Python bug? Indexing to matrices

[sturlamolden]

On 12 Jul, 07:39, David <davidb... at gmail.com> wrote:
> Should the following line work for defining a matrix with zeros?
>
> c= [[0]*col]*row

No. The rows will be aliased.

This will work:

c = [[0]*col for i in range(row)]

Note that Python lists are not ment to be used as matrices. We have
NumPy or the array module for that.


> If this a valid way of initializing a matrix in Python 3.2.1, then it
> appears to me that a bug surfaces in Python when performing this line:
>
> c[i][j] = c[i][j] + a[i][k] * b[k][j]
>
> It writes to the jth column rather than just the i,j cell.

That is due to aliasing.


> I'm new at Python and am not sure if I'm just doing something wrong if
> there is really a bug in Python.  The script works fine if I
> initialize the matrix with numpy instead:
>
> c = np.zeros((row,col))
>
> So, I know my matrix multiply algorithm is correct (I know I could use
> numpy for matrix multiplication, this was just to learn Python).

Like so:

   np.dot(a,b)

or

   ma = np.matrix(a)
   mb = np.matrix(b)
   a*b

or call BLAS directly:

   scipy.linalg.fblas.dgemm


Sturla