M01 Chapter Contents
M01 Chapter Introduction
NAG Library Manual

NAG Library Routine DocumentM01CCF

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

M01CCF rearranges a vector of character data so that a specified substring is in ASCII or reverse ASCII order.

2  Specification

 SUBROUTINE M01CCF ( CH, M1, M2, L1, L2, ORDER, IFAIL)
 INTEGER M1, M2, L1, L2, IFAIL CHARACTER(*) CH(M2) CHARACTER(1) ORDER

3  Description

M01CCF is based on Singleton's implementation of the ‘median-of-three’ Quicksort algorithm (see Singleton (1969)), but with two additional modifications. First, small subfiles are sorted by an insertion sort on a separate final pass (see Sedgewick (1978)) Second, if a subfile is partitioned into two very unbalanced subfiles, the larger of them is flagged for special treatment: before it is partitioned, its end points are swapped with two random points within it; this makes the worst case behaviour extremely unlikely.
Only the substring (L1:L2) of each element of the array CH is used to determine the sorted order, but the entire elements are rearranged into sorted order.

4  References

Sedgewick R (1978) Implementing Quicksort programs Comm. ACM 21 847–857
Singleton R C (1969) An efficient algorithm for sorting with minimal storage: Algorithm 347 Comm. ACM 12 185–187

5  Parameters

1:     CH(M2) – CHARACTER(*) arrayInput/Output
On entry: elements M1 to M2 of CH must contain character data to be sorted.
Constraint: the length of each element of CH must not exceed $255$.
On exit: these values are rearranged into sorted order.
2:     M1 – INTEGERInput
On entry: the index of the first element of CH to be sorted.
Constraint: ${\mathbf{M1}}>0$.
3:     M2 – INTEGERInput
On entry: the index of the last element of CH to be sorted.
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 sorted 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 sorted into ASCII order.
If ${\mathbf{ORDER}}=\text{'R'}$, into reverse ASCII order.
Constraint: ${\mathbf{ORDER}}=\text{'A'}$ or $\text{'R'}$.
7:     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 worst case time is proportional to ${n}^{2}$, but this is extremely unlikely to occur.
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 sorts them into reverse ASCII order on characters $7$ to $12$.

9.1  Program Text

Program Text (m01ccfe.f90)

9.2  Program Data

Program Data (m01ccfe.d)

9.3  Program Results

Program Results (m01ccfe.r)