c++ - Boost Using pow function with cpp_dec_float -

i using pow() function , trying compare return cpp_dec_float getting error

code:

pow(sqrt(172.601), 2) != n); 

error:

userpath\main.cpp:21: error: no match 'operator!=' (operand types '__gnu_cxx::__promote_2<double, int, double, double>::__type {aka double}' , 'boost::multiprecision::cpp_int {aka boost::multiprecision::number<boost::multiprecision::backends::cpp_int_backend<> >}')     pow(sqrt(172.601), 2) != n))                           ^ 

there number of pitfalls here. see added links @ bottom.

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something tells me you've been using backend type, not frontend adaptor (like number<> or rational_adaptor<>).

the thing works without change:

live on coliru

#include <boost/multiprecision/cpp_dec_float.hpp> #include <iostream>  int main() {     boost::multiprecision::cpp_dec_float_50 n = 3;     bool ok = pow(sqrt(172.601), 2) != n;      std::cout << std::boolalpha << ok; } 

prints

true 

however

you mixing double , cpp_dec_float. means not gain - if in scenario - enhanced accuracy or decimal representation of boost multiprecision.

instead consider going whole way:

live on coliru

#include <boost/multiprecision/cpp_dec_float.hpp> #include <iostream>  int main() {     typedef boost::multiprecision::cpp_dec_float_50 decimal;      decimal n("172.601");      decimal other = pow(sqrt(decimal("172.601")), 2);      std::cout << std::setprecision(50) << n << "\n";     std::cout << std::setprecision(50) << other << "\n";      bool equal = (abs(other - n) < std::numeric_limits<decimal>::epsilon());      std::cout << std::boolalpha << equal; } 

prints:

172.601 172.601 true 

note crucial initialization text, not double literal!

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background info:

• what effective way float , double comparison?
• higher precision floating point using boost lib (higher 16 digits)