NAG Library Routine Document

m01ncf examines an ordered vector of null terminated strings and returns the index of the first value equal to the sought-after item. Character items are compared according to the ASCII collating sequence.

2

Specification

Fortran Interface

Function m01ncf (

valid, ch, m1, m2, item, ifail)

Integer	::	m01ncf
Integer, Intent (In)	::	m1, m2
Integer, Intent (Inout)	::	ifail
Logical, Intent (In)	::	valid
Character (*), Intent (In)	::	ch(m2), item

C Header Interface

#include <nagmk26.h>

Integer

m01ncf_ (const logical *valid, const char ch[], const Integer *m1, const Integer *m2, const char *item, Integer *ifail, const Charlen length_ch, const Charlen length_item)

3

Description

m01ncf is based on Professor Niklaus Wirth's implementation of the Binary Search algorithm (see Wirth (2004)), but with two modifications. First, if the sought-after item is less than the value of the first element of the array to be searched,

0

is returned. Second, if a value equal to the sought-after item is not found, the index of the immediate lower value is returned.

4

References

Wirth N (2004) Algorithms and Data Structures 35–36 Prentice Hall

5

Arguments

1: $valid$ – LogicalInput: On entry: if valid is set to .TRUE. argument checking will be performed. If valid is set to .FALSE. m01ncf will be called without argument checking, which includes checking that array ch is sorted in ascending order and the routine will return with $ifail = 0$ . See Section 9 for further details.
2: $ch (m2)$ – Character(*) arrayInput: On entry: elements m1 to m2 contain character data to be searched.

Constraint: elements m1 to m2 of ch must be sorted in ascending order. The length of each element of ch must not exceed $255$ . Trailing space characters are ignored.
3: $m1$ – IntegerInput: On entry: the index of the first element of ch to be searched.

Constraint: $m1 \geq 1$ .
4: $m2$ – IntegerInput: On entry: the index of the last element of ch to be searched.

Constraint: $m2 \geq m1$ .
5: $item$ – Character(*)Input: On entry: the sought-after item. Trailing space characters are ignored.
6: $ifail$ – IntegerInput/Output: On entry: ifail must be set to $0$ , $- 1 or 1$ . 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 $- 1 or 1$ 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 $- 1 or 1$ 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:

(Note: these errors will only be returned if

valid = .TRUE.

$ifail = 2$: On entry, ch must be sorted in ascending order: $ch element 〈value〉 > element 〈value〉$ .

$ifail = 3$: On entry, $m1 = 〈value〉$ .
Constraint: $m1 \geq 1$ .

$ifail = 4$: On entry, $m1 = 〈value〉$ , $m2 = 〈value〉$ .
Constraint: $m2 \geq m1$ .

$ifail = 5$: On entry, the length of each element of ch must be at most $255$ : maximum string length $= 〈value〉$ .

$ifail = - 99$: 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.

$ifail = - 399$: 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.

$ifail = - 999$: 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

m01ncf is not threaded in any implementation.

9

Further Comments

The argument valid should be used with caution. Set it to .FALSE. only if you are confident that the other arguments are correct, in particular that array ch is in fact arranged in ascending order. If you wish to search the same array ch many times, you are recommended to set valid to .TRUE. on first call of m01ncf and to .FALSE. on subsequent calls, in order to minimize the amount of time spent checking ch, which may be significant if ch is large.

The time taken by m01ncf is

O (\log (n))

, where

n = m2 - m1 + 1

, when

valid = .FALSE.

10

Example

This example reads a list of character data and sought-after items and performs the search for these items.

NAG Library Routine Document

m01ncf (charvec_search)

▸▿ 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

1

Purpose