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devel / comp.lang.python / Re: Unexpected behaviour of math.floor, round and int functions (rounding)

SubjectAuthor
* Unexpected behaviour of math.floor, round and int functions (rounding)René Silva Valdés
`- Re: Unexpected behaviour of math.floor, round and int functions (rounding)Julio Di Egidio

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Unexpected behaviour of math.floor, round and int functions (rounding)

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From: rene.sil...@gmail.com (René Silva Valdés)
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Subject: Unexpected behaviour of math.floor, round and int functions (rounding)
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 by: René Silva Valdés - Fri, 19 Nov 2021 02:16 UTC

Hello, I would like to report the following issue:

Working with floats i noticed that:

int(23.99999999999999/12) returns 1, and
int(23.999999999999999/12) returns 2

This implies that int() function is rounding, which doesn't appear to be
expected (documentation doesn't say anything about it). Looking further i
noticed that

0.5+0.49999999999999994 returns 1. This seems to be related to double
numbers' operations in C language, where 0.49999999999999994 is the
greatest floating-point value less than 0.5. Counting on this several
examples can be deduced, like:

round(0+0.49999999999999994) returns 0, and
round(1+0.49999999999999994) returns 2

This seems to be a known issue in Java (see reference)

Reference:

https://bugs.java.com/bugdatabase/view_bug.do?bug_id=6430675

I hope this information is helpful, thanks in advance for reading this,

Kind regards,

René

Re: Unexpected behaviour of math.floor, round and int functions (rounding)

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Subject: Re: Unexpected behaviour of math.floor, round and int functions (rounding)
From: jul...@diegidio.name (Julio Di Egidio)
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 by: Julio Di Egidio - Fri, 19 Nov 2021 07:05 UTC

On Friday, 19 November 2021 at 03:30:51 UTC+1, René Silva Valdés wrote:

> Hello, I would like to report the following issue:
> This seems to be related to double numbers' [...]

What every *programmer* should know about floating point:

<https://docs.oracle.com/cd/E19957-01/806-3568/ncg_goldberg.html>

Yes, it's definitely a known issue... ;)

HTH,

Julio

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