[SciPy-user] small error problem
Travis Oliphant
oliphant at ee.byu.edu
Sat Oct 29 13:12:23 CDT 2005
Ryan Krauss wrote:
>I have a script that uses row reduction to mind the null space of a
>matrix. I am experiencing some strange behavior. At the begining of
>the script, I have a 2x2 matrix, but it is of rank 1. (I realize that
>find the null space of a 2x2 could be much simpler, but I will
>generally have higher ranked matrices. I have also found the
>nullspace using svd, but have gotten the best results from rref in the
>past.)
>
>
which version of scipy are you using?
>The fact that the matrix is rank one can be verified early in the
>script. Row 2 is a multiple of row 1:
>(Pdb) mat[1,1]/mat[1,0]-mat[0,1]/mat[0,0]
>Out[2]: NumPy array, format: long
>0.0
>(Pdb) '%0.30f'%(mat[1,1]/mat[1,0]-mat[0,1]/mat[0,0])
>Out[2]: '0.000000000000000000000000000000'
>
>In order to do the row reduction, I append a column of zeros:
>(Pdb) vect
>Out[2]: NumPy array, format: long
>[[ 0.,]
> [ 0.,]]
> 15 -> mat=c_[mat,vect]
>(Pdb) mat
>Out[2]: NumPy array, format: long
>[[ 1.50155914,-1.24770975,]
> [-2.98977017, 2.48432803,]]
>
>But now there is some small error:
>(Pdb) '%0.30f'%(mat[1,1]/mat[1,0]-mat[0,1]/mat[0,0])
>Out[2]: '-0.000000013035565737951060327759'
>(Pdb) mat[1,1]/mat[1,0]-mat[0,1]/mat[0,0]
>Out[2]: -1.303556573795106e-08
>
>and the output is now a scalar instead of NumPy array, format: long
>
>
I'm not sure what NumPy array, format: long means
It does seem strange that the expression is different things in
different places.
Could you post more information. What is the result of type(mat), for
example.
Thanks,
-Travis
More information about the SciPy-user
mailing list