# NAG FL Interfacem01def (realmat_​rank_​rows)

## 1Purpose

m01def ranks the rows of a matrix of real numbers in ascending or descending order.

## 2Specification

Fortran Interface
 Subroutine m01def ( rm, ldm, m1, m2, n1, n2,
 Integer, Intent (In) :: ldm, m1, m2, n1, n2 Integer, Intent (Inout) :: ifail Integer, Intent (Out) :: irank(m2) Real (Kind=nag_wp), Intent (In) :: rm(ldm,n2) Character (1), Intent (In) :: order
#include <nag.h>
 void m01def_ (const double rm[], const Integer *ldm, const Integer *m1, const Integer *m2, const Integer *n1, const Integer *n2, const char *order, Integer irank[], Integer *ifail, const Charlen length_order)
The routine may be called by the names m01def or nagf_sort_realmat_rank_rows.

## 3Description

m01def ranks rows m1 to m2 of a matrix, using the data in columns n1 to n2 of those rows. The ordering is determined by first ranking the data in column n1, then ranking any tied rows according to the data in column ${\mathbf{n1}}+1$, and so on up to column n2.
m01def uses a variant of list-merging, as described on pages 165–166 in Knuth (1973). The routine takes advantage of natural ordering in the data, and uses a simple list insertion in a preparatory pass to generate ordered lists of length at least $10$. The ranking is stable: equal rows preserve their ordering in the input data.

## 4References

Knuth D E (1973) The Art of Computer Programming (Volume 3) (2nd Edition) Addison–Wesley

## 5Arguments

1: $\mathbf{rm}\left({\mathbf{ldm}},{\mathbf{n2}}\right)$Real (Kind=nag_wp) array Input
On entry: columns n1 to n2 of rows m1 to m2 of rm must contain real data to be ranked.
2: $\mathbf{ldm}$Integer Input
On entry: the first dimension of the array rm as declared in the (sub)program from which m01def is called.
Constraint: ${\mathbf{ldm}}\ge {\mathbf{m2}}$.
3: $\mathbf{m1}$Integer Input
On entry: the index of the first row of rm to be ranked.
Constraint: ${\mathbf{m1}}>0$.
4: $\mathbf{m2}$Integer Input
On entry: the index of the last row of rm to be ranked.
Constraint: ${\mathbf{m2}}\ge {\mathbf{m1}}$.
5: $\mathbf{n1}$Integer Input
On entry: the index of the first column of rm to be used.
Constraint: ${\mathbf{n1}}>0$.
6: $\mathbf{n2}$Integer Input
On entry: the index of the last column of rm to be used.
Constraint: ${\mathbf{n2}}\ge {\mathbf{n1}}$.
7: $\mathbf{order}$Character(1) Input
On entry: if ${\mathbf{order}}=\text{'A'}$, the rows will be ranked in ascending (i.e., nondecreasing) order.
If ${\mathbf{order}}=\text{'D'}$, into descending order.
Constraint: ${\mathbf{order}}=\text{'A'}$ or $\text{'D'}$.
8: $\mathbf{irank}\left({\mathbf{m2}}\right)$Integer array Output
On exit: elements m1 to m2 of irank contain the ranks of the corresponding rows of rm. Note that the ranks are in the range m1 to m2: thus, if the $i$th row of rm is the first in the rank order, ${\mathbf{irank}}\left(i\right)$ is set to m1.
9: $\mathbf{ifail}$Integer Input/Output
On entry: ifail must be set to $0$, . If you are unfamiliar with this argument you should refer to Section 4 in the Introduction to the NAG Library FL Interface for details.
For environments where it might be inappropriate to halt program execution when an error is detected, the value is recommended. If the output of error messages is undesirable, then the value $1$ is recommended. Otherwise, if you are not familiar with this argument, the recommended value is $0$. When the value is used it is essential to test the value of ifail on exit.
On exit: ${\mathbf{ifail}}={\mathbf{0}}$ unless the routine detects an error or a warning has been flagged (see Section 6).

## 6Error Indicators and Warnings

If on entry ${\mathbf{ifail}}=0$ or $-1$, explanatory error messages are output on the current error message unit (as defined by x04aaf).
Errors or warnings detected by the routine:
${\mathbf{ifail}}=1$
On entry, ${\mathbf{m1}}=〈\mathit{\text{value}}〉$.
Constraint: ${\mathbf{m1}}\ge 1$.
On entry, ${\mathbf{m1}}=〈\mathit{\text{value}}〉$ and ${\mathbf{m2}}=〈\mathit{\text{value}}〉$.
Constratint: ${\mathbf{m1}}\le {\mathbf{m2}}$.
On entry, ${\mathbf{m2}}=〈\mathit{\text{value}}〉$.
Constraint: ${\mathbf{m2}}\ge 1$.
On entry, ${\mathbf{m2}}=〈\mathit{\text{value}}〉$ and ${\mathbf{ldm}}=〈\mathit{\text{value}}〉$.
Constraint: ${\mathbf{m2}}\le {\mathbf{ldm}}$.
On entry, ${\mathbf{n1}}=〈\mathit{\text{value}}〉$.
Constraint: ${\mathbf{n1}}\ge 1$.
On entry, ${\mathbf{n1}}=〈\mathit{\text{value}}〉$ and ${\mathbf{n2}}=〈\mathit{\text{value}}〉$.
Constraint: ${\mathbf{n1}}\le {\mathbf{n2}}$.
On entry, ${\mathbf{n2}}=〈\mathit{\text{value}}〉$.
Constraint: ${\mathbf{n2}}\ge 1$.
${\mathbf{ifail}}=2$
On entry, order has an illegal value: ${\mathbf{order}}=〈\mathit{\text{value}}〉$.
${\mathbf{ifail}}=-99$
See Section 7 in the Introduction to the NAG Library FL Interface for further information.
${\mathbf{ifail}}=-399$
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.
${\mathbf{ifail}}=-999$
Dynamic memory allocation failed.
See Section 9 in the Introduction to the NAG Library FL Interface for further information.

Not applicable.

## 8Parallelism and Performance

m01def is not threaded in any implementation.

The average time taken by the routine is approximately proportional to $n×\mathrm{log}\left(n\right)$, where $n={\mathbf{m2}}-{\mathbf{m1}}+1$.

## 10Example

This example reads a matrix of real numbers and ranks the rows in ascending order.

### 10.1Program Text

Program Text (m01defe.f90)

### 10.2Program Data

Program Data (m01defe.d)

### 10.3Program Results

Program Results (m01defe.r)