program

check pow overflow

Source code

// Create by matrix207 2016-03-09
// g++ -o t check-pow-overflow.cc

#include <iostream>
#include <typeinfo>
#include <cmath>
#include <limits>

/*
 * Check if pow overflow for type T (int,float,long,double)
 * a^b <= m  <=> log(a^b) <= log(m) <=> b*log(a) <= log(m) <=> b <= log(m)/log(a)
 * @param a
 *     base number
 * @param b
 *     exponent number
 * @return
 *     1 if overflow, otherwise 0
 */
template<typename T1, typename T2>
int check_pow(T1 a, T2 b)
{
    int ret = 0;
    // i:int, f:float, d:double, l:long
    std::cout << "Type: " << typeid(a).name() << "," << typeid(b).name() << " ";
    double max_a = std::log(std::numeric_limits<T1>::max());
    double max_b = std::log(std::numeric_limits<T2>::max());
    double max_max = max_a < max_b ? max_b : max_a;
    if (b * std::log(a) < max_max) {
        double c = std::pow(a, b);
        std::cout << a << "^" << b << "=" << c << std::endl;
    } else {
        std::cout << a << "^" << b << " overflow" << std::endl;
        ret = 1;
    }
    return ret;
}

int main(int argc, char* argv[])
{
    std::cout << "\n>>> Test int type <<<" << std::endl;
    check_pow(25, 1);
    check_pow(25, 10);
    check_pow(25, 100);
    check_pow(25, 1000);

    std::cout << "\n>>> Test long type <<<" << std::endl;
    check_pow(25L, 1L);
    check_pow(25L, 10L);
    check_pow(25L, 100L);
    check_pow(25L, 1000L);

    std::cout << "\n>>> Test float type <<<" << std::endl;
    check_pow(25.3f, 1.0f);
    check_pow(25.3f, 10.0f);
    check_pow(25.3f, 100.0f);
    check_pow(25.3f, 1000.0f);

    std::cout << "\n>>> Test double type <<<" << std::endl;
    check_pow(25.3, 1.0);
    check_pow(25.3, 10.0);
    check_pow(25.3, 100.0);
    check_pow(25.3, 1000.0);

    std::cout << "\n>>> Test mix type <<<" << std::endl;
    check_pow(25, 10.0); // int, double
    check_pow(25, 100.0); // int, double
    check_pow(25, 1000.0); // int, double
    check_pow(25.0f, 10.0); // float, double
    check_pow(25.0f, 100.0); // float, double
    check_pow(25.0f, 1000.0); // float, double

    std::cout << "\n>>> Test others <<<" << std::endl;
    check_pow(0, 100);
    check_pow(0, -2);
    check_pow(-1, -2);
    check_pow(-1, -20);
    check_pow(25, -20);

    return 0;
}

Test

[dennis@localhost code]$ g++ -o t check-pow-overflow.cc
[dennis@localhost code]$ ./t

>>> Test int type <<<
Type: i,i 25^1=25
Type: i,i 25^10 overflow
Type: i,i 25^100 overflow
Type: i,i 25^1000 overflow

>>> Test long type <<<
Type: l,l 25^1=25
Type: l,l 25^10=9.53674e+13
Type: l,l 25^100 overflow
Type: l,l 25^1000 overflow

>>> Test float type <<<
Type: f,f 25.3^1=25.3
Type: f,f 25.3^10=1.0745e+14
Type: f,f 25.3^100 overflow
Type: f,f 25.3^1000 overflow

>>> Test double type <<<
Type: d,d 25.3^1=25.3
Type: d,d 25.3^10=1.0745e+14
Type: d,d 25.3^100=2.05141e+140
Type: d,d 25.3^1000 overflow

>>> Test mix type <<<
Type: i,d 25^10=9.53674e+13
Type: i,d 25^100=6.22302e+139
Type: i,d 25^1000 overflow
Type: f,d 25^10=9.53674e+13
Type: f,d 25^100=6.22302e+139
Type: f,d 25^1000 overflow

>>> Test other <<<
Type: i,i 0^100=0
Type: i,i 0^-2 overflow
Type: i,i -1^-2 overflow
Type: i,i -1^-20 overflow
Type: i,i 25^-20=1.09951e-28

Reference