# NAG Library Function Document

## 1Purpose

nag_sparse_nherm_sort (f11znc) sorts the nonzero elements of a complex sparse non-Hermitian matrix, represented in coordinate storage format.

## 2Specification

 #include #include
 void nag_sparse_nherm_sort (Integer n, Integer *nnz, Complex a[], Integer irow[], Integer icol[], Nag_SparseNsym_Dups dup, Nag_SparseNsym_Zeros zero, Integer istr[], NagError *fail)

## 3Description

nag_sparse_nherm_sort (f11znc) takes a coordinate storage (CS) representation (see Section 2.1.1 in the f11 Chapter Introduction) of a sparse $n$ by $n$ complex non-Hermitian 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, or the values may be summed. Any entries with zero values may optionally be removed.
The function also returns a pointer array istr to the starting address of each row in $A$.

None.

## 5Arguments

1:    $\mathbf{n}$IntegerInput
On entry: $n$, the order of the matrix $A$.
Constraint: ${\mathbf{n}}\ge 1$.
2:    $\mathbf{nnz}$Integer *Input/Output
On entry: the number of nonzero elements in the matrix $A$.
Constraint: ${\mathbf{nnz}}\ge 0$.
On exit: the number of nonzero elements with unique row and column indices.
3:    $\mathbf{a}\left[\mathit{dim}\right]$ComplexInput/Output
Note: the dimension, dim, of the array a must be at least $\mathrm{max}\phantom{\rule{0.125em}{0ex}}\left(1,{\mathbf{nnz}}\right)$.
On entry: the nonzero elements of the 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 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:    $\mathbf{irow}\left[\mathit{dim}\right]$IntegerInput/Output
Note: the dimension, dim, of the array irow must be at least $\mathrm{max}\phantom{\rule{0.125em}{0ex}}\left(1,{\mathbf{nnz}}\right)$.
On entry: the row indices corresponding to the nonzero elements supplied in the array a.
Constraint: $1\le {\mathbf{irow}}\left[\mathit{i}\right]\le {\mathbf{n}}$, for $\mathit{i}=0,1,\dots ,{\mathbf{nnz}}-1$.
On exit: the first nnz elements contain the row indices corresponding to the nonzero elements returned in the array a.
5:    $\mathbf{icol}\left[\mathit{dim}\right]$IntegerInput/Output
Note: the dimension, dim, of the array icol must be at least $\mathrm{max}\phantom{\rule{0.125em}{0ex}}\left(1,{\mathbf{nnz}}\right)$.
On entry: the column indices corresponding to the nonzero elements supplied in the array a.
Constraint: $1\le {\mathbf{icol}}\left[\mathit{i}\right]\le {\mathbf{n}}$, for $\mathit{i}=0,1,\dots ,{\mathbf{nnz}}-1$.
On exit: the first nnz elements contain the column indices corresponding to the nonzero elements returned in the array a.
6:    $\mathbf{dup}$Nag_SparseNsym_DupsInput
On entry: indicates how any nonzero elements with duplicate row and column indices are to be treated.
${\mathbf{dup}}=\mathrm{Nag_SparseNsym_RemoveDups}$
The entries are removed.
${\mathbf{dup}}=\mathrm{Nag_SparseNsym_SumDups}$
The relevant values in a are summed.
${\mathbf{dup}}=\mathrm{Nag_SparseNsym_FailDups}$
The function fails with ${\mathbf{fail}}\mathbf{.}\mathbf{code}=$ NE_NON_ZERO_DUP on detecting a duplicate.
Constraint: ${\mathbf{dup}}=\mathrm{Nag_SparseNsym_RemoveDups}$, $\mathrm{Nag_SparseNsym_SumDups}$ or $\mathrm{Nag_SparseNsym_FailDups}$.
7:    $\mathbf{zero}$Nag_SparseNsym_ZerosInput
On entry: indicates how any elements with zero values in array a are to be treated.
${\mathbf{zero}}=\mathrm{Nag_SparseNsym_RemoveZeros}$
The entries are removed.
${\mathbf{zero}}=\mathrm{Nag_SparseNsym_KeepZeros}$
The entries are kept.
${\mathbf{zero}}=\mathrm{Nag_SparseNsym_FailZeros}$
The function fails with ${\mathbf{fail}}\mathbf{.}\mathbf{code}=$ NE_ZERO_COEFF on detecting a zero.
Constraint: ${\mathbf{zero}}=\mathrm{Nag_SparseNsym_RemoveZeros}$, $\mathrm{Nag_SparseNsym_KeepZeros}$ or $\mathrm{Nag_SparseNsym_FailZeros}$.
8:    $\mathbf{istr}\left[{\mathbf{n}}+1\right]$IntegerOutput
On exit: ${\mathbf{istr}}\left[\mathit{i}-1\right]-1$, for $\mathit{i}=1,2,\dots ,{\mathbf{n}}$, is the starting address in the arrays a, irow and icol of row $i$ of the matrix $A$. ${\mathbf{istr}}\left[{\mathbf{n}}\right]-1$ is the address of the last nonzero element in $A$ plus one.
9:    $\mathbf{fail}$NagError *Input/Output
The NAG error argument (see Section 3.7 in How to Use the NAG Library and its Documentation).

## 6Error Indicators and Warnings

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.
On entry, argument $〈\mathit{\text{value}}〉$ had an illegal value.
NE_INT
On entry, ${\mathbf{n}}=〈\mathit{\text{value}}〉$.
Constraint: ${\mathbf{n}}\ge 1$.
On entry, ${\mathbf{nnz}}=〈\mathit{\text{value}}〉$.
Constraint: ${\mathbf{nnz}}\ge 0$.
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_CS
On entry, $i=〈\mathit{\text{value}}〉$, ${\mathbf{icol}}\left[i-1\right]=〈\mathit{\text{value}}〉$ and ${\mathbf{n}}=〈\mathit{\text{value}}〉$.
Constraint: ${\mathbf{icol}}\left[i-1\right]\ge 1$ and ${\mathbf{icol}}\left[i-1\right]\le {\mathbf{n}}$.
On entry, $i=〈\mathit{\text{value}}〉$, ${\mathbf{irow}}\left[i-1\right]=〈\mathit{\text{value}}〉$ and ${\mathbf{n}}=〈\mathit{\text{value}}〉$.
Constraint: ${\mathbf{irow}}\left[i-1\right]\ge 1$ and ${\mathbf{irow}}\left[i-1\right]\le {\mathbf{n}}$.
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_NON_ZERO_DUP
On entry, a duplicate entry has been found in row $\mathit{I}$ and column $\mathit{J}$: $\mathit{I}=〈\mathit{\text{value}}〉$, $\mathit{J}=〈\mathit{\text{value}}〉$.
NE_ZERO_COEFF
On entry, a zero entry has been found in row $\mathit{I}$ and column $\mathit{J}$: $\mathit{I}=〈\mathit{\text{value}}〉$, $\mathit{J}=〈\mathit{\text{value}}〉$.

Not applicable.

## 8Parallelism and Performance

nag_sparse_nherm_sort (f11znc) is not threaded in any implementation.

The time taken for a call to nag_sparse_nherm_sort (f11znc) is proportional to nnz.
Note that the resulting matrix may have either rows or columns with no entries. If row $i$ has no entries then ${\mathbf{istr}}\left[i-1\right]={\mathbf{istr}}\left[i\right]$.

## 10Example

This example reads the CS representation of a complex sparse matrix $A$, calls nag_sparse_nherm_sort (f11znc) to reorder the nonzero elements, and outputs the original and the reordered representations.

### 10.1Program Text

Program Text (f11znce.c)

### 10.2Program Data

Program Data (f11znce.d)

### 10.3Program Results

Program Results (f11znce.r)

© The Numerical Algorithms Group Ltd, Oxford, UK. 2017