[Numpy-discussion] numpy.arange() error?
Thu Feb 9 13:47:51 CST 2012
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:
>>> Am I wrong or the numpy.arange() function is not correct 100%?
>>> Try to do this:
>>> In : len(np.arange(3.1, 4.9, 0.1))
>>> Out: 18
>>> In : len(np.arange(8.1, 9.9, 0.1))
>>> Out: 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.
>>> 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
> 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.
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