[SciPy-User] <transferred from Scipy-Dev>Re: Seeking help/ advice for applying functions

eat e.antero.tammi@gmail....
Tue Mar 9 15:53:35 CST 2010


Anne Archibald <peridot.faceted <at> gmail.com> writes:

> Not especially. They are compiled to bytecode, whose execution is not
> particularly fast. But the big problem is all the baggage of python's
> nature as a dynamic language: for example each value is allocated with
> malloc() and contains type information; for another example, each
> access to a list involves identifying that the object really is a
> list, dispatching to the list-lookup function, determining the type of
> the argument (integer, slice object, long integer, other), and bounds
> checking before finally returning the list element. Thus even tools
> like cython that let you write, effectively, python code that gets
> compiled to machine code are not much faster unless you can turn off
> the dynamic features of python (which cython lets you do, selectively;
> it's great).

Good to know, I'll better postpone studying cython while I'm still
learning 'the very basics' of Scipy/ Numpy.

> Think about whether you can write each 'base' function to take arrays
> as arguments, for example:
> 
> def F(x, mu, sigma):
>     return np.exp(-((x-mu)/sigma)**2)
> 
> If your current code does something like
> 
> fis = [lambda x: F(x, mui, sigmai) for (mui, sigmai) in zip(muis, sigmais)]
> 
> r = [f(7) for f in fis]
> 
> you can rewrite it as the single line
> 
> r = F(7, muis, sigmais)
> 
> (if muis and sigmais are numpy arrays). Now you have just a couple of
> lines of python, and the heavy lifting all happens inside numpy loops.

Good quidelines in general to drive ones design!

> If you have several different functions, look into separating your
> input arrays based on which function needs to be applied to them;
                                  
Hmm, but I think that's what not possible in my particular case! That's
depends on what happens externally. I don't have any control of it.

> remember numpy lets you select out all the elements of an array
> meeting a criterion.
                                  
As far as I understod, select and where gives you only the first one.
In my case I need all of them.

> I realize this kind of rewriting will mess up a nice clean functional
> design, but as is often the case, that is the price you pay for
> performance. If your code works and is fast enough, I recommend
> leaving it as is.
                                  
True, I'll better figure out my toughts more clearly in higher level and
worry the performance later (if ever).
                                  
 
> Anne

Thanks a lot,
eat







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