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

Drew Frank drewfrank@gmail....
Thu Feb 9 13:20:11 CST 2012


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?





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