Why is rounding 0.5 (decimal) not exact?

Question

One half, i.e. 0.5 in decimal, has an exact binary representation: 0.1

Nevertheless, if I round it to integer, I get 0 instead of 1. I tried in Python and C, which behave the same. E.g. the python code:

>>> a,b,c = 0.49, 0.5, 0.51
>>> [round(x) for x in (a,b,c)]
[0, 0, 1]
>>> "%.0f %.0f %.0f" % (a,b,c)
'0 0 1'

Interestingly,

>>> a,b,c = 0.049, 0.05, 0.051
>>> [round(x,1) for x in (a,b,c)]
[0.0, 0.1, 0.1]
>>> "%.1f %.1f %.1f" % (a,b,c)
'0.0 0.1 0.1'

If a number has an exact binary representation, such as (decimal) 0.5, shouldn't it be rounded correctly?

edit: The issue occurs in version 3.4.3, but not in version 2.7.6

I don't get those results - I see 0, 1, 1. What version of Python are you using, and on what platform? — Jon Skeet, Jun 29 '16 at 12:49
Note that while `0.5` has an exact binary representation `0.05` does not — piet.t, Jun 29 '16 at 12:52
Question has already been answered: http://stackoverflow.com/questions/10825926/python-3-x-rounding-behavior — HelloWorld, Jun 29 '16 at 12:52

score 9 · Accepted Answer · edited May 23 '17 at 12:22

9

I know they changed the round method in python 3.

So, under v2.7.3:

In [85]: round(2.5)
Out[85]: 3.0

In [86]: round(3.5)
Out[86]: 4.0

under v3.2.3:

In [32]: round(2.5)
Out[32]: 2

In [33]: round(3.5)
Out[33]: 4

I don't know if it helps you but I am posting it as answer since I can't comment due to my low reputation.

The question is answered more properly here : Python 3.x rounding behavior

edited May 23 '17 at 12:22

Community

answered Jun 29 '16 at 12:50

Sorry, this doesn't answer the question – Ishay Peled Jun 29 '16 at 12:51
2

Good answer. See http://stackoverflow.com/questions/10825926/python-3-x-rounding-behavior for more information. – mtrw Jun 29 '16 at 12:53

1 Answers1