[Numpy-discussion] masked index surprise

Keith Goodman kwgoodman@gmail....
Fri Aug 14 14:20:33 CDT 2009


On Fri, Aug 14, 2009 at 11:52 AM, Robert Kern<robert.kern@gmail.com> wrote:
> On Fri, Aug 14, 2009 at 13:05, John Hunter<jdh2358@gmail.com> wrote:
>> I just tracked down a subtle bug in my code, which is equivalent to
>>
>>
>> In [64]: x, y = np.random.rand(2, n)
>>
>> In [65]: z = np.zeros_like(x)
>>
>> In [66]: mask = x>0.5
>>
>> In [67]: z[mask] = x/y
>>
>>
>>
>> I meant to write
>>
>>  z[mask] = x[mask]/y[mask]
>>
>> so I can fix my code, but why is line 67 allowed
>>
>>  In [68]: z[mask].shape
>>  Out[68]: (54,)
>>
>>  In [69]: (x/y).shape
>>  Out[69]: (100,)
>>
>> it seems like broadcasting would fail
>
> Broadcasting doesn't take place with boolean masks. Instead, the
> values repeat if there are too few and extra values are ignored.
> Boolean indexing derives from Numeric's putmask() implementation,
> which had these semantics, rather than other forms of indexing.
>
> You may consider this a wart or a bad design decision (and I would
> probably agree), but it is not a bug.

Are the last two, x[[1]] and x[np.array([1])], broadcasting?

>> x = np.array([1,2,3])
>> x[1] = np.array([4,5,6])
ValueError: setting an array element with a sequence.
>> x[(1,)] = np.array([4,5,6])
ValueError: array dimensions are not compatible for copy
>> x[[1]] = np.array([4,5,6])
>> x
   array([1, 4, 3])
>> x[np.array([1])] = np.array([4,5,6])
>> x
   array([1, 4, 3])


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