round() for float in C++


I need a simple floating point rounding function, thus:

double round(double);

round(0.1) = 0
round(-0.1) = 0
round(-0.9) = -1

I can find ceil() and floor() in the math.h - but not round().

Is it present in the standard C++ library under another name, or is it missing??

3/16/2017 1:55:02 AM

Accepted Answer

There's no round() in the C++98 standard library. You can write one yourself though. The following is an implementation of round-half-up:

double round(double d)
  return floor(d + 0.5);

The probable reason there is no round function in the C++98 standard library is that it can in fact be implemented in different ways. The above is one common way but there are others such as round-to-even, which is less biased and generally better if you're going to do a lot of rounding; it's a bit more complex to implement though.

9/27/2017 9:16:53 AM

Boost offers a simple set of rounding functions.

#include <boost/math/special_functions/round.hpp>

double a = boost::math::round(1.5); // Yields 2.0
int b = boost::math::iround(1.5); // Yields 2 as an integer

For more information, see the Boost documentation.

Edit: Since C++11, there are std::round, std::lround, and std::llround.

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