On Thu, Feb 9, 2012 at 2:47 PM, Eric Firing <efir...@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. > > Eric > > On a related note: would an `endpoint` parameter for `arange` be desirable? For example, `linspace` already has this as a parameter. When using `arange` with int values, I often want the endpoint. Of course, adding 1 is easy to do (and shorter), but in some ways it's less readable (are you adding 1 because the variable itself is off by 1 or because of the fact the limit is exclusive; sometimes I end up adding 2 because both are true). It's a subtle difference, I know. And it's debatable whether `endpoint` would be more readable (though I think it is). This is somewhat tangential to how `arange` behaves with floats, in which the second limit sometimes *appears* inclusive and sometimes *appears* exclusive. I never use `arange` with floats because of this unpredictably, so I like the idea of adding a tolerance. -Tony
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