[Numpy-discussion] numpy.arange() error?

eat e.antero.tammi@gmail....
Thu Feb 9 15:44:01 CST 2012


Hi,

On Thu, Feb 9, 2012 at 9:47 PM, Eric Firing <efiring@hawaii.edu> wrote:

> On 02/09/2012 09:20 AM, Drew Frank wrote:
> > Eric Firing<efiring<at>  hawaii.edu>  writes:
> >
> >>
> >> On 02/08/2012 09:31 PM, teomat wrote:
> >>>
> >>> Hi,
> >>>
> >>> Am I wrong or the numpy.arange() function is not correct 100%?
> >>>
> >>> Try to do this:
> >>>
> >>> In [7]: len(np.arange(3.1, 4.9, 0.1))
> >>> Out[7]: 18
> >>>
> >>> In [8]: len(np.arange(8.1, 9.9, 0.1))
> >>> Out[8]: 19
> >>>
> >>> I would expect the same result for each command.
> >>
> >> Not after more experience with the wonders of floating point!
> >> Nice-looking decimal numbers often have long, drawn-out, inexact
> >> floating point (base 2) representations.  That leads to exactly this
> >> sort of problem.
> >>
> >> numpy.linspace is provided to help get around some of these surprises;
> >> or you can use an integer sequence and then scale and shift it.
> >>
> >> Eric
> >>
> >>>
> >>> All the best
> >>>
> >>>
> >>
> > I also found this surprising -- not because I lack experience with
> floating
> > point, but because I do have experience with MATLAB.  In MATLAB, the
> > corresponding operation 3.1:0.1:4.9 has length 19 because of an explicit
> > tolerance parameter used in the implmentation
> > (
> http://www.mathworks.com/support/solutions/en/data/1-4FLI96/index.html?solution=1-4FLI96
> ).
> >
> > Of course, NumPy is not MATLAB :).  That said, I prefer the MATLAB
> behavior in
> > this case -- even though it has a bit of a "magic" feel to it, I find it
> hard to
> > imagine code that operates correctly given the Python semantics and
> incorrectly
> > under MATLAB's.  Thoughts?
>
> You raise a good point.  Neither arange nor linspace provides a close
> equivalent to the nice behavior of the Matlab colon, even though that is
> often what one really wants.  Adding this, either via an arange kwarg, a
> linspace kwarg, or a new function, seems like a good idea.
>
Maybe this issue is raised also earlier, but wouldn't it be more consistent
to let arange operate only with integers (like Python's range) and let
linspace handle the floats as well?


My 2 cents,
eat

>
> Eric
>
> >
> >
> >
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