calculation bug in long() function

[must21]

BPO	2140
Nosy	@facundobatista

^{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 2008-02-18.12:25:04.698>
created_at = <Date 2008-02-18.12:18:06.057>
labels = ['type-bug', 'invalid']
title = 'calculation bug in long() function'
updated_at = <Date 2008-02-18.12:55:36.779>
user = 'https://bugs.python.org/must21'

bugs.python.org fields:

activity = <Date 2008-02-18.12:55:36.779>
actor = 'must21'
assignee = 'none'
closed = True
closed_date = <Date 2008-02-18.12:25:04.698>
closer = 'facundobatista'
components = ['None']
creation = <Date 2008-02-18.12:18:06.057>
creator = 'must21'
dependencies = []
files = []
hgrepos = []
issue_num = 2140
keywords = []
message_count = 3.0
messages = ['62523', '62524', '62526']
nosy_count = 2.0
nosy_names = ['facundobatista', 'must21']
pr_nums = []
priority = 'normal'
resolution = 'not a bug'
stage = None
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue2140'
versions = ['Python 2.5', 'Python 2.4', 'Python 2.3', 'Python 2.2.3', 'Python 2.2.2', 'Python 2.2.1', 'Python 2.2', 'Python 2.1.2', 'Python 2.1.1']

[must21]

betrag = 146.95
betrag = float(betrag)
betrag = betrag * 100.0
betrag = long(betrag)
print betrag

gives 14694 as result rather than 14695

[facundobatista]

In short:

>>> long(100 * 146.95)
14694L

This is NOT a bug, but a behaviour of binary floating point:

>>> 146.95
146.94999999999999

In binary you can not express this number exactly.

>>> 146.95 * 100
14694.999999999998

When you long() that, you truncate the number, so it goes to 14694.

Please address further discussion through python-list.

Thanks!

[must21]

thank you for fast reply

I see the point and do not want to argue about that (rounding is fine
for me). I just think this behaviour makes the long() function pretty
much useless.

Regards, Markus
Am 18.02.2008 um 13:25 schrieb Facundo Batista:

Facundo Batista added the comment:

In short:

>>> long(100 * 146.95)
14694L

This is NOT a bug, but a behaviour of binary floating point:

>>> 146.95
146.94999999999999

In binary you can not express this number exactly.

>>> 146.95 * 100
14694.999999999998

When you long() that, you truncate the number, so it goes to 14694.

Please address further discussion through python-list.

Thanks!

----------
nosy: +facundobatista
resolution: -> invalid
status: open -> closed

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