[Numpy-discussion] Problem with correlate
David Cournapeau
david@ar.media.kyoto-u.ac...
Sun May 31 20:18:31 CDT 2009
Charles R Harris wrote:
>
>
> On Sun, May 31, 2009 at 11:54 AM, rob steed <rjsteed@talk21.com
> <mailto:rjsteed@talk21.com>> wrote:
>
>
> Hi,
> After my previous email, I have opened a ticket #1117 (correlate
> not order dependent)
>
> I have found that the correlate function is defined in
> multiarraymodule.c and
> that inputs are being swapped using the following code
>
> n1 = ap1->dimensions[0];
> n2 = ap2->dimensions[0];
> if (n1 < n2) {
> ret = ap1;
> ap1 = ap2;
> ap2 = ret;
> ret = NULL;
> i = n1;
> n1 = n2;
> n2 = i;
> }
>
> I do not know the code well enough to see whether this could just
> be removed (I don't know c either).
> Maybe the algorithmn requires the inputs to be length ordered? I
> will try to work it out.
>
>
> If the correlation algorithm doesn't use an fft and is done
> explicitly, then the maximum overlap for any shift is the length of
> the shortest input. Swapping the arrays makes that logic easier to
> implement, but it isn't necessary.
But this logic is also wrong if the swapping is not taken into account -
as the OP mentioned, correlate(a, b) is not equal to correlate(b, a) in
the general case. The output is reversed in the second case compared to
the first case.
cheers,
David
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