calculating on matrix indices

[Paul Probert]

Brian Blais wrote:
> Hello,
> 
> In my attempt to learn python, migrating from matlab, I have the following problem. 
> Here is what I want to do, (with the wrong syntax):
> 
> from numpy import *
> 
> t=arange(0,20,.1)
> x=zeros(len(t),'f')
> 
> idx=(t>5)
> tau=5
> x[idx]=exp(-t[idx]/tau)  # <---this line is wrong (gives a TypeError)
> 
> #------------------
> 
> what is the best way to replace the wrong line with something that works: replace all 
> of the values of x at the indices idx with exp(-t/tau) for values of t at indices idx?
> 
> I do this all the time in matlab scripts, but I don't know that the pythonic 
> preferred method is.
> 
Brian,
First, let me warn you I only know the old Numeric, and I haven't made 
the jump to Numpy yet. I'm told the syntax is about the same though.
Using nonzero, put, and take, I can do what you want like this:

import Numeric as num
t=num.arange(0,20,.1)
x=num.zeros(len(t),'f')
idx=num.nonzero(t>5)
tau=5.
num.put(x,idx,num.exp(-num.take(t,idx)/tau))

Kinda messy compared to Matlab. Do you know about Octave 
(www.octave.org)? It tries to be compatible with Matlab syntax. Of 
course it lacks the other niceties of python.

Paul Probert