Qt: Roundoff of 12.625 by 2 returns 12.62

Question

I can't get it why the ff code returns 12.62 instead of 12.63

 QDebug << QString::number(12.625, 'f', 2);

and also what will be the solution for this? Currently my solution is this

QDebug << QString::number(12.625 + 0.0001, 'f', 2);

and it will return 12.63.

btw my OS is ubuntu 11.04

Answer 1

GCC uses "Round to Even" or "Banker's Rounding" as it's default mode while Visual Studio uses "Round away from Zero"

From the GCC docs: Rounding

This is the default mode. It should be used unless there is a specific need for one of the others. In this mode results are rounded to the nearest representable value. If the result is midway between two representable values, the even representable is chosen. Even here means the lowest-order bit is zero. This rounding mode prevents statistical bias and guarantees numeric stability: round-off errors in a lengthy calculation will remain smaller than half of FLT_EPSILON.

To change the rounding mode, use fesetround(FE_UPWARD) to round upwards. If you aren't using GCC, your compiler will have a similar option.

Rick Regan provides a list of programming environments and their default rounding behaviours. Inconsistent Rounding

Answer 2

There is an open bug report for this:

https://bugreports.qt-project.org/browse/QTBUG-38171