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.

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 Arguments

1: $ch (m2)$ – 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: $m1$ – Integer Input: On entry: the index of the first element of ch to be sorted.

Constraint: $m1 > 0$ .
3: $m2$ – Integer Input: On entry: the index of the last element of ch to be sorted.

Constraint: $m2 \geq m1$ .
4: $l1$ – Integer Input
5: $l2$ – Integer Input: On entry: only the substring (l1:l2) of each element of ch is to be used in determining the sorted order.

Constraint: $0 < l1 \leq l2 \leq len (ch (1))$ .
6: $order$ – Character(1) Input: On entry: if $order ='A'$ , the values will be sorted into ASCII order.
If $order ='R'$ , into reverse ASCII order.

Constraint: $order ='A'$ or $'R'$ .
7: $ifail$ – 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 $- 1$ or $1$ is used it is essential to test the value of ifail on exit.

On exit: $ifail = 0$ unless the routine detects an error or a warning has been flagged (see Section 6).

6 Error Indicators and Warnings

If on entry

ifail = 0

−1

, explanatory error messages are output on the current error message unit (as defined by x04aaf).

Errors or warnings detected by the routine:

$ifail = 1$: On entry, $l1 = ⟨ value ⟩$ .
Constraint: $l1 \geq 1$ .

On entry, $l1 = ⟨ value ⟩$ and $l2 = ⟨ value ⟩$ .
Constraint: $l1 \leq l2$ .

On entry, $l2 = ⟨ value ⟩$ .
Constraint: $l2 \geq 1$ .

On entry, $l2 = ⟨ value ⟩$ and $len (ch (1)) = ⟨ value ⟩$ .
Constraint: $l2 \leq len (ch (1))$ .

On entry, $m1 = ⟨ value ⟩$ .
Constraint: $m1 \geq 1$ .

On entry, $m1 = ⟨ value ⟩$ and $m2 = ⟨ value ⟩$ .
Constraint: $m1 \leq m2$ .

On entry, $m2 = ⟨ value ⟩$ .
Constraint: $m2 \geq 1$ .

$ifail = 2$: On entry, order has an illegal value: $order = ⟨ value ⟩$ .

$ifail = 3$: On entry, $len (ch (1)) = ⟨ value ⟩$ .
Constraint: $len (ch (1)) \leq 255$ .

$ifail = - 99$: An unexpected error has been triggered by this routine. Please contact NAG.
See Section 7 in the Introduction to the NAG Library FL Interface for further information.

$ifail = - 399$: 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.

$ifail = - 999$: Dynamic memory allocation failed.
See Section 9 in the Introduction to the NAG Library FL Interface for further information.

7 Accuracy

Not applicable.

8 Parallelism and Performance

Background information to multithreading can be found in the Multithreading documentation.

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.

9 Further Comments

The average time taken by the routine is approximately proportional to

n \times \log (n)

, where

n = m2 - 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

12

m01cc: FL CL CPP AD PY MB

NAG FL Interfacem01ccf (charvec_​sort)

▸▿ Contents

1 Purpose

2 Specification

3 Description

4 References

5 Arguments

6 Error Indicators and Warnings

7 Accuracy

8 Parallelism and Performance

9 Further Comments

10 Example

10.1 Program Text

10.2 Program Data

10.3 Program Results

NAG FL Interface
m01ccf (charvec_sort)