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Title: The built-in round() function giving a wrong output
Type: behavior Stage: resolved
Components: Versions: Python 3.6
Status: closed Resolution: not a bug
Dependencies: Superseder:
Assigned To: Nosy List: Goodester, SilentGhost, remi.lapeyre, xtreak
Priority: normal Keywords:

Created on 2019-02-22 17:02 by Goodester, last changed 2022-04-11 14:59 by admin. This issue is now closed.

Messages (4)
msg336329 - (view) Author: Juho Pesonen (Goodester) Date: 2019-02-22 17:02
As the title says I have got some wrong outputs with the round() built-in function.
The bug occurs only with certain numbers though. (I've had problems only with the number 2.5, but I assume that there is more than one number that gives wrong output.)
Here are some outputs with the round built-in function:


As you can see the number 2.5 gives 2 instead of 3.
msg336330 - (view) Author: Karthikeyan Singaravelan (xtreak) * (Python committer) Date: 2019-02-22 17:07

> The round() function rounding strategy and return type have changed. Exact halfway cases are now rounded to the nearest even result instead of away from zero. (For example, round(2.5) now returns 2 rather than 3.) round(x[, n]) now delegates to x.__round__([n]) instead of always returning a float. It generally returns an integer when called with a single argument and a value of the same type as x when called with two arguments.
msg336331 - (view) Author: SilentGhost (SilentGhost) * (Python triager) Date: 2019-02-22 17:08
This is the documented behaviour. It might seem counter-intuitive, but it's is not likely to change.
msg336333 - (view) Author: Rémi Lapeyre (remi.lapeyre) * Date: 2019-02-22 17:24
This is called banker rounding and is done on purpose to limit the accumulation of errors when summing a list of rounded integers. You can read and look at IEEE 754 for more information.
Date User Action Args
2022-04-11 14:59:11adminsetgithub: 80263
2019-02-22 17:24:47remi.lapeyresetnosy: + remi.lapeyre
messages: + msg336333
2019-02-22 17:08:55SilentGhostsetstatus: open -> closed

nosy: + SilentGhost
messages: + msg336331

resolution: not a bug
stage: resolved
2019-02-22 17:07:34xtreaksetnosy: + xtreak
messages: + msg336330
2019-02-22 17:02:23Goodestercreate