Program for power of a complex number in O(log n)

Given a complex number of the form x + yi and an integer n, the task is to calculate the value of this complex number raised to the power n.

Examples:

Input: num = 17 - 12i, n = 3
Output: -2431 + i ( -8676 )

Input: num = 18 - 13i, n = 8
Output: 16976403601 + i ( 56580909840 )

Approach: This algorithm works in O(log n) time complexity. Let c = a + bi and n is the exponent then,



power(x, n) {
    if(n == 1)
        return x
    sq = power(x, n/2);
    if(n % 2 == 0)
        return cmul(sq, sq);
    if(n % 2 != 0)
        return cmul(x, cmul(sq, sq));
}

As complex number has 2 fields one is real and other is complex hence we store it in an array. We use cmul() function to multiply two arrays where cmul() implements the below multiplication.

If x1 = a + bi and x2 = c + di
then x1 * x2 = a * c - b * d + (b * c + d * a )i 

As we can see in power() function the same function is called for input decreased by 1/2 times.
T(n) = T(n / 2) + c therefore, T(n) = log n

Below is the implementation of the above approach:

C++

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// C++ implementation of the approach
#include <bits/stdc++.h>
using namespace std;
  
// Function to return the product
// of two complex numbers
long long* cmul(long long* sq1, long long* sq2)
{
    long long* ans = new long long[2];
  
    // For real part
    ans[0] = (sq1[0] * sq2[0]) - (sq1[1] * sq2[1]);
  
    // For imaginary part
    ans[1] = (sq1[1] * sq2[0]) + sq1[0] * sq2[1];
  
    return ans;
}
  
// Function to return the complex number
// raised to the power n
long long* power(long long* x, long long n)
{
    long long* ans = new long long[2];
    if (n == 0) {
        ans[0] = 0;
        ans[1] = 0;
        return ans;
    }
    if (n == 1)
        return x;
  
    // Recursive call for n/2
    long long* sq = power(x, n / 2);
    if (n % 2 == 0)
        return cmul(sq, sq);
    return cmul(x, cmul(sq, sq));
}
  
// Driver code
int main()
{
    int n;
    long long* x = new long long[2];
  
    // Real part of the complex number
    x[0] = 18;
  
    // Imaginary part of the complex number
    x[1] = -13;
    n = 8;
  
    // Calculate and print the result
    long long* a = power(x, n);
    cout << a[0] << " + i ( " << a[1] << " )" << endl;
  
    return 0;
}

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Python 3

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# Python3 implementation of the approach
  
# Function to return the product
# of two complex numbers
def cmul(sq1, sq2):
  
    ans = [0] * 2
  
    # For real part
    ans[0] = ((sq1[0] * sq2[0]) - 
              (sq1[1] * sq2[1]))
  
    # For imaginary part
    ans[1] = ((sq1[1] * sq2[0]) + 
               sq1[0] * sq2[1])
  
    return ans
  
# Function to return the complex 
# number raised to the power n
def power(x, n):
  
    ans = [0] * 2
    if (n == 0):
        ans[0] = 0
        ans[1] = 0
        return ans
      
    if (n == 1):
        return x
  
    # Recursive call for n/2
    sq = power(x, n // 2)
    if (n % 2 == 0):
        return cmul(sq, sq)
    return cmul(x, cmul(sq, sq))
  
# Driver code
if __name__ == "__main__":
  
    x = [0] * 2
  
    # Real part of the complex number
    x[0] = 18
  
    # Imaginary part of the
    # complex number
    x[1] = -13
    n = 8
  
    # Calculate and print the result
    a = power(x, n)
    print(a[0], " + i ( ", a[1], " )")
  
# This code is contributed by ita_c

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PHP

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<?php
// PHP implementation of the approach 
  
// Function to return the product 
// of two complex numbers 
function cmul($sq1, $sq2
    $ans = array(); 
  
    // For real part 
    $ans[0] = ($sq1[0] * $sq2[0]) - 
              ($sq1[1] * $sq2[1]); 
  
    // For imaginary part 
    $ans[1] = ($sq1[1] * $sq2[0]) + 
               $sq1[0] * $sq2[1]; 
  
    return $ans
  
// Function to return the complex 
// number raised to the power n 
function power($x, $n
    $ans = array(); 
    if ($n == 0) 
    
        $ans[0] = 0; 
        $ans[1] = 0; 
        return $ans
    
    if ($n == 1) 
        return $x
  
    // Recursive call for n/2 
    $sq = power($x, $n / 2); 
    if ($n % 2 == 0) 
        return cmul($sq, $sq); 
    return cmul($x, cmul($sq, $sq)); 
  
// Driver code 
$x = array(); 
  
// Real part of the complex number 
$x[0] = 18; 
  
// Imaginary part of the complex number 
$x[1] = -13; 
$n = 8; 
  
// Calculate and print the result 
$a = power($x, $n); 
echo $a[0], " + i ( ", $a[1], " )";
  
// This code is contributed by Ryuga
?>

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Output:

16976403601 + i ( 56580909840 )

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