[Numpy-discussion] swap elements in two arrays

Alan G Isaac aisaac@american....
Sun Feb 7 09:39:12 CST 2010

On 2/7/2010 10:21 AM, Alan Isaac wrote:
>> I have two 1d arrays, say `a` and `b`.
>> I need to swap elements if a 1d boolean criterion `to_swap` is met.
> [clip]
>> Here is a much faster way:
>>           a[to_swap], b[to_swap] = b[to_swap], a[to_swap]

On 2/7/2010 10:21 AM, Pauli Virtanen wrote:
> That doesn't necessarily work -- the above code expands to
> 	tmp = a[to_swap]
> 	a[to_swap] = b[to_swap]
> 	b[to_swap] = tmp
> It'll work provided `to_swap` is such that `tmp` is not a view on `a`...

I thought that if `to_swap` is a boolean array that `a[to_swap]`
will always own its own data.  Can that fail?


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