# NAG FL Interfacem01ncf (charvec_​search)

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## 1Purpose

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.

## 2Specification

Fortran Interface
 Function m01ncf ( ch, m1, m2, item,
 Integer :: m01ncf Integer, Intent (In) :: m1, m2 Integer, Intent (Inout) :: ifail Logical, Intent (In) :: valid Character (*), Intent (In) :: ch(m2), item
#include <nag.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)
The routine may be called by the names m01ncf or nagf_sort_charvec_search.

## 3Description

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.

## 4References

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

## 5Arguments

1: $\mathbf{valid}$Logical Input
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 ${\mathbf{ifail}}={\mathbf{0}}$. See Section 9 for further details.
2: $\mathbf{ch}\left({\mathbf{m2}}\right)$Character(*) array Input
On entry: elements ${\mathbf{m1}}$ to ${\mathbf{m2}}$ contain character data to be searched.
Constraint: elements ${\mathbf{m1}}$ to ${\mathbf{m2}}$ 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: $\mathbf{m1}$Integer Input
On entry: the index of the first element of ch to be searched.
Constraint: ${\mathbf{m1}}\ge 1$.
4: $\mathbf{m2}$Integer Input
On entry: the index of the last element of ch to be searched.
Constraint: ${\mathbf{m2}}\ge {\mathbf{m1}}$.
5: $\mathbf{item}$Character(*) Input
On entry: the sought-after item. Trailing space characters are ignored.
6: $\mathbf{ifail}$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:
Note: these errors will only be returned if ${\mathbf{valid}}=\mathrm{.TRUE.}$.).
${\mathbf{ifail}}=2$
On entry, ch must be sorted in ascending order: ${\mathbf{ch}}\text{​ element ​}⟨\mathit{\text{value}}⟩>\text{​ element ​}⟨\mathit{\text{value}}⟩$.
${\mathbf{ifail}}=3$
On entry, ${\mathbf{m1}}=⟨\mathit{\text{value}}⟩$.
Constraint: ${\mathbf{m1}}\ge 1$.
${\mathbf{ifail}}=4$
On entry, ${\mathbf{m1}}=⟨\mathit{\text{value}}⟩$, ${\mathbf{m2}}=⟨\mathit{\text{value}}⟩$.
Constraint: ${\mathbf{m2}}\ge {\mathbf{m1}}$.
${\mathbf{ifail}}=5$
On entry, the length of each element of ch must be at most $255$: maximum string length $\text{}=⟨\mathit{\text{value}}⟩$.
${\mathbf{ifail}}=-99$
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

m01ncf is not threaded in any implementation.

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 $\mathit{O}\left(\mathrm{log}\left(n\right)\right)$, where $n={\mathbf{m2}}-{\mathbf{m1}}+1$, when ${\mathbf{valid}}=\mathrm{.FALSE.}$.

## 10Example

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

### 10.1Program Text

Program Text (m01ncfe.f90)

### 10.2Program Data

Program Data (m01ncfe.d)

### 10.3Program Results

Program Results (m01ncfe.r)