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Different division results with / and // operators with large numbers #62021

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PhilippeRouquier mannequin opened this issue Apr 23, 2013 · 5 comments
Closed

Different division results with / and // operators with large numbers #62021

PhilippeRouquier mannequin opened this issue Apr 23, 2013 · 5 comments
Labels
interpreter-core (Objects, Python, Grammar, and Parser dirs) type-bug An unexpected behavior, bug, or error

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@PhilippeRouquier
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Mannequin

PhilippeRouquier mannequin commented Apr 23, 2013

BPO 17821
Nosy @mdickinson

Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.

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GitHub fields:

assignee = None
closed_at = <Date 2013-04-23.11:47:14.010>
created_at = <Date 2013-04-23.09:19:59.016>
labels = ['interpreter-core', 'type-bug', 'invalid']
title = 'Different division results with / and // operators with large numbers'
updated_at = <Date 2013-04-23.11:47:14.006>
user = 'https://bugs.python.org/PhilippeRouquier'

bugs.python.org fields:

activity = <Date 2013-04-23.11:47:14.006>
actor = 'sbt'
assignee = 'none'
closed = True
closed_date = <Date 2013-04-23.11:47:14.010>
closer = 'sbt'
components = ['Interpreter Core']
creation = <Date 2013-04-23.09:19:59.016>
creator = 'Philippe.Rouquier'
dependencies = []
files = []
hgrepos = []
issue_num = 17821
keywords = []
message_count = 5.0
messages = ['187623', '187624', '187625', '187626', '187628']
nosy_count = 3.0
nosy_names = ['mark.dickinson', 'sbt', 'Philippe.Rouquier']
pr_nums = []
priority = 'normal'
resolution = 'not a bug'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue17821'
versions = ['Python 3.3']
@PhilippeRouquier
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PhilippeRouquier mannequin commented Apr 23, 2013

Hi,

the following statement yields different results in python2 and python3:
284397269195572115652769428988866694680//17 - int(284397269195572115652769428988866694680/17)

In python3 it yields:
309657313492949847071

In python2 it yields:
OL

Python2's result seems to be correct as (284397269195572115652769428988866694680//17) and (int(284397269195572115652769428988866694680/17)) should return the same result (as far as I understand).

With smaller numbers, this difference in results does not appear with python3.

Note: I noticed this, while working on RSA; 284397269195572115652769428988866694680 is (p-1)(q-1) and 17 is e. I just mention this in case it could help.

I used linux version 3.3.3.0 and 2.7.3 for the tests on a 64 bits processor.

Sorry if I am missing something here.

@PhilippeRouquier PhilippeRouquier mannequin added interpreter-core (Objects, Python, Grammar, and Parser dirs) type-bug An unexpected behavior, bug, or error labels Apr 23, 2013
@sbt
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sbt mannequin commented Apr 23, 2013

In Python 3 "/" gives float division, whereas in Python 2 it is integer division, the same as "//".

@sbt
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sbt mannequin commented Apr 23, 2013

Just to clarify, if you use float division then you get rounding errors. 309657313492949847071 is a rounding error:

>>> x = 284397269195572115652769428988866694680//17
>>> x - int(float(x))
309657313492949847071L

@PhilippeRouquier
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PhilippeRouquier mannequin commented Apr 23, 2013

Does your comment mean that this is bug should be closed as notabug since anyone wanting to avoid such rounding error should use // operator?

@sbt
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sbt mannequin commented Apr 23, 2013

Yes.

@sbt sbt mannequin closed this as completed Apr 23, 2013
@sbt sbt mannequin added the invalid label Apr 23, 2013
@ezio-melotti ezio-melotti transferred this issue from another repository Apr 10, 2022
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