# NAG Library Routine Document

## 1Purpose

m01dbf ranks a vector of integer numbers in ascending or descending order.

## 2Specification

Fortran Interface
 Subroutine m01dbf ( iv, m1, m2,
 Integer, Intent (In) :: iv(m2), m1, m2 Integer, Intent (Inout) :: ifail Integer, Intent (Out) :: irank(m2) Character (1), Intent (In) :: order
#include nagmk26.h
 void m01dbf_ ( const Integer iv[], const Integer *m1, const Integer *m2, const char *order, Integer irank[], Integer *ifail, const Charlen length_order)

## 3Description

m01dbf 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 elements preserve their ordering in the input data.
Knuth D E (1973) The Art of Computer Programming (Volume 3) (2nd Edition) Addison–Wesley

## 5Arguments

1:     $\mathbf{iv}\left({\mathbf{m2}}\right)$ – Integer arrayInput
On entry: elements m1 to m2 of iv must contain integer values to be ranked.
2:     $\mathbf{m1}$ – IntegerInput
On entry: the index of the first element of iv to be ranked.
Constraint: ${\mathbf{m1}}>0$.
3:     $\mathbf{m2}$ – IntegerInput
On entry: m2 must specify the index of the last element of iv to be ranked.
Constraint: ${\mathbf{m2}}\ge {\mathbf{m1}}$.
4:     $\mathbf{order}$ – Character(1)Input
On entry: if ${\mathbf{order}}=\text{'A'}$, the values will be ranked in ascending (i.e., nondecreasing) order.
If ${\mathbf{order}}=\text{'D'}$, into descending order.
Constraint: ${\mathbf{order}}=\text{'A'}$ or $\text{'D'}$.
5:     $\mathbf{irank}\left({\mathbf{m2}}\right)$ – Integer arrayOutput
On exit: elements m1 to m2 of irank contain the ranks of the corresponding elements of iv. Note that the ranks are in the range m1 to m2: thus, if ${\mathbf{iv}}\left(i\right)$ is the first element in the rank order, ${\mathbf{irank}}\left(i\right)$ is set to m1.
6:     $\mathbf{ifail}$ – IntegerInput/Output
On entry: ifail must be set to $0$, $-1\text{​ or ​}1$. If you are unfamiliar with this argument you should refer to Section 3.4 in How to Use the NAG Library and its Documentation for details.
For environments where it might be inappropriate to halt program execution when an error is detected, the value $-1\text{​ or ​}1$ 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 $-\mathbf{1}\text{​ or ​}\mathbf{1}$ 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, ${\mathbf{m2}}<1$, or ${\mathbf{m1}}<1$, or ${\mathbf{m1}}>{\mathbf{m2}}$.
${\mathbf{ifail}}=2$
 On entry, order is not 'A' or 'D'.
${\mathbf{ifail}}=-99$
See Section 3.9 in How to Use the NAG Library and its Documentation for further information.
${\mathbf{ifail}}=-399$
Your licence key may have expired or may not have been installed correctly.
See Section 3.8 in How to Use the NAG Library and its Documentation for further information.
${\mathbf{ifail}}=-999$
Dynamic memory allocation failed.
See Section 3.7 in How to Use the NAG Library and its Documentation for further information.

Not applicable.

## 8Parallelism and Performance

m01dbf 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 list of integers and ranks them in descending order.

### 10.1Program Text

Program Text (m01dbfe.f90)

### 10.2Program Data

Program Data (m01dbfe.d)

### 10.3Program Results

Program Results (m01dbfe.r)

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