[Numpy-discussion] arange and floating point arguments

[lorenzo bolla]

this is really annoying.
Matlab handles the "ceil" weirdness quite well, though.

--------------------------------------------------------------

>> ceil(0.6/0.1)

ans =

     6

>> ceil((0.4+0.2)/0.1)

ans =

     7

>> 0:0.1:0.6

ans =

                         0    1.000000000000000e-001
2.000000000000000e-001    3.000000000000000e-001    4.000000000000000e-001
5.000000000000000e-001    6.000000000000000e-001

>> 0:0.1:(0.4+0.2)

ans =

                         0    1.000000000000000e-001
2.000000000000000e-001    3.000000000000000e-001    4.000000000000001e-001
5.000000000000001e-001    6.000000000000001e-001


>> length(0:0.1:0.6) == length(0:0.1:(0.4+0.2))

ans =

     1

--------------------------------------------------------------

hth,

L.



On 9/14/07, Ed Schofield <edschofield at gmail.com> wrote:
>
> Hi everyone,
>
> This was reported yesterday as a bug in Debian's numpy package:
>
> >>> len(numpy.arange(0, 0.6, 0.1)) == len(numpy.arange(0, 0.4+0.2, 0.1))
> False
>
> The cause is this:
>
> >>> ceil((0.4+0.2)/0.1)
> 7.0
>
> >>> ceil(0.6/0.1)
> 6.0
>
> which holds for both numpy's and the standard library's ceil().
>
> Using arange in this way is a fundamentally unreliable thing to do,
> but is there anything we want to do about this? Should numpy emit a
> warning when using arange with floating point values when
> (stop-start)/step is close to an integer?
>
> -- Ed
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