On Mon Jul 27 22:51:52 2015 GMT+0200, Sturla Molden wrote:
On 27/07/15 22:10, Anton Akhmerov wrote:
Hi everyone,
I have encountered an initially rather confusing problem in a piece of code that attempted to symmetrize a matrix: `h += h.T` The problem of course appears due to `h.T` being a view of `h`, and some elements being overwritten during the __iadd__ call.
I think the typical proposal is to raise a warning. Note there is np.may_share_memoty. But the logic to give the warning is possibly not quite easy, since this is ok to use sometimes. If someone figures it out (mostly) I would be very happy zo see such warnings.
Here is another example
a = np.ones(10) a[1:] += a[:-1] a array([ 1., 2., 3., 2., 3., 2., 3., 2., 3., 2.])
I am not sure I totally dislike this behavior. If it could be made constent it could be used to vectorize recursive algorithms. In the case above I would prefer the output to be:
array([ 1., 2., 3., 4., 5., 6., 7., 8., 9., 10.])
It does not happen because we do not enforce that the result of one operation is stored before the next two operands are read. The only way to speed up recursive equations today is to use compiled code.
Sturla
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