On 3/24/07, Colin J. Williams <[EMAIL PROTECTED]> wrote:
Charles R Harris wrote: > > > On 3/24/07, *Alan G Isaac* <[EMAIL PROTECTED] > <mailto:[EMAIL PROTECTED]>> 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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