[Numpy-discussion] Putmask/take ?

[Stefan van der Walt]

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
Stéfan