NAG FL Interface
f11zbf (real_​symm_​sort)

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1 Purpose

f11zbf sorts the nonzero elements of a real sparse symmetric matrix, represented in symmetric coordinate storage format.

2 Specification

Fortran Interface
Subroutine f11zbf ( n, nnz, a, irow, icol, dup, zer, istr, iwork, ifail)
Integer, Intent (In) :: n
Integer, Intent (Inout) :: nnz, irow(*), icol(*), ifail
Integer, Intent (Out) :: istr(n+1), iwork(n)
Real (Kind=nag_wp), Intent (Inout) :: a(*)
Character (1), Intent (In) :: dup, zer
C Header Interface
#include <nag.h>
void  f11zbf_ (const Integer *n, Integer *nnz, double a[], Integer irow[], Integer icol[], const char *dup, const char *zer, Integer istr[], Integer iwork[], Integer *ifail, const Charlen length_dup, const Charlen length_zer)
The routine may be called by the names f11zbf or nagf_sparse_real_symm_sort.

3 Description

f11zbf takes a symmetric coordinate storage (SCS) representation (see Section 2.1.2 in the F11 Chapter Introduction) of a real n×n sparse symmetric matrix A, and reorders the nonzero elements by increasing row index and increasing column index within each row. Entries with duplicate row and column indices may be removed. Alternatively, duplicate entries may be summed, which facilitates spare matrix addition (see Section 9). Any entries with zero values may optionally be removed.
f11zbf also returns a pointer array istr to the starting address of each row in A.

4 References

None.

5 Arguments

1: n Integer Input
On entry: n, the order of the matrix A.
Constraint: n1.
2: nnz Integer Input/Output
On entry: the number of elements supplied in the array a.
Constraint: nnz0.
On exit: the number of elements with unique row and column indices.
3: a(*) Real (Kind=nag_wp) array Input/Output
Note: the dimension of the array a must be at least max(1,nnz).
On entry: the nonzero elements of the lower triangular part of the real matrix A. These may be in any order and there may be multiple nonzero elements with the same row and column indices.
On exit: the lower triangular nonzero elements ordered by increasing row index, and by increasing column index within each row. Each nonzero element has a unique row and column index.
4: irow(*) Integer array Input/Output
Note: the dimension of the array irow must be at least max(1,nnz).
On entry: the row indices corresponding to the elements supplied in the array a.
Constraint: 1irow(i)n, for i=1,2,,nnz.
On exit: the first nnz elements contain the row indices corresponding to the elements returned in the array a.
5: icol(*) Integer array Input/Output
Note: the dimension of the array icol must be at least max(1,nnz).
On entry: the column indices corresponding to the elements supplied in the array a.
Constraint: 1icol(i)irow(i), for i=1,2,,nnz.
On exit: the first nnz elements contain the column indices corresponding to the elements returned in the array a.
6: dup Character(1) Input
On entry: indicates how elements in a with duplicate row and column indices are to be treated.
dup='R'
Duplicate entries are removed, only the first entry is kept.
dup='S'
The relevant values in a are summed.
dup='F'
The routine fails with ifail=3 on detecting a duplicate.
Constraint: dup='R', 'S' or 'F'.
7: zer Character(1) Input
On entry: indicates how elements in a with zero values are to be treated.
zer='R'
The entries are removed.
zer='K'
The entries are kept.
zer='F'
The routine fails with ifail=4 on detecting a zero.
Constraint: zer='R', 'K' or 'F'.
8: istr(n+1) Integer array Output
On exit: istr(i), for i=1,2,,n, is the starting address in the arrays a, irow and icol of row i of the matrix A. istr(n+1) is the address of the last element in a plus one.
9: iwork(n) Integer array Workspace
10: ifail Integer Input/Output
On entry: ifail must be set to 0, -1 or 1 to set behaviour on detection of an error; these values have no effect when no error is detected.
A value of 0 causes the printing of an error message and program execution will be halted; otherwise program execution continues. A value of -1 means that an error message is printed while a value of 1 means that it is not.
If halting is not appropriate, the value -1 or 1 is recommended. If message printing is undesirable, then the value 1 is recommended. Otherwise, the value 0 is recommended. When the value -1 or 1 is used it is essential to test the value of ifail on exit.
On exit: ifail=0 unless the routine detects an error or a warning has been flagged (see Section 6).

6 Error Indicators and Warnings

If on entry ifail=0 or -1, explanatory error messages are output on the current error message unit (as defined by x04aaf).
Errors or warnings detected by the routine:
ifail=1
On entry, dup=value.
Constraint: dup='R', 'S' or 'F'.
On entry, n=value.
Constraint: n1.
On entry, nnz=value.
Constraint: nnz0.
On entry, zer=value.
Constraint: zer='R', 'K' or 'F'.
ifail=2
On entry, i=value, icol(i)=value and irow(i)=value.
Constraint: icol(i)1 and icol(i)irow(i).
On entry, i=value, irow(i)=value and n=value.
Constraint: irow(i)1 and irow(i)n.
ifail=3
On entry, a duplicate entry has been found in row I and column J: I=value, J=value.
ifail=4
On entry, a zero entry has been found in row I and column J: I=value, J=value.
ifail=-99
An unexpected error has been triggered by this routine. Please contact NAG.
See Section 7 in the Introduction to the NAG Library FL Interface for further information.
ifail=-399
Your licence key may have expired or may not have been installed correctly.
See Section 8 in the Introduction to the NAG Library FL Interface for further information.
ifail=-999
Dynamic memory allocation failed.
See Section 9 in the Introduction to the NAG Library FL Interface for further information.

7 Accuracy

Not applicable.

8 Parallelism and Performance

f11zbf is not threaded in any implementation.

9 Further Comments

The time taken for a call to f11zbf is the sum of two contributions, where one is proportional to nnz and the other is proportional to n.
Note that the resulting matrix may have either rows or columns with no entries. If row i has no entries then istr(i)=istr(i+1).
Two sparse matrices can be added by concatenating the three pairs of SCS format arrays, representing the two matrices, and passing these new arrays to f11zbf, specifying that duplicates should be summed. This functionality is illustrated in Section 10 in f11zaf.

10 Example

This example reads the SCS representation of a real sparse symmetric matrix A, calls f11zbf to reorder the nonzero elements, and outputs the original and the reordered representations.

10.1 Program Text

Program Text (f11zbfe.f90)

10.2 Program Data

Program Data (f11zbfe.d)

10.3 Program Results

Program Results (f11zbfe.r)