M01 Chapter Contents
M01 Chapter Introduction
NAG Library Manual

# NAG Library Routine DocumentM01DBF

Note:  before using this routine, please read the Users' Note for your implementation to check the interpretation of bold italicised terms and other implementation-dependent details.

## 1  Purpose

M01DBF ranks a vector of integer numbers in ascending or descending order.

## 2  Specification

 SUBROUTINE M01DBF ( IV, M1, M2, ORDER, IRANK, IFAIL)
 INTEGER IV(M2), M1, M2, IRANK(M2), IFAIL CHARACTER(1) ORDER

## 3  Description

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.

## 4  References

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

## 5  Parameters

1:     IV(M2) – INTEGER arrayInput
On entry: elements M1 to M2 of IV must contain integer values to be ranked.
2:     M1 – INTEGERInput
On entry: the index of the first element of IV to be ranked.
Constraint: ${\mathbf{M1}}>0$.
3:     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:     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:     IRANK(M2) – 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:     IFAIL – INTEGERInput/Output
On entry: IFAIL must be set to $0$, $-1\text{​ or ​}1$. If you are unfamiliar with this parameter you should refer to Section 3.3 in the Essential Introduction 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 parameter, 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).

## 6  Error Indicators and Warnings

If on entry ${\mathbf{IFAIL}}={\mathbf{0}}$ or $-{\mathbf{1}}$, 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'.

## 7  Accuracy

Not applicable.

The average time taken by the routine is approximately proportional to $n×\mathrm{log}n$, where $n={\mathbf{M2}}-{\mathbf{M1}}+1$.

## 9  Example

This example reads a list of integers and ranks them in descending order.

### 9.1  Program Text

Program Text (m01dbfe.f90)

### 9.2  Program Data

Program Data (m01dbfe.d)

### 9.3  Program Results

Program Results (m01dbfe.r)