On Sun, May 31, 2009 at 9:08 PM, David Cournapeau < david@ar.media.kyoto-u.ac.jp> wrote:
Charles R Harris wrote:
On Sun, May 31, 2009 at 7:18 PM, David Cournapeau <david@ar.media.kyoto-u.ac.jp <mailto:david@ar.media.kyoto-u.ac.jp>> wrote:
Charles R Harris wrote: > > > On Sun, May 31, 2009 at 11:54 AM, rob steed <rjsteed@talk21.com <mailto:rjsteed@talk21.com> > <mailto: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.
I didn't say it was *correctly* implemented ;)
:) So I gave it a shot
http://github.com/cournape/numpy/commits/fix_correlate
(It took me a while to realize that PyArray_ISFLEXIBLE returns false for array object. Is this expected ? The documentation concerning copyswap says that it is necessary for flexible arrays, but I think it is necessary for object arrays as well).
Don't know. PyArray_ISFLEXIBLE looks like a macro... #define PyArray_ISFLEXIBLE(obj) PyTypeNum_ISFLEXIBLE(PyArray_TYPE(obj)) #define PyTypeNum_ISFLEXIBLE(type) (((type) >=NPY_STRING) && \ ((type) <=NPY_VOID)) And the typecodes are '?bhilqpBHILQPfdgFDGSUVO'. So 'SUV' are flexible and O is not. I'm not clear on how correlate should apply to any of 'SUV' but it might be worth having it work for objects.
It still bothers me that correlate does not conjugate the second argument for complex arrays...
It bothers me also... Chuck