[Numpy-discussion] Putmask/take ?

[Robert Kern]

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?

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().

-- 
Robert Kern

"I have come to believe that the whole world is an enigma, a harmless enigma
  that is made terrible by our own mad attempt to interpret it as though it had
  an underlying truth."
   -- Umberto Eco