[Numpy-discussion] swap elements in two arrays

[Alan G Isaac]

On 2/7/2010 11:16 AM, Keith Goodman wrote:
> Bad:
  
> >>>  a = np.array([1, 2, 3])
> >>>  b = a[1:]
> >>>
> >>>  a[to_swap], b[to_swap] = b[to_swap], a[to_swap]
> >>>
> >>>  a
>     array([2, 1, 2])
> >>>  b
>     array([1, 2])



So that is an important point:
if `a` and `b` share data, the
"swap" is not well defined.

But that affects the alternative idiom as well:

>>> to_swap
array([ True,  True], dtype=bool)
>>> a = np.array([1, 2, 3])
>>> b = a[1:]
>>> temp = a.copy()
>>> a[to_swap] = b[to_swap]
>>> b[to_swap] = temp
>>> a, b
(array([2, 1, 2]), array([1, 2]))

Thanks,
Alan