Re: [Numpy-discussion] Getting indices from numpy array with condition

2008/11/19 Robert Kern :

On Wed, Nov 19, 2008 at 01:31, David Warde-Farley wrote:

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On 18-Nov-08, at 3:06 PM, Robert Kern wrote:

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I like to discourage this use of where(). For some reason, back in Numeric's days, where() got stuck with two functionalities. nonzero() is the preferred function for this functionality. IMO, where(cond, if_true, if_false) should be the only use of where().

Hmm. nonzero() seems semantically awkward to be calling on a boolean array, n'est pas?

Why? In Python and numpy, False==0 and True==1.

Well, using nonzero() isn't actually all that obvious until you understand that 1) a conditional expression like (a < 3) returns a boolean array *and* 2) that False==0 and True==1. These two things are not necessarily known to users who are scientists/engineers etc. I've added an example of this use case at http://docs.scipy.org/numpy/docs/numpy.core.fromnumeric.nonzero/ and added an FAQ http://www.scipy.org/FAQ , so that there's something to point to in future. Cheers, Scott