[Numpy-discussion] Extracting column vectors from matrix.

Konrad Hinsen hinsen at cnrs-orleans.fr
Thu Sep 4 05:37:05 CDT 2003


On Thursday 04 September 2003 11:58, Andrew Nesbit wrote:
> Say that I have a rank-2 array (matrix) of shape (m,n).  Call it A.
>
> When extracting column vectors from A, I often want to keep the result
> as a rank-2 column vector (for subsequent matrix multiplications,
> etc), so I usually end up writing something ugly like this:
>
> column_vector = reshape(A[:,col],(m,1))
>
> I've got a function to wrap this, but is there a builtin (or cleaner)
> way to do this sort of operation?

1) A[:, col:col+1]

or perhaps

2) A[:, col][:, NewAxis]

Konrad.
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