[SciPy-user] Help with array functions

Bob.Cowdery at CGI-Europe.com Bob.Cowdery at CGI-Europe.com
Tue May 4 10:36:16 CDT 2004


Eric Jones wrote:
> Bob.Cowdery at CGI-Europe.com wrote:
> 
> The general ideas of uisng Numeric in Python is to avoid loops, even 
> at
> the expense of sometimes doing extra computations.  In this case, the 
> signal processing calculations can be done for every entry in the array 
> (except the first).  The conditions checked for in the "if" statements 
> can then be filtered out in subsequent array processing steps.
> 
> 
> Here is some (totally untested) code that should give you some ideas:
> 
>     gain = self.m_procGain
>     abs_complexout = abs(self.m_complexout)
> 
>     # Do singal processing calculation for all elements in array
>     gain[1:] = ( (gain[:-1] * (1 - attack)) +
>                  ((attack * peak) / abs_complexout[:-1]) )
>

Perry Greenfield wrote: 
> Does this do what is needed? I think the point he was making was that
gain[i] depends on gain[i-1] which in turn depends on gain[i-2] 
> and so on. The expression above doesn't capture that since from his code
he doesn't show any initial value for gain (implicitly the 
> gain[0] must exist). So using an array expression which uses a shifted
gain array in its input won't work. 

> This is the sort of problem that doesn't fit the array model very well
unless there are some primitive array operations that happen to > do the
iterative thing needed (sometimes the accumulate or convolve functions can
be used for this thing, but I can't see any obvious 
> existing function. Seems like a candidate for a C extension but maybe I'm
missing 
> something.

That is correct, my basic problem was how to use the same array in input and
output which I guess I should have mentioned. The gain array is initialised
to zeros.

Bob
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