[Numpy-discussion] Re: calculating on matrix indices

Travis Oliphant oliphant.travis at ieee.org
Thu Feb 16 19:02:02 EST 2006

Brian Blais wrote:

> Colin J. Williams wrote:
>> Brian Blais wrote:
>>> 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)                # <---this produces a Boolean array, 
>>> probably not what you want.
>>> tau=5
>>> x[idx]=exp(-t[idx]/tau)  # <---this line is wrong (gives a TypeError)
>> What are you trying to do?  It is most unlikely that you need Boolean 
>> values in x[idx]
> in this example, as in many that I would do in matlab, I want to 
> replace part of a vector with values from another vector.  In this 
> case, I want x to be zero from t=0 to 5, and then have a value of 
> exp(-t/tau) for t>5.  I could do it with an explicit for-loop, but 
> that would be both inefficient and unpython-like.  For those who know 
> matlab, what I am doing here is:
from numpy import *

t = r_[0:20:0.1]
idx = t>5
tau = 5
x = zeros_like(t)
x[idx] = exp(-t[idx]/tau)

Should do it.


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