[Numpy-discussion] step paramter for linspace
Fri Mar 1 08:24:57 CST 2013
On 3/1/13, Henry Gomersall <email@example.com> 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
>> > 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
Here you go:
In : end = 2.2
In : x = arange(0.1, end, 0.3)
In : x[-1]
In : x[-1] > end
> 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.
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