MATLAB Function Reference |
Sparse column permutation based on nonzero count
Syntax
Description
j = colperm(S)
generates a permutation vector j
such that the columns of S(:,j)
are ordered according to increasing count of nonzero entries. This is sometimes useful as a preordering for LU factorization; in this case use lu(S(:,j))
.
If S
is symmetric, then j
=
colperm(S)
generates a permutation j
so that both the rows and columns of S(j,j)
are ordered according to increasing count of nonzero entries. If S
is positive definite, this is sometimes useful as a preordering for Cholesky factorization; in this case use chol(S(j,j))
.
Algorithm
The algorithm involves a sort on the counts of nonzeros in each column.
Examples
has a full first row and column. Its LU factorization, lu(A)
, is almost completely full. The statement
returns j
=
[2:n
1]
. So A(j,j)
sends the full row and column to the bottom and the rear, and lu(A(j,j))
has the same nonzero structure as A
itself.
On the other hand, the Bucky ball example,
has exactly three nonzero elements in each row and column, so j = colperm(B)
is the identity permutation and is no help at all for reducing fill-in with subsequent factorizations.
See Also
chol
, colamd
, colmmd
, lu
, spparms
, symamd
, symmmd
, symrcm
ColorSpec | comet |
© 1994-2005 The MathWorks, Inc.