fast_power.cpp File Reference

Faster computation for \(a^b\). More...

#include <cassert>
#include <cmath>
#include <cstdint>
#include <cstdlib>
#include <ctime>
#include <iostream>

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Functions
template<typename T >
double	fast_power_recursive (T a, T b)
template<typename T >
double	fast_power_linear (T a, T b)
int	main ()

Detailed Description

Faster computation for \(a^b\).

Program that computes \(a^b\) in \(O(logN)\) time. It is based on formula that:

1. if \(b\) is even: \(a^b = a^\frac{b}{2} \cdot a^\frac{b}{2} = {a^\frac{b}{2}}^2\)
2. if \(b\) is odd: \(a^b = a^\frac{b-1}{2} \cdot a^\frac{b-1}{2} \cdot a = {a^\frac{b-1}{2}}^2 \cdot a\)

We can compute \(a^b\) recursively using above algorithm.

Function Documentation

fast_power_linear()

template<typename T >

double fast_power_linear	(	T	a,
		T	b )

Same algorithm with little different formula. It still calculates in \(O(\log N)\)

                                   {
    // negative power. a^b = 1 / (a^-b)
    if (b < 0)
        return 1.0 / fast_power_linear(a, -b);
 
    double result = 1;
    while (b) {
        if (b & 1)
            result = result * a;
        a = a * a;
        b = b >> 1;
    }
    return result;
}

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fast_power_recursive()

template<typename T >

double fast_power_recursive	(	T	a,
		T	b )

algorithm implementation for \(a^b\)

                                      {
    // negative power. a^b = 1 / (a^-b)
    if (b < 0)
        return 1.0 / fast_power_recursive(a, -b);
 
    if (b == 0)
        return 1;
    T bottom = fast_power_recursive(a, b >> 1);
    // Since it is integer division b/2 = (b-1)/2 where b is odd.
    // Therefore, case2 is easily solved by integer division.
 
    double result;
    if ((b & 1) == 0)  // case1
        result = bottom * bottom;
    else  // case2
        result = bottom * bottom * a;
    return result;
}

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main()

int main

(

void

)

Main function

           {
    std::srand(std::time(nullptr));
    std::ios_base::sync_with_stdio(false);
 
    std::cout << "Testing..." << std::endl;
    for (int i = 0; i < 20; i++) {
        int a = std::rand() % 20 - 10;
        int b = std::rand() % 20 - 10;
        std::cout << std::endl << "Calculating " << a << "^" << b << std::endl;
        assert(fast_power_recursive(a, b) == std::pow(a, b));
        assert(fast_power_linear(a, b) == std::pow(a, b));
 
        std::cout << "------ " << a << "^" << b << " = "
                  << fast_power_recursive(a, b) << std::endl;
    }
 
    int64_t a, b;
    std::cin >> a >> b;
 
    std::cout << a << "^" << b << " = " << fast_power_recursive(a, b)
              << std::endl;
 
    std::cout << a << "^" << b << " = " << fast_power_linear(a, b) << std::endl;
 
    return 0;
}

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