ + 
7*10² + 6*10 + 4
Remainder with 999 can be written as : 
2*100 + 3*10 + 5*1 + 7*100 + 6*10 + 4 
The above expression is basically sum of
groups of size 3.

Since the sum is divisible by 999, answer is yes.

A simple and efficient method is to take input in form of string (make its length in form of 3*m by adding 0 to left of number if required) and then you have to add the digits in blocks of three from right to left until it become a 3 digit number and if that result is 999 we can say that number is divisible by 999.

As in the case of “divisibility by 9” we check that sum of all digit is divisible by 9 or not, the same thing follows within the case of divisibility by 999. We sum up all 3-digits group from right to left and check whether the final result is 999 or not.

Implementation:

C++

// CPP for divisibility of number by 999
#include<bits/stdc++.h>
using namespace std;
  
// function to check divisibility
bool isDivisible999(string num)
{
    int n = num.length();
    if (n == 0 && num[0] == '0')
       return true;
  
    // Append required 0s at the beginning.
    if (n % 3 == 1)
       num = "00" + num;
    if (n % 3 == 2)
       num = "0" + num;
  
    // add digits in group of three in gSum
    int gSum = 0;
    for (int i = 0; i<n; i++)
    {
        // group saves 3-digit group
        int group = 0;
        group += (num[i++] - '0') * 100;
        group += (num[i++] - '0') * 10;
        group += num[i] - '0';
        gSum += group;
    }
  
    // calculate result till 3 digit sum
    if (gSum > 1000)
    {
        num = to_string(gSum);
        n = num.length();
        gSum = isDivisible999(num);
    }
    return (gSum == 999);
}
  
// driver program
int main()
{
    string num = "1998";
    int n = num.length();
    if (isDivisible999(num))
        cout << "Divisible";
    else
        cout << "Not divisible";
    return 0;
}

Java

//Java for divisibility of number by 999
  
class Test
{    
    // Method to check divisibility
    static boolean isDivisible999(String num)
    {
        int n = num.length();
        if (n == 0 && num.charAt(0) == '0')
           return true;
       
        // Append required 0s at the beginning.
        if (n % 3 == 1)
           num = "00" + num;
        if (n % 3 == 2)
           num = "0" + num;
       
        // add digits in group of three in gSum
        int gSum = 0;
        for (int i = 0; i<n; i++)
        {
            // group saves 3-digit group
            int group = 0;
            group += (num.charAt(i++) - '0') * 100;
            group += (num.charAt(i++) - '0') * 10;
            group += num.charAt(i) - '0';
            gSum += group;
        }
       
        // calculate result till 3 digit sum
        if (gSum > 1000)
        {
            num = Integer.toString(gSum);
            n = num.length();
            gSum = isDivisible999(num) ? 1 : 0;
        }
        return (gSum == 999);
    }
      
    // Driver method
    public static void main(String args[]) 
    {
        String num = "1998";
       
        System.out.println(isDivisible999(num) ? "Divisible" : "Not divisible");
    }
}

Python 3

# Python3 program for divisibility 
# of number by 999 
  
# function to check divisibility 
def isDivisible999(num):
    n = len(num);
    if(n == 0 or num[0] == '0'):
        return true
  
    # Append required 0s at the beginning.
    if((n % 3) == 1):
        num = "00" + num
    if((n % 3) == 2):
        num = "0" + num
  
    # add digits in group of three in gSum     
    gSum = 0
    for i in range(0, n, 3):
          
        # group saves 3-digit group 
        group = 0
        group += (ord(num[i]) - 48) * 100
        group += (ord(num[i + 1]) - 48) * 10
        group += (ord(num[i + 2]) - 48)
        gSum += group
  
    # calculate result till 3 digit sum     
    if(gSum > 1000):
        num = str(gSum)
        n = len(num)
        gSum = isDivisible999(num)
    return (gSum == 999) 
  
# Driver code 
if __name__=="__main__":
    num = "1998"
    n = len(num)
    if(isDivisible999(num)):
        print("Divisible")
    else:
        print("Not divisible")
          
# This code is contributed
# by Sairahul Jella

C#

// C# code for divisibility of number by 999 
  
using System;
class Test 
{ 
    // Method to check divisibility 
    static bool isDivisible999(String num) 
    { 
        int n = num.Length; 
        if (n == 0 && num[0] == '0') 
        return true; 
      
        // Append required 0s at the beginning. 
        if (n % 3 == 1) 
        num = "00" + num; 
        if (n % 3 == 2) 
        num = "0" + num; 
      
        // add digits in group of three in gSum 
        int gSum = 0; 
        for (int i = 0; i<n; i++) 
        { 
            // group saves 3-digit group 
            int group = 0; 
            group += (num[i++] - '0') * 100; 
            group += (num[i++] - '0') * 10; 
            group += num[i] - '0'; 
            gSum += group; 
        } 
      
        // calculate result till 3 digit sum 
        if (gSum > 1000) 
        { 
            num = Convert.ToString(gSum); 
            n = num.Length ; 
            gSum = isDivisible999(num) ? 1 : 0; 
        } 
        return (gSum == 999); 
    } 
      
    // Driver method 
    public static void Main() 
    { 
        String num = "1998"; 
      
        Console.WriteLine(isDivisible999(num) ? "Divisible" : "Not divisible"); 
    } 
    // This code is contributed by Ryuga
} 

PHP

<?php
// PHP for divisibility of number by 999
  
// function to check divisibility
function isDivisible999($num)
{
    $n = strlen($num);
    if ($n == 0 && $num[0] == '0')
    return true;
  
    // Append required 0s at the beginning.
    if ($n % 3 == 1)
        $num = "00" . $num;
    if ($n % 3 == 2)
        $num = "0" . $num;
  
    // add digits in group of three in gSum
    $gSum = 0;
    for ($i = 0; $i < $n; $i += 3)
    {
        // group saves 3-digit group
        $group = 0;
        $group += (ord($num[$i]) - 48) * 100;
        $group += (ord($num[$i + 1]) - 48) * 10;
        $group += (ord($num[$i + 2]) - 48);
        $gSum += $group;
    }
      
    // calculate result till 3 digit sum
    if ($gSum > 1000)
    {
        $num = strval($gSum);
        $n = strlen($num);
        $gSum = isDivisible999($num);
    }
    return ($gSum == 999);
}
  
// Driver Code
$num = "1998";
if (isDivisible999($num))
    echo "Divisible";
else
    echo "Not divisible";
  
// This code is contributed by mits
?>

Javascript

<script>
// Javascript for divisibility of number by 999
  
// function to check divisibility
function isDivisible999(num)
{
    let n = num.length;
    if (n == 0 && num[0] == '0')
    return true;
  
    // Append required 0s at the beginning.
    if (n % 3 == 1)
        num = "00" + num;
    if (n % 3 == 2)
        num = "0" + num;
  
    // add digits in group of three in gSum
    let gSum = 0;
    for (let i = 0; i < n; i += 3)
    {
        // group saves 3-digit group
        group = 0;
        group += (num.charCodeAt(i) - 48) * 100;
        group += (num.charCodeAt(i + 1) - 48) * 10;
        group += (num.charCodeAt(i + 2) - 48);
        gSum += group;
    }
      
    // calculate result till 3 digit sum
    if (gSum > 1000)
    {
        num = String(gSum);
        n = strlen(num);
        gSum = isDivisible999(num);
    }
    return (gSum == 999);
}
  
// Driver Code
let num = "1998";
if (isDivisible999(num))
    document.write("Divisible");
else
    document.write("Not divisible");
  
// This code is contributed by _saurabh_jaiswal.
</script>

Output

Divisible

Time complexity : O(n)
Auxiliary Space : O(1)

Method 2: Checking given any number is divisible by 999 or not by using modulo division operator “%”.

Implementation:

C++

#include <iostream>
using namespace std;
int main()
{
    //input
    long long int n=235764;
       
       
    // finding given number is divisible by 999 or not
    if (n%999==0)
    {
        cout << "Yes";
    }
    else
    {
        cout << "No";
    }
     
    return 0;
}
  
// This code is contributed by satwik4409.

Java

/*package whatever //do not write package name here */
import java.io.*;
  
class GFG {
  public static void main (String[] args) {
  
    // input 
    long n = 235764;
  
    // finding given number is divisible by 999 or not
    if (n % 999 == 0)
      System.out.println("Yes");
    else
      System.out.println("No"); 
  
  }
}
  
// This code is contributed by ksrikanth0498.

Python3

# Python code 
# To check whether the given number is divisible by 999 or not
  
#input 
n=235764
# the above input can also be given as n=input() -> taking input from user
# finding given number is divisible by 999 or not
if int(n)%999==0:
  print("Yes") 
else: 
  print("No") 
    
  # this code is contributed by gangarajula laxmi

C#

// C# code 
// To check whether the given number is divisible by 999 or not
using System;
  
public class GFG{
  
    static public void Main ()
    {
        
       // input 
       long n = 235764;
        
        // finding given number is divisible by 999 or not
       if (n % 999 == 0)
            Console.Write("Yes");
        else
            Console.Write("No"); 
    } 
}
  
// This code is contributed by ksrikanth0498

PHP

<?php
    $num =235764;
    // checking if the given number is divisible by 999 or
    // not using modulo division operator if the output of
    // num%999 is equal to 0 then given number is divisible
    // by 999 otherwise not divisible by 999
    if ($num % 999 == 0) {
        echo "Yes";
    }
    else {
        echo "No";
    }
    
?>

Javascript

<script>
        // JavaScript code for the above approach
        //input 
        let n = 235764
          
        // finding given number is divisible by 999 or not
        if (n % 999 == 0)
            document.write("Yes")
        else
            document.write("No")
    // This code is contributed by Potta Lokesh
    </script>

Output

Yes

Time Complexity : O(1)

Auxiliary Space: O(1)

Method 3: The function returns a Boolean value indicating whether the input integer is divisible by 999 or not. The function calculates this by checking if the sum of the digits of num is divisible by 9 and if the last three digits of num are divisible by 27. This code then tests the function with the examples: 999, 998 and 9987 and outputs the values True, False and False respectively.

C++

convert C++ code into Javascript code
  
#include <bits/stdc++.h>
using namespace std;
  
bool is_divisible_by_999(int num) {
    string num_str = to_string(num);
    int sum = 0;
    for (char digit : num_str) {
        sum += digit - '0';
    }
    return sum % 9 == 0 && stoi(num_str.substr(num_str.length() - 3)) % 27 == 0;
}
  
int main() {
    cout << is_divisible_by_999(999) << endl;  // True
    cout << is_divisible_by_999(998) << endl;  // False
    cout << is_divisible_by_999(9987) << endl;  // False
    return 0;
}

Java

public class Main {
    public static boolean isDivisibleBy999(int num) {
        String numStr = String.valueOf(num);
        int sum = 0;
        for (int i = 0; i < numStr.length(); i++) {
            char digit = numStr.charAt(i);
            sum += Character.getNumericValue(digit);
        }
        return sum % 9 == 0 && Integer.parseInt(numStr.substring(numStr.length() - 3)) % 27 == 0;
    }
  
    public static void main(String[] args) {
        System.out.println(isDivisibleBy999(999));  // true
        System.out.println(isDivisibleBy999(998));  // false
        System.out.println(isDivisibleBy999(9987)); // false
    }
}

Python3

def is_divisible_by_999(num):
    num_str = str(num)
    return sum(int(digit) for digit in num_str) % 9 == 0 and int(num_str[-3:]) % 27 == 0
  
print(is_divisible_by_999(999)) # True
print(is_divisible_by_999(998)) # False
print(is_divisible_by_999(9987)) # False

C#

using System;
  
class Program {
    static bool IsDivisibleBy999(int num) {
        string numStr = num.ToString();
        int digitSum = 0;
        foreach (char digitChar in numStr) {
            digitSum += digitChar - '0';
        }
        return digitSum % 9 == 0 && int.Parse(numStr.Substring(numStr.Length - 3)) % 27 == 0;
    }
  
    static void Main() {
        Console.WriteLine(IsDivisibleBy999(999)); // True
        Console.WriteLine(IsDivisibleBy999(998)); // False
        Console.WriteLine(IsDivisibleBy999(9987)); // False
    }
}

Javascript

function is_divisible_by_999(num) {
    // Convert the input number to a string
    var num_str = num.toString();
  
    // Initialize a variable to hold the sum of the digits
    var sum = 0;
  
    // Iterate over each character in the string representation of the number
    for (var i = 0; i < num_str.length; i++) {
        // Add the numeric value of the current digit to the sum variable
        sum += parseInt(num_str[i]);
    }
  
    // Check if the sum is divisible by 9 and the last three digits of the number are divisible by 27
    return (sum % 9 == 0 && parseInt(num_str.substring(num_str.length - 3)) % 27 == 0);
}
  
// Test the function with some sample inputs
console.log(is_divisible_by_999(999));   // true
console.log(is_divisible_by_999(998));   // false
console.log(is_divisible_by_999(9987));  // false
  
// This code is contributed by bhardwajji.

Output

True
False
False

Time complexity: O(1),
Auxiliary Space : O(1)

Method 4: Using string manipulation

1. Convert the number to a string.
2. Find the sum of every third digit from the right end to the left end.
3. If the sum is divisible by 3, then the original number is divisible by 999.

Java

public class GFG {
    public static boolean isDivisibleBy999(int n) {
        String s = Integer.toString(n);
        int l = s.length();
        int sum = 0;
        for (int i = l - 1; i >= 0; i -= 3) {
            int j = Math.max(i - 2, -1);
            sum += Integer.parseInt(s.substring(j + 1, i + 1));
        }
        return sum % 3 == 0;
    }
  
    public static void main(String[] args) {
        int n1 = 998;
        int n2 = 999;
        System.out.println(isDivisibleBy999(n1));
        System.out.println(isDivisibleBy999(n2));
    }
}

Python3

# python code to check if given number is divisible by 998 or not
def is_divisible_by_999(n):
    s = str(n)
    l = len(s)
    sum = 0
    for i in range(l-1, -1, -3):
        j = max(i-2, -1)
        sum += int(s[j+1:i+1])
    return sum % 3 == 0
  
# Example usage
n1 = 998
n2 = 999
print(is_divisible_by_999(n1))  # Output: True
print(is_divisible_by_999(n2))  # Output: False

C++

#include <iostream>
#include <string>
  
using namespace std;
  
bool isDivisibleBy999(int n)
{
    string s = to_string(n);
    int l = s.length();
    int sum = 0;
    for (int i = l - 1; i >= 0; i -= 3) {
        int j = max(i - 2, -1);
        sum += stoi(s.substr(j + 1, i - j));
    }
    return sum % 3 == 0;
}
  
int main()
{
    int n1 = 998;
    int n2 = 999;
    cout << isDivisibleBy999(n1) << endl;
    cout << isDivisibleBy999(n2) << endl;
    return 0;
}

C#

using System;
  
public class GFG {
    public static bool IsDivisibleBy999(int n) {
        string s = n.ToString();
        int l = s.Length;
        int sum = 0;
        for (int i = l - 1; i >= 0; i -= 3) {
            int j = Math.Max(i - 2, -1);
            sum += int.Parse(s.Substring(j + 1, i - j));
        }
        return sum % 3 == 0;
    }
  
    public static void Main(string[] args) {
        int n1 = 998;
        int n2 = 999;
        Console.WriteLine(IsDivisibleBy999(n1));
        Console.WriteLine(IsDivisibleBy999(n2));
    }
}
// This code is contributed by shiv1o43g

Javascript

// JavaScript code to check if given number is divisible by 998 or not
function isDivisibleBy998(n) {
  let s = n.toString();
  let l = s.length;
  let sum = 0;
  for (let i = l - 1; i >= 0; i -= 3) {
    let j = Math.max(i - 2, -1);
    sum += parseInt(s.substring(j + 1, i + 1));
  }
  return sum % 3 === 0;
}
  
// Example usage
let n1 = 998;
let n2 = 999;
console.log(isDivisibleBy998(n1));  
console.log(isDivisibleBy998(n2));  

Output

False
True

The time complexity of the given function is_divisible_by_999 is O(N/3) where N is the number of digits in the input number n. This is because the loop iterates over every third digit of the input number, and performs constant time operations (such as string slicing and integer conversion) on each iteration.

The auxiliary space used by the function is O(1), since it uses a constant amount of extra space to store the loop variables and the running sum.

More Divisibility Algorithms.
This article is contributed by Shivam Pradhan (anuj_charm). If you like GeeksforGeeks and would like to contribute, you can also write an article using write.geeksforgeeks.org or mail your article to review-team@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.

Last Updated : 15 Sep, 2023