# NAG FL Interfacem01ccf (charvec_​sort)

## 1Purpose

m01ccf rearranges a vector of character data so that a specified substring is in ASCII or reverse ASCII order.

## 2Specification

Fortran Interface
 Subroutine m01ccf ( ch, m1, m2, l1, l2,
 Integer, Intent (In) :: m1, m2, l1, l2 Integer, Intent (Inout) :: ifail Character (*), Intent (Inout) :: ch(m2) Character (1), Intent (In) :: order
#include <nag.h>
 void m01ccf_ (char ch[], const Integer *m1, const Integer *m2, const Integer *l1, const Integer *l2, const char *order, Integer *ifail, const Charlen length_ch, const Charlen length_order)
The routine may be called by the names m01ccf or nagf_sort_charvec_sort.

## 3Description

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.

## 4References

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

## 5Arguments

1: $\mathbf{ch}\left({\mathbf{m2}}\right)$Character(*) array Input/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: $\mathbf{m1}$Integer Input
On entry: the index of the first element of ch to be sorted.
Constraint: ${\mathbf{m1}}>0$.
3: $\mathbf{m2}$Integer Input
On entry: the index of the last element of ch to be sorted.
Constraint: ${\mathbf{m2}}\ge {\mathbf{m1}}$.
4: $\mathbf{l1}$Integer Input
5: $\mathbf{l2}$Integer Input
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: $\mathbf{ifail}$Integer Input/Output
On entry: ifail must be set to $0$, $-1$ or $1$ to set behaviour on detection of an error; these values have no effect when no error is detected.
A value of $0$ causes the printing of an error message and program execution will be halted; otherwise program execution continues. A value of $-1$ means that an error message is printed while a value of $1$ means that it is not.
If halting is not appropriate, the value $-1$ or $1$ is recommended. If message printing is undesirable, then the value $1$ is recommended. Otherwise, the value $0$ is recommended. When the value $-\mathbf{1}$ 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, ${\mathbf{l1}}=〈\mathit{\text{value}}〉$.
Constraint: ${\mathbf{l1}}\ge 1$.
On entry, ${\mathbf{l1}}=〈\mathit{\text{value}}〉$ and ${\mathbf{l2}}=〈\mathit{\text{value}}〉$.
Constraint: ${\mathbf{l1}}\le {\mathbf{l2}}$.
On entry, ${\mathbf{l2}}=〈\mathit{\text{value}}〉$.
Constraint: ${\mathbf{l2}}\ge 1$.
On entry, ${\mathbf{l2}}=〈\mathit{\text{value}}〉$ and $\mathrm{len}\left({\mathbf{ch}}\left(1\right)\right)=〈\mathit{\text{value}}〉$.
Constraint: ${\mathbf{l2}}\le \mathrm{len}\left({\mathbf{ch}}\left(1\right)\right)$.
On entry, ${\mathbf{m1}}=〈\mathit{\text{value}}〉$.
Constraint: ${\mathbf{m1}}\ge 1$.
On entry, ${\mathbf{m1}}=〈\mathit{\text{value}}〉$ and ${\mathbf{m2}}=〈\mathit{\text{value}}〉$.
Constraint: ${\mathbf{m1}}\le {\mathbf{m2}}$.
On entry, ${\mathbf{m2}}=〈\mathit{\text{value}}〉$.
Constraint: ${\mathbf{m2}}\ge 1$.
${\mathbf{ifail}}=2$
On entry, order has an illegal value: ${\mathbf{order}}=〈\mathit{\text{value}}〉$.
${\mathbf{ifail}}=3$
On entry, $\mathrm{len}\left({\mathbf{ch}}\left(1\right)\right)=〈\mathit{\text{value}}〉$.
Constraint: $\mathrm{len}\left({\mathbf{ch}}\left(1\right)\right)\le 255$.
${\mathbf{ifail}}=-99$
See Section 7 in the Introduction to the NAG Library FL Interface for further information.
${\mathbf{ifail}}=-399$
Your licence key may have expired or may not have been installed correctly.
See Section 8 in the Introduction to the NAG Library FL Interface for further information.
${\mathbf{ifail}}=-999$
Dynamic memory allocation failed.
See Section 9 in the Introduction to the NAG Library FL Interface for further information.

Not applicable.

## 8Parallelism 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 implementation-specific information.

The average time taken by the routine is approximately proportional to $n×\mathrm{log}\left(n\right)$, 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.

## 10Example

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

### 10.1Program Text

Program Text (m01ccfe.f90)

### 10.2Program Data

Program Data (m01ccfe.d)

### 10.3Program Results

Program Results (m01ccfe.r)