[Numpy-discussion] Re: calculating on matrix indices

Brian Blais bblais at bryant.edu
Thu Feb 16 18:47:03 CST 2006

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:

t=0:.1:20;
idx=find(t>5);
tau=5;

x=zeros(size(t));
x(idx)=exp(-t(idx)/tau)

is that clearer?  I am sure there is a nice method to do this in python, but I
haven't found it in the python or numpy docs.

thanks,

bb

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