(C++) TestFloatVersusDouble

February 24, 2017 · View on GitHub

(C++) TestFloatVersusDouble

A simple benchmark to test how much faster or slower the use of float is compared to the use of doubles.

Prefer a double over a float [1].

Technical facts

Application type(s)

Desktop application

Operating system(s) or programming environment(s)

Lubuntu 12.10 (quantal)

Qt Creator 2.5.2

Console application

C++11

G++ 4.7.2

Libraries used:

STL: GNU ISO C++ Library, version 4.7.2

Qt project file: CppTestFloatVersusDouble.pro

`TEMPLATE = app CONFIG += console CONFIG -= qt QMAKE_CXXFLAGS += -std=c++11 SOURCES += main.cpp`

main.cpp

#include <cstdlib> #include <iostream> #include <vector> #include <boost/timer.hpp> template <class T> double Test( const int nRepeat, const int size, const std::vector<T>& a, const std::vector<T>& b, std::vector<T>& c) { boost::timer t; for (int i=0; i!=nRepeat; ++i) { for (int x=0; x!=size; ++x) { c[x] = a[x] * b[x]; } } return t.elapsed(); } ///Benchmark to compare the speed of float versus double //From http://www.richelbilderbeek.nl/CppTestFloatVersusDouble.htm int main() { const int size = 1000; const int n_repeat = 1000; std::vector<double> vd1(size); std::vector<double> vd2(size); std::vector<double> vd3(size); std::vector<float> vf1(size); std::vector<float> vf2(size); std::vector<float> vf3(size); //Initialize the std::vector<double> with random doubles for (int i=0; i!=size; ++i) { vd1[i] = static_cast<double>(std::rand()) / static_cast<double>(3 + (std::rand() % 100)); vd2[i] = static_cast<double>(std::rand()) / static_cast<double>(3 + (std::rand() % 100)); } //Copy the std::vector<double> to std::vector<float>, //so that the values are the same for (int i=0; i!=size; ++i) { vf1[i] = static_cast<float>(vd1[i]); vf2[i] = static_cast<float>(vd2[i]); } //Some tallies int n_float_faster = 0; int n_double_faster = 0; int n_draw = 0; double sum_time_float = 0.0; double sum_time_double = 0.0; while (1) { double time_float = 0.0; double time_double = 0.0; if ((std::rand() >> 4) % 2 == 0) { //Floats first time_float = Test(n_repeat,size,vf1,vf2,vf3); time_double = Test(n_repeat,size,vd1,vd2,vd3); } else { //Doubles first time_double = Test(n_repeat,size,vd1,vd2,vd3); time_float = Test(n_repeat,size,vf1,vf2,vf3); } std::swap(vf1,vf3); std::swap(vd1,vd3); sum_time_float += time_float; sum_time_double += time_double; if (time_double < time_float) { ++n_double_faster; } else if (time_float < time_double) { ++n_float_faster; } else { ++n_draw; } const int sum = n_double_faster + n_float_faster + n_draw; if (sum == 10000) break; if (sum % 100 == 0) { std::cout << "Draw " << n_draw << " times. " << '\n' << "Float has been faster " << n_float_faster << " times. " << "Sum time: " << sum_time_float << '\n' << "Double has been faster " << n_double_faster << " times. " << "Sum time: " << sum_time_double << std::endl; } } }

Results

Version 1.0

On a Windows XP (built in around 2005) computer, using version 1.0:

`Draw 0 times. Float has been faster 465 times. Sum time: 385.621 Double has been faster 106 times. Sum time: 439.11`

Version 2.0

On a laptop (built in around 2011) with Lubuntu using version 2.0:

`Draw 0 times. Float has been faster 8702 times. Sum time: 102.81 Double has been faster 1198 times. Sum time: 101.54`

On the same laptop (built in around 2011) with Lubuntu using Wine using version 2.0:

`Draw 0 times. Float has been faster 7406 times. Sum time: 86.62 Double has been faster 994 times. Sum time: 86.45`

This result has been taken from this screenshot (notice that I used the top-left one, as the program was running four times).

Version 3.0

On a laptop (built in around 2011)(same as used in the version 2.0 test) with Lubuntu using version 3.0:

`Draw 3 times. Float has been faster 570 times. Sum time: 217.33 Double has been faster 627 times. Sum time: 210.45`

On the same laptop (built in around 2011) with Lubuntu using Wine using version 3.0:

`Draw 7 times. Float has been faster 247 times. Sum time: 142.88 Double has been faster 546 times. Sum time: 120.65`

This result has been taken from this screenshot (notice that I used the top-left one, as the program was running four times).

References

Bjarne Stroustrup. The C++ Programming Language (3rd edition). 1997. ISBN: 0-201-88954-4. Item 4.10.15: 'Prefer a double over a float or a long double.'