# [Numpy-discussion] Detect subclass of ndarray

Charles R Harris charlesr.harris@gmail....
Sat Mar 24 14:15:57 CDT 2007

```On 3/24/07, Colin J. Williams <cjw@sympatico.ca> wrote:
>
> Charles R Harris wrote:
> >
> >
> > On 3/24/07, *Alan G Isaac* <aisaac@american.edu
> > <mailto:aisaac@american.edu>> wrote:
> >
> >     On Fri, 23 Mar 2007, Charles R Harris apparently wrote:
> >      > the following gives the wrong result:
> >      > In [15]: I = matrix(eye(2))
> >      > In [16]: I*ones(2)
> >      > Out[16]: matrix([[ 1.,  1.]])
> >      > where the output should be a column vector.
> >
> >     Why should this output a column?
> >     I would prefer an exception.
> >     Add the axis if you want it:
> >     I*ones(2)[:,None]
> >     works fine.
> >
> >
> > Because it is mathematically correct. You can't multiply a vector by a
> > 2x2 matrix and get a 1x2 matrix as the result. Sure, there are work
> > arounds, but if matrix multiplication is going to work when mixed with
> > arrays, it should work correctly.
> >
> > Chuck
> >
> It depends on the convention you use when working with matrices.
>
> Suppose you adopt the notion, for matrices, a vector is always
> represented by a matrix.  This a row vector would have the shape (1, n)
> and the column vector would have (n, 1).
>
> If A were a (3, 4) matrix  and b were a 4 element column vector, then
> the product of A by b, using matrix arithmetic, would give a 3 element
> column vector.

Yes, that is what I am thinking. Given that there are only the two
possibilities, row or column, choose the only one that is compatible with
the multiplying matrix. The result will not always be a column vector, for
instance, mat([[1]])*ones(3) will be a 1x3 row vector.

Chuck
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