NumArray array-indexing
Christopher T King
squirrel at WPI.EDU
Thu Aug 12 20:40:30 CEST 2004
On 12 Aug 2004, Michael Drumheller wrote:
> I am new to NumArray and I wonder if someone can help me with
> array-indexing. Here's the basic situation: Given a rank-2 array
> (i.e., a matrix) it seems to be trivial, with array indexing,
> to extract a subset of its *columns*. But it does not seem
> to be trivial to extract a subset of its *rows*.
You can do this using slices, like so:
>>> from numarray import *
>>> a = array([[1, 2], [3, 4]])
>>> a[:,1]
array([2, 4])
':' means 'take all values along this axis', just like how with standard
Python lists it means 'take all values in the list'.
> Basically, it seems to me that NumArray simply does not support
> the distinction between a column vector and a row vector. That
> is, if you have x=[1,2,3], then transpose(x) is a no-op. True?
False. You have to supply numarray with a two-dimensional array in order
to perform a two-dimensional transpose:
>>> transpose([[1, 2, 3]])
array([[1],
[2],
[3]])
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