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Modular Exponentiation (Power in Modular Arithmetic)

  • Difficulty Level : Medium
  • Last Updated : 22 Apr, 2021

Given three numbers x, y and p, compute (xy) % p. 

Examples : 

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Input:  x = 2, y = 3, p = 5
Output: 3
Explanation: 2^3 % 5 = 8 % 5 = 3.

Input:  x = 2, y = 5, p = 13
Output: 6
Explanation: 2^5 % 13 = 32 % 13 = 6.

We have discussed recursive and iterative solutions for power.



Below is discussed iterative solution. 

C++




/* Iterative Function to calculate (x^y) in O(log y) */
int power(int x, int y)
{
     
    // Initialize answer
    int res = 1;
     
    // Check till the number becomes zero
    while (y)
    {
         
        // If y is odd, multiply x with result
        if (y % 2 == 1)
            res = (res * x);
             
        // y = y/2
        y = y >> 1;
         
        // Change x to x^2
        x = (x * x);
    }
    return res;
}
 
// This code is contributed by yaswanth0412


C




/* Iterative Function to calculate (x^y) in O(log y) */
int power(int x, unsigned int y)
{
    int res = 1;     // Initialize result
  
    while (y > 0)
    {
        // If y is odd, multiply x with result
        if (y & 1)
            res = res*x;
  
        // y must be even now
        y = y>>1; // y = y/2
        x = x*x;  // Change x to x^2
    }
    return res;
}


Java




/* Iterative Function to calculate (x^y) in O(log y) */
static int power(int x, int y)
{
    int res = 1;     // Initialize result
  
    while (y > 0)
    {
       
        // If y is odd, multiply x with result
        if ((y & 1) != 0)
            res = res * x;
  
        // y must be even now
        y = y >> 1; // y = y/2
        x = x * x;  // Change x to x^2
    }
    return res;
}
 
// This code is contributed by Dharanendra L V.


Python3




# Iterative Function to calculate (x^y) in O(log y)
def power(x, y):
     
    # Initialize result
    res = 1   
  
    while (y > 0):
       
        # If y is odd, multiply x with result
        if ((y & 1) != 0):
            res = res * x
  
        # y must be even now
        y = y >> 1 # y = y/2
        x = x * # Change x to x^2
         
    return res
 
# This code is contributed by Khushboogoyal499


C#




/* Iterative Function to calculate (x^y) in O(log y) */
static int power(int x, int y)
{
    int res = 1;     // Initialize result
  
    while (y > 0)
    {
        // If y is odd, multiply x with result
        if ((y & 1) != 0)
            res = res * x;
  
        // y must be even now
        y = y >> 1; // y = y/2
        x = x * x;  // Change x to x^2
    }
    return res;
}
 
// This code is contributed by Dharanendra L V.


Javascript




<script>
/* Iterative Function to calculate (x^y) in O(log y) */
function power(x, y)
{
    let res = 1;     // Initialize result
 
    while (y > 0)
    {
        // If y is odd, multiply x with result
        if (y & 1)
            res = res*x;
 
        // y must be even now
        y = y>>1; // y = y/2
        x = x*x; // Change x to x^2
    }
    return res;
}
 
// This code is contributed by _saurabh_jaiswal
</script>


Efficient Approach:

The problem with above solutions is, overflow may occur for large value of n or x. Therefore, power is generally evaluated under modulo of a large number.

Below is the fundamental modular property that is used for efficiently computing power under modular arithmetic. 

(ab) mod p = ( (a mod p) (b mod p) ) mod p 

For example a = 50,  b = 100, p = 13
50  mod 13  = 11
100 mod 13  = 9

(50 * 100) mod 13 = ( (50 mod 13) * (100 mod 13) ) mod 13 
or (5000) mod 13 = ( 11 * 9 ) mod 13
or 8 = 8

Below is the implementation based on above property.  

C++14




// Iterative C++ program to compute modular power
#include <iostream>
using namespace std;
 
/* Iterative Function to calculate (x^y)%p in O(log y) */
int power(long long x, unsigned int y, int p)
{
    int res = 1;     // Initialize result
 
    x = x % p; // Update x if it is more than or
                // equal to p
  
    if (x == 0) return 0; // In case x is divisible by p;
 
    while (y > 0)
    {
        // If y is odd, multiply x with result
        if (y & 1)
            res = (res*x) % p;
 
        // y must be even now
        y = y>>1; // y = y/2
        x = (x*x) % p;
    }
    return res;
}
 
// Driver code
int main()
{
    int x = 2;
    int y = 5;
    int p = 13;
    cout << "Power is " << power(x, y, p);
    return 0;
}
 
// This code is contributed by shubhamsingh10


Java




// Iterative Java program to compute modular power
import java.io.*;
class GFG
{
 
  /* Iterative Function to calculate (x^y) in O(log y) */
  static int power(int x, int y, int p)
  {
    int res = 1; // Initialize result
 
    x = x % p; // Update x if it is more than or
    // equal to p
 
    if (x == 0)
      return 0; // In case x is divisible by p;
 
    while (y > 0)
    {
 
      // If y is odd, multiply x with result
      if ((y & 1) != 0)
        res = (res * x) % p;
 
      // y must be even now
      y = y >> 1; // y = y/2
      x = (x * x) % p;
    }
    return res;
  }
 
  // Driver Code
  public static void main(String[] args)
  {
    int x = 2;
    int y = 5;
    int p = 13;
    System.out.print("Power is " + power(x, y, p));
  }
}
 
// This code is contributed by Dharanendra L V.


Python3




# Iterative Python3 program
# to compute modular power
 
# Iterative Function to calculate
# (x^y)%p in O(log y)
def power(x, y, p) :
    res = 1     # Initialize result
 
    # Update x if it is more
    # than or equal to p
    x = x % p
     
    if (x == 0) :
        return 0
 
    while (y > 0) :
         
        # If y is odd, multiply
        # x with result
        if ((y & 1) == 1) :
            res = (res * x) % p
 
        # y must be even now
        y = y >> 1      # y = y/2
        x = (x * x) % p
         
    return res
     
 
# Driver Code
 
x = 2; y = 5; p = 13
print("Power is ", power(x, y, p))
 
 
# This code is contributed by Nikita Tiwari.


C#




using System;
public class GFG
{
 
  /* Iterative Function to calculate (x^y) in O(log y) */
  static int power(int x, int y, int p)
  {
    int res = 1; // Initialize result
 
    x = x % p; // Update x if it is more than or
    // equal to p
 
    if (x == 0)
      return 0; // In case x is divisible by p;
 
    while (y > 0)
    {
 
      // If y is odd, multiply x with result
      if ((y & 1) != 0)
        res = (res * x) % p;
 
      // y must be even now
      y = y >> 1; // y = y/2
      x = (x * x) % p;
    }
    return res;
  }
 
  // Driver Code
  static public void Main ()
  {
    int x = 2;
    int y = 5;
    int p = 13;
    Console.Write("Power is " + power(x, y, p));
  }
}
 
// This code is contributed by Dharanendra L V.


PHP




<?php
// Iterative PHP program to
// compute modular power
 
// Iterative Function to
// calculate (x^y)%p in O(log y)
function power($x, $y, $p)
{
    // Initialize result
    $res = 1;
 
    // Update x if it is more
    // than or equal to p
    $x = $x % $p;
 
    if ($x == 0)
        return 0;
 
    while ($y > 0)
    {
        // If y is odd, multiply
        // x with result
        if ($y & 1)
            $res = ($res * $x) % $p;
 
        // y must be even now
         
        // y = $y/2
        $y = $y >> 1;
        $x = ($x * $x) % $p;
    }
    return $res;
}
 
// Driver Code
$x = 2;
$y = 5;
$p = 13;
echo "Power is ", power($x, $y, $p);
 
// This code is contributed by aj_36
?>


Javascript




// Iterative Javascript program to
// compute modular power
 
// Iterative Function to
// calculate (x^y)%p in O(log y)
function power(x, y, p)
{
    // Initialize result
    let res = 1;
 
    // Update x if it is more
    // than or equal to p
    x = x % p;
 
    if (x == 0)
        return 0;
 
    while (y > 0)
    {
        // If y is odd, multiply
        // x with result
        if (y & 1)
            res = (res * x) % p;
 
        // y must be even now
         
        // y = $y/2
        y = y >> 1;
        x = (x * x) % p;
    }
    return res;
}
 
// Driver Code
let x = 2;
let y = 5;
let p = 13;
document.write("Power is " + power(x, y, p));
 
// This code is contributed by _saurabh_jaiswal


Output

Power is 6

Time Complexity of above solution is O(Log y). 

Modular exponentiation (Recursive)
This article is contributed by Shivam Agrawal. Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above. 




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