CODEWITHSUNDEEP
c++mathboostrounding
Bing
Bing

Reputation: 83

boost::format gives different results than round

I would like to understand why 

double nbr = 102262.5;

boost::format("%.0f") % nbr;

gives 102262 while round(102262.5) gives 102263.0

If we take another example value 34087.5

boost::format("%.0f") % nbr

gives 34088 and round(34087.5) gives the same 34088

Does it mean round implements a more sophisticated algorithm to ensure the nearest while format or printf does not?

Upvotes: 4

Views: 410

Answers (2)

Michael Aramini
Michael Aramini

Reputation: 185

BTW, if you want round half to even behavior in C++ without relying on Boost, you could call the C++ standard library function std::rint which is available if your compiler supports C++11 (or later) C++ standard.

If you're using a pre-C++11 compiler which supports calling C standard library functions, you could call one of the C standard library functions rintf, rint, or rintl, depending on which floating-point type you are using.

Upvotes: 1

Andreas H.
Andreas H.

Reputation: 6105

There is a thing called "round half to even" or "round half to odd" (link).

This is a rule to reduce the bias of rounding errors and boost::format seems to implement such a strategy. Essentially this is to round the tie-breaking cases (such as 1.5 or 2.5) up and down equally often in a deterministic way (depending on the number itself). If one would always round up or round down these cases all, a statistical bias could be introduced by rounding. The latter is the more "classical" way of rounding which seems to be implemented by round.

Note that the strategy implemented by boost::format (round half to even) corresponds to the default rounding mode in the IEEE 754 standard.

Upvotes: 8

Related Questions