NAG Library Routine Document
m01ccf (charvec_sort)
1
Purpose
m01ccf rearranges a vector of character data so that a specified substring is in ASCII or reverse ASCII order.
2
Specification
Fortran Interface
Integer, Intent (In)  ::  m1, m2, l1, l2  Integer, Intent (Inout)  ::  ifail  Character (*), Intent (Inout)  ::  ch(m2)  Character (1), Intent (In)  ::  order 

3
Description
m01ccf is based on Singleton's implementation of the ‘medianofthree’ 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
Arguments
 1: $\mathbf{ch}\left({\mathbf{m2}}\right)$ – 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: $\mathbf{m1}$ – IntegerInput

On entry: the index of the first element of
ch to be sorted.
Constraint:
${\mathbf{m1}}>0$.
 3: $\mathbf{m2}$ – IntegerInput

On entry: the index of the last element of
ch to be sorted.
Constraint:
${\mathbf{m2}}\ge {\mathbf{m1}}$.
 4: $\mathbf{l1}$ – IntegerInput
 5: $\mathbf{l2}$ – 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: $\mathbf{order}$ – 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: $\mathbf{ifail}$ – 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).
6
Error 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  ${\mathbf{m1}}>{\mathbf{m2}}$, 
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 $255$. 
 ${\mathbf{ifail}}=99$
An unexpected error has been triggered by this routine. Please
contact
NAG.
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.
7
Accuracy
Not applicable.
8
Parallelism and Performance
m01ccf is threaded by NAG for parallel execution in multithreaded implementations of the NAG Library.
Please consult the
X06 Chapter Introduction for information on how to control and interrogate the OpenMP environment used within this routine. Please also consult the
Users' Note for your implementation for any additional implementationspecific information.
The average time taken by the routine is approximately proportional to $n\times \mathrm{log}\left(n\right)$, 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.
10
Example
This example reads a file of $12$character records, and sorts them into reverse ASCII order on characters $7$ to $12$.
10.1
Program Text
Program Text (m01ccfe.f90)
10.2
Program Data
Program Data (m01ccfe.d)
10.3
Program Results
Program Results (m01ccfe.r)