M01 Chapter Contents
M01 Chapter Introduction
NAG Library Manual

# NAG Library Routine DocumentM01DCF

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

M01DCF ranks a vector of character data in ASCII or reverse ASCII order of a specified substring.

## 2  Specification

 SUBROUTINE M01DCF ( CH, M1, M2, L1, L2, ORDER, IRANK, IFAIL)
 INTEGER M1, M2, L1, L2, IRANK(M2), IFAIL CHARACTER(*) CH(M2) CHARACTER(1) ORDER

## 3  Description

M01DCF 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.
Only the substring (L1:L2) of each element of the array CH is used to determine the rank order.

## 4  References

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

## 5  Parameters

1:     CH(M2) – CHARACTER(*) arrayInput
On entry: elements M1 to M2 of CH must contain character data to be ranked.
Constraint: the length of each element of CH must not exceed $255$.
2:     M1 – INTEGERInput
On entry: the index of the first element of CH to be ranked.
Constraint: ${\mathbf{M1}}>0$.
3:     M2 – INTEGERInput
On entry: the index of the last element of CH to be ranked.
Constraint: ${\mathbf{M2}}\ge {\mathbf{M1}}$.
4:     L1 – INTEGERInput
5:     L2 – INTEGERInput
On entry: only the substring (L1:L2) of each element of CH is to be used in determining the rank order.
Constraint: $0<{\mathbf{L1}}\le {\mathbf{L2}}\le \mathrm{LEN}\left({\mathbf{CH}}\left(1\right)\right)$.
6:     ORDER – CHARACTER(1)Input
On entry: if ${\mathbf{ORDER}}=\text{'A'}$, the values will be ranked in ASCII order.
If ${\mathbf{ORDER}}=\text{'R'}$, in reverse ASCII order.
Constraint: ${\mathbf{ORDER}}=\text{'A'}$ or $\text{'R'}$.
7:     IRANK(M2) – INTEGER arrayOutput
On exit: elements M1 to M2 of IRANK contain the ranks of the corresponding elements of CH. Note that the ranks are in the range M1 to M2: thus, if ${\mathbf{CH}}\left(i\right)$ is the first element in the rank order, ${\mathbf{IRANK}}\left(i\right)$ is set to M1.
8:     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 ${\mathbf{M1}}>{\mathbf{M2}}$, or ${\mathbf{L2}}<1$, or ${\mathbf{L1}}<1$, or ${\mathbf{L1}}>{\mathbf{L2}}$, or ${\mathbf{L2}}>\mathrm{LEN}\left({\mathbf{CH}}\left(1\right)\right)$.
${\mathbf{IFAIL}}=2$
 On entry, ORDER is not 'A' or 'R'.
${\mathbf{IFAIL}}=3$
 On entry, the length of each element of CH exceeds $255$.

## 7  Accuracy

Not applicable.

The average time taken by the routine is approximately proportional to $n×\mathrm{log}n$, where $n={\mathbf{M2}}-{\mathbf{M1}}+1$.
The routine relies on the Fortran intrinsic functions LLT and LGT to order characters according to the ASCII collating sequence.

## 9  Example

This example reads a file of $12$-character records, and ranks them in reverse ASCII order on characters $7$ to $12$.

### 9.1  Program Text

Program Text (m01dcfe.f90)

### 9.2  Program Data

Program Data (m01dcfe.d)

### 9.3  Program Results

Program Results (m01dcfe.r)