On Fri, Sep 22, 2006 at 02:17:57AM -0500, Robert Kern wrote:

Stefan van der Walt wrote:

Hi P.,

On Thu, Sep 21, 2006 at 07:40:39PM -0400, PGM wrote:

I'm running into the following problem with putmask on take.

import numpy x = N.arange(12.) m = [1, 0, 0, 0, 0, 0, 1, 0, 0, 1, 0, 1] i = N.nonzero(m)[0] w = N.array([-1, -2, -3, -4.]) x.putmask(w,m) x.take(i) N.allclose(x.take(i),w)

According to the putmask docstring:

`a.putmask(values, mask) sets a.flat[n] = v[n] for each n where mask.flat[n] is true. v can be scalar.`

This would mean that 'w' is not of the right length.

There are 4 true values in m and 4 values in w. What's the wrong

length?

The way I read the docstring, you use putmask like this:

In [4]: x = N.array([1,2,3,4])

In [5]: x.putmask([4,3,2,1],[1,0,0,1])

In [6]: x Out[6]: array([4, 2, 3, 1])

For the sake of clarity:

In [1]: from numpy import *

In [3]: m = [1, 0, 0, 0, 0, 0, 1, 0, 0, 1, 0, 1]

In [4]: i = nonzero(m)[0]

In [5]: i Out[5]: array([ 0, 6, 9, 11])

In [6]: w = array([-1, -2, -3, -4.])

In [7]: x = arange(12.)

In [8]: x.putmask(w, m)

In [9]: x Out[9]: array([ -1., 1., 2., 3., 4., 5., -3., 7., 8., -2., 10., -4.])

In [17]: x[array(m, dtype=bool)] = w

In [18]: x Out[18]: array([ -1., 1., 2., 3., 4., 5., -2., 7., 8., -3., 10., -4.])

Out[9] and Out[18] should have been the same, but elements 6 and 9 are flipped. It's pretty clear that this is a bug in .putmask().

Based purely on what I read in the docstring, I would expect the above to do

x[0] = w[0] x[6] = w[6] x[9] = w[9] x[11] = w[11]

Since w is of length 4, you'll probably get indices modulo 4:

w[6] == w[2] == -3 w[9] == w[1] == -2 w[11] == w[3] == -4

Which seems to explain what you are seeing.

Regards StÃ©fan