[Numpy-discussion] using the functions nonzero and where

Timothy Hochberg tim.hochberg@ieee....
Tue Apr 24 12:22:35 CDT 2007


On 4/20/07, Dennis Cooke <dennis.a.cooke@gmail.com> wrote:
>
> I'm an ex-Matlab user trying to come up to speed with python and numpy.
> I'm confused on how to use the Numpy functions nonzero() and where().  In
> matlab, the equivalent function (find(a>0) ) would return an array, whereas
> in numpy, where() or nonzero() will return a single element tuple.  For
> example:
>
> In [35]: x = [ 0, 0, 0, 99, 0, 1, 5]
> # and I wish to file the index of the first non-zero element of x - which
> is 3.
>
> In [36]: from numpy import nonzero
> In [37]: i=nonzero(x)
> In [38]: i
> Out[38]: (array([3, 5, 6]),)
> In [39]: i[0]
> Out[39]: array([3, 5, 6])
>
> so nonzero() has output a tuple with a single element = the string
> 'array([3, 5, 6])'


What makes you think that this is a string? It is in fact the array that you
seek:

>>> from numpy import nonzero
>>> x = [ 0, 0, 0, 99, 0, 1, 5]
>>> i = nonzero(x)
>>> i
(array([3, 5, 6]),)
>>> i[0]
array([3, 5, 6])
>>> type(i[0])
<type 'numpy.ndarray'>
>>> i[0][0]
3


How do I convert this string (or the original tuple)  into an array where
> the first element = 3 (the index of the first non-zero element of my
> original array x).


I fairly certain that you just want i[0].

-tim



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-- 

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tim.hochberg@ieee.org
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