# [Numpy-discussion] Detect subclass of ndarray

Colin J. Williams cjw@sympatico...
Sat Mar 24 13:06:16 CDT 2007

```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.

Colin W.
>
>
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```