[Numpy-discussion] Selecting columns of a matrix

Bill Baxter wbaxter at gmail.com
Wed Jun 21 22:23:38 CDT 2006


On 6/22/06, Alan G Isaac <aisaac at american.edu> wrote:
>
> > Alan G Isaac wrote:
> >> M.transpose()[V>0]
> >> If you want the columns as columns,
> >> you can transpose again.
>
>
> On Wed, 21 Jun 2006, Keith Goodman apparently wrote:
> > I can't get that to work when M is a n by m matrix:
>
> The problem is not M being a matrix.
> You made V a matrix (i.e., 2d).
> So you need to ravel() it first.
> >> M.transpose()[V.ravel()>0]


No dice, V.ravel() returns a matrix still.  Looks like you'll need
M.T[V.A.ravel()>0].T

Just lovely.  Is the new bool conversion thingy going to help make the
syntax more reasonable for matrices, too?  Seems like it will still require
M[:,V.A.ravel() > 0]  or M[:, V.A.squeeze() > 0] or M[:,V.A[:,0]>0]

Anyway, this seems to me just more evidence that one is better off getting
used to the 'array' way of doing things rather than clinging to Matlab ways
by using 'matrix'.  Is it worth dealing with the extra A's and asmatrix()'s
and squeeze()'s that seem to crop up just to be able to write A*B instead of
dot(A,B)  (*)?

--Bill

(*) Ok, there's also the bit about being able to tell column vectors from
row vectors and getting useful errors when you try to use a row that should
have been a column.  And then there's also the .T, .I, .H convenience
factor.
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