NAG Library Function Document
nag_superlu_column_permutation (f11mdc)
1
Purpose
nag_superlu_column_permutation (f11mdc) computes a column permutation suitable for
factorization (by
nag_superlu_lu_factorize (f11mec)) of a real sparse matrix in compressed column (Harwell–Boeing) format and applies it to the matrix. This function must be called prior to
nag_superlu_lu_factorize (f11mec).
2
Specification
#include <nag.h> |
#include <nagf11.h> |
void |
nag_superlu_column_permutation (Nag_ColumnPermutationType spec,
Integer n,
const Integer icolzp[],
const Integer irowix[],
Integer iprm[],
NagError *fail) |
|
3
Description
Given a sparse matrix in compressed column (Harwell–Boeing) format
and a choice of column permutation schemes, the function computes those data structures that will be needed by the
factorization function
nag_superlu_lu_factorize (f11mec) and associated functions
nag_superlu_diagnostic_lu (f11mmc),
nag_superlu_solve_lu (f11mfc) and
nag_superlu_refine_lu (f11mhc). The column permutation choices are:
- original order (that is, no permutation);
- user-supplied permutation;
- a permutation, computed by the function, designed to minimize fill-in during the factorization.
The algorithm for this computed permutation is based on the approximate minimum degree column ordering algorithm COLAMD. The computed permutation is not sensitive to the magnitude of the nonzero values of .
4
References
Amestoy P R, Davis T A and Duff I S (1996) An approximate minimum degree ordering algorithm SIAM J. Matrix Anal. Appl. 17 886–905
Gilbert J R and Larimore S I (2004) A column approximate minimum degree ordering algorithm ACM Trans. Math. Software 30,3 353–376
Gilbert J R, Larimore S I and Ng E G (2004) Algorithm 836: COLAMD, an approximate minimum degree ordering algorithm ACM Trans. Math. Software 30, 3 377–380
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Arguments
- 1:
– Nag_ColumnPermutationTypeInput
-
On entry: indicates the permutation to be applied.
- The identity permutation is used (i.e., the columns are not permuted).
- The permutation in the iprm array is used, as supplied by you.
- The permutation computed by the COLAMD algorithm is used
Constraint:
, or .
- 2:
– IntegerInput
-
On entry: , the order of the matrix .
Constraint:
.
- 3:
– const IntegerInput
-
On entry:
contains the index in
of the start of a new column. See
Section 2.1.3 in the f11 Chapter Introduction.
- 4:
– const IntegerInput
-
Note: the dimension,
dim, of the array
irowix
must be at least
.
On entry:
contains the row index in
for element
. See
Section 2.1.3 in the f11 Chapter Introduction.
- 5:
– IntegerInput/Output
-
On entry: the first entries contain the column permutation supplied by you. This will be used if , and ignored otherwise. If used, it must consist of a permutation of all the integers in the range , the leftmost column of the matrix denoted by and the rightmost by . Labelling columns in this way, means that column of is in position in , where expresses the factorization to be performed.
On exit: a new permutation is returned in the first
entries. The rest of the array contains data structures that will be used by other functions. The function computes the column elimination tree for
and a post-order permutation on the tree. It then compounds the
iprm permutation given or computed by the COLAMD algorthm with the post-order permutation. This array is needed by the
factorization function
nag_superlu_lu_factorize (f11mec) and associated functions
nag_superlu_solve_lu (f11mfc),
nag_superlu_refine_lu (f11mhc) and
nag_superlu_diagnostic_lu (f11mmc) and should be passed to them unchanged.
- 6:
– NagError *Input/Output
-
The NAG error argument (see
Section 3.7 in How to Use the NAG Library and its Documentation).
6
Error Indicators and Warnings
- NE_ALG_FAIL
-
COLAMD algorithm failed.
- NE_ALLOC_FAIL
-
Dynamic memory allocation failed.
See
Section 2.3.1.2 in How to Use the NAG Library and its Documentation for further information.
- NE_BAD_PARAM
-
On entry, argument had an illegal value.
- NE_INT
-
On entry, .
Constraint: .
- NE_INTERNAL_ERROR
-
An internal error has occurred in this function. Check the function call and any array sizes. If the call is correct then please contact
NAG for assistance.
See
Section 2.7.6 in How to Use the NAG Library and its Documentation for further information.
- NE_INVALID_PERM_COL
-
Incorrect column permutations in array
iprm.
- NE_NO_LICENCE
-
Your licence key may have expired or may not have been installed correctly.
See
Section 2.7.5 in How to Use the NAG Library and its Documentation for further information.
- NE_SPARSE_COL
-
Incorrect specification of argument
icolzp.
- NE_SPARSE_ROW
-
Incorrect specification of argument
irowix.
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Accuracy
Not applicable. This computation does not use floating-point numbers.
8
Parallelism and Performance
nag_superlu_column_permutation (f11mdc) is threaded by NAG for parallel execution in multithreaded implementations of the NAG Library.
Please consult the
x06 Chapter Introduction for information on how to control and interrogate the OpenMP environment used within this function. Please also consult the
Users' Note for your implementation for any additional implementation-specific information.
We recommend calling this function with
before calling
nag_superlu_lu_factorize (f11mec). The COLAMD algorithm computes a sparsity-preserving permutation
solely from the pattern of
such that the
factorization
remains as sparse as possible, regardless of the subsequent choice of
. The algorithm takes advantage of the existence of super-columns (columns with the same sparsity pattern) to reduce running time.
10
Example
This example computes a sparsity preserving column permutation for the
factorization of the matrix
, where
10.1
Program Text
Program Text (f11mdce.c)
10.2
Program Data
Program Data (f11mdce.d)
10.3
Program Results
Program Results (f11mdce.r)