[Numpy-discussion] step paramter for linspace

[Warren Weckesser]

On 3/1/13, Henry Gomersall <heng at cantab.net> wrote:
> On Fri, 2013-03-01 at 13:34 +0000, Nathaniel Smith wrote:
>> > My usual hack to deal with the numerical bounds issue is to
>> add/subtract
>> > half the step.
>>
>> Right. Which is exactly the sort of annoying, content-free code that a
>> library is supposed to handle for you, so you can save mental energy
>> for more important things :-).
>
> I agree with the sentiment (I sometimes wish a library could read my
> mind ;) but putting this sort of logic into the library seems dangerous
> to me.
>
> The point is that the coder _should_ understand the subtleties of
> floating point numbers. IMO arange _should_ be well specified and
> actually operate on the half open interval; continuing to add a step
> until >= the limit is clear and always unambiguous.
>
> Unfortunately, the docs tell me that this isn't the case:
> "For floating point arguments, the length of the result is
>  ``ceil((stop - start)/step)``.  Because of floating point overflow,
>  this rule may result in the last element of `out` being greater
>  than `stop`."
>
> In my jet-lag addled state, i can't see when this out[-1] > stop case
> will occur, but I can take it as true. It does seem to be problematic
> though.


Here you go:

In [32]: end = 2.2

In [33]: x = arange(0.1, end, 0.3)

In [34]: x[-1]
Out[34]: 2.2000000000000006

In [35]: x[-1] > end
Out[35]: True



Warren



>
> As soon as you allow freeform setting of the stop value, problems are
> going to be encountered. Who's to say that the stop - delta is actually
> _meant_ to be below the limit, or is meant to be the limit? Certainly
> not the library!
>
> It just seems to me that this will lead to lots of bad code in which the
> writer has glossed over an ambiguous case.
>
> Henry
>
>
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