[Numpy-discussion] slow numpy.clip ?

Travis Oliphant oliphant at ee.byu.edu
Tue Dec 19 19:21:31 CST 2006


Robert Kern wrote:
> David Cournapeau wrote:
>
>   
>> Basically, at least from those figures, both versions are pretty 
>> similar, and not worth improving much anyway for matplotlib. There is 
>> something funny with numpy version, though.
>>     
>
> Looking at the code, it's certainly not surprising that the current
> implementation of clip() is slow. It is a direct numpy C API translation of the
> following (taken from numarray, but it is the same in Numeric):
>
>
> def clip(m, m_min, m_max):
>     """clip()  returns a new array with every entry in m that is less than m_min
>     replaced by m_min, and every entry greater than m_max replaced by m_max.
>     """
>     selector = ufunc.less(m, m_min)+2*ufunc.greater(m, m_max)
>     return choose(selector, (m, m_min, m_max))
>
>   

There are a lot of functions that are essentially this.   Many things 
were done to just get something working.  It would seem like a good idea 
to re-code many of these to speed them up.
> Creating that integer selector array is probably the most expensive part.
> Copying the array, then using putmask() or similar is certainly a better
> approach, and I can see no drawbacks to it.
>
> If anyone is up to translating their faster clip() into C, I'm more than happy
> to check it in. I might also entertain adding a copy=True keyword argument, but
> I'm not entirely certain we should be expanding the API during the 1.0.x series.
>
>   
The problem with the copy=True keyword is that it would imply needing to 
expand the C-API for PyArray_Clip and should not be done until 1.1 IMHO.

We would probably be better off not expanding the keyword arguments to 
methods as well until that time.

-Travis



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