Question

How to implement an integer-based power function pow(int, int) in C++

Answer 1

#include <iostream>

// Function to calculate integer power
int intpow(int base, int exp) {
    int power = 1;

    while (true) {
        if (exp & 1)
            power *= base;
        exp >>= 1;
        if (!exp)
            break;
        base *= base;
    }

    return power;
}

int main() {
    std::cout << intpow(2, 3) << std::endl;  // 8
    std::cout << intpow(3, 3) << std::endl;  // 27
    std::cout << intpow(3, 2) << std::endl;  // 9
    std::cout << intpow(2, 2) << std::endl;  // 4
    std::cout << intpow(5, 2) << std::endl;  // 25
    std::cout << intpow(-2, 4) << std::endl; // 16
}



/*
run:

8
27
9
4
25
16

*/

 

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Answer 2

#include <iostream>
#include <stdexcept>

/*
    Integer power function: intpow(base, exponent)

    - Computes base^exponent for integer inputs.
    - Uses exponentiation by squaring:
        • This reduces the number of multiplications dramatically.
        • Runs in O(log exponent) time.
    - Handles negative bases naturally.
    - Rejects negative exponents because the result would be fractional.
      (You can extend this if you want integer reciprocals or fixed‑point.)
*/

int intpow(int base, int exp) {
    // Guard against negative exponents: not representable as int
    if (exp < 0) {
        throw std::invalid_argument("intpow: negative exponent not supported");
    }

    // Fast path: anything to the power of 0 is 1
    if (exp == 0) {
        return 1;
    }

    // Use exponentiation by squaring
    long long result = 1;     // use wider type internally to reduce overflow risk
    long long current = base; // current multiplier

    while (exp > 0) {
        // If the current exponent bit is set, multiply result by current
        if (exp & 1) {
            result *= current;
        }

        // Square the current multiplier for the next bit
        current *= current;

        // Shift exponent right by one bit
        exp >>= 1;
    }

    // Cast back to int; caller is responsible for avoiding overflow
    return static_cast<int>(result);
}

/*
    Test main
*/
int main() {
    std::cout << intpow(2, 3) << std::endl;  // 8
    std::cout << intpow(3, 3) << std::endl;  // 27
    std::cout << intpow(3, 2) << std::endl;  // 9
    std::cout << intpow(2, 2) << std::endl;  // 4
    std::cout << intpow(5, 2) << std::endl;  // 25
    std::cout << intpow(-2, 4) << std::endl; // 16
}



/*
run:

8
27
9
4
25
16

*/

 

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