c++ - float to int conversion going wrong (even though the float is already an int) -

i writing little function calculate binomial coefficiant using tgamma function provided c++. tgamma returns float values, wanted return integer. please take @ example program comparing 3 ways of converting float int:

#include <iostream> #include <cmath>  int bincoeffnear(int n,int k){     return std::nearbyint( std::tgamma(n+1) / (std::tgamma(k+1)*std::tgamma(n-k+1)) ); }  int bincoeffcast(int n,int k){     return static_cast<int>( std::tgamma(n+1) / (std::tgamma(k+1)*std::tgamma(n-k+1)) ); }  int bincoeff(int n,int k){     return (int) std::tgamma(n+1) / (std::tgamma(k+1)*std::tgamma(n-k+1)); }  int main() {     int n = 7;     int k = 2;     std::cout << "correct: " << std::tgamma(7+1) / (std::tgamma(2+1)*std::tgamma(7-2+1)); //returns 21     std::cout << " bincoeff: " << bincoeff(n,k); //returns 20      std::cout << " staticcast: " << bincoeffcast(n,k); //returns 20     std::cout << " nearby int: " << bincoeffnear(n,k); //returns 21      return 0;    } 

why it, though calculation returns float equal 21, 'normal' conversion fails , nearbyint returns correct value. nicest way implement this?

edit: according c++ documentation here tgamma(int) returns double.

from this std::tgamma reference:

if arg natural number, std::tgamma(arg) factorial of arg-1. many implementations calculate exact integer-domain factorial if argument sufficiently small integer.

it seems compiler you're using doing that, calculating factorial of 7 expression std::tgamma(7+1).

the result might differ between compilers, , between optimization levels. demonstrated jonas there big difference between optimized , unoptimized builds.

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