[Numpy-discussion] Re: What am I missing about concatenate? (original) (raw)

Tim Hochberg tim.hochberg at cox.net
Thu Jun 1 12:59:02 EDT 2006

Christopher Barker wrote:

Thanks all,

Robert Kern wrote: Look at vstack() (and also its friends hstack(), dstack() and columnstack() for completeness). I like this, but need to keep Numeric/numarray compatibility for the moment -- I think, I've just sent out a query to my users.

Tim Hochberg wrote: If you are using real arrays, use newaxis: >>> a array([0, 1, 2]) >>> b array([3, 4, 5]) >>> concatenate([a[newaxis], b[newaxis]], 0) array([[0, 1, 2], [3, 4, 5]]) I like this, but again, not in Numeric -- I really need to dump that as soon as I can!

In Numeric, you can use NewAxis instead for the same effect.

hate newaxis, wrap the arrays in [] to give them an extra dimension. This tends to look nicer, but I suspect has poorer performance than above (haven't timed it though):

>>> concatenate([[a], [b]], 0) array([[0, 1, 2], [3, 4, 5]]) Lovely. much cleaner. By they way, wouldn't wrapping in a tuple, be slightly better, performance-wise (I know, probably negligible, but I always feel that I should use a tuple when I don't need mutability)

I doubt it would make a signifigant difference and the square brackets are much easier to read IMO. Your mileage may vary.

-tim

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