[Numpy-discussion] What am I missing about concatenate? (original) (raw)
Tim Hochberg tim.hochberg at cox.net
Thu Jun 1 11:47:07 EDT 2006
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Christopher Barker wrote:
I want to take two (2,) arrays and put them together into one (2,2) array. I thought one of these would work:
>>> N.concatenate(((1,2),(3,4)),0) array([1, 2, 3, 4]) >>> N.concatenate(((1,2),(3,4)),1) array([1, 2, 3, 4]) Is this the best I can do? >>> N.concatenate(((1,2),(3,4))).reshape(2,2) array([[1, 2], [3, 4]]) Is it because the arrays I'm putting together are rank-1?
Yes.
You need to add a dimension somehow. There are (at least) two ways to do this. 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]])Alternatively, if you don't know that 'a' and 'b' are arrays or you just 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]])-tim
>>> N.version '0.9.6' -Chris
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