[Numpy-discussion] Multiple Boolean Operations

[Andrea Gavana]

Hi Peter & All,

On Fri, May 23, 2008 at 4:16 PM, Peter Creasey wrote:
> Hi Andrea,
>
> 2008/5/23  "Andrea Gavana" <andrea.gavana at gmail.com>:
>> And so on. The probelm with this approach is that I lose the original
>> indices for which I want all the inequality tests to succeed:
>
> To have the original indices you just need to re-index your indices, as it were
>
> idx = flatnonzero(xCent >= xMin)
> idx = idx[flatnonzero(xCent[idx] <= xMax)]
> idx = idx[flatnonzero(yCent[idx] >= yMin)]
> idx = idx[flatnonzero(yCent[idx] <= yMax)]
> ...
> (I haven't tested this code, apologies for bugs)
>
> However, there is a performance penalty for doing all this re-indexing
> (I once fell afoul of this), and if these conditions "mostly" evaluate
> to True you can often be better off with one of the solutions already
> suggested.

Thank you for your answer. I have tried your suggestion, and the
performances are more or less comparable with the other NumPy
implementations (yours is roughly 1.2 times slower than the others),
but I do gain some advantage when the subgrids are very small (i.e.,
most of the values in the first array are already False). I'll go and
implement your solution when I have many small subgrids in my model.

Thank you!

Andrea.

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