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.
||M1, M2, IFAIL
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,
is returned. Second, if a value equal to the sought-after item is not found, the index of the immediate lower value is returned.
Wirth N (2004) Algorithms and Data Structures 35–36 Prentice Hall
- 1: VALID – LOGICALInput
: if VALID
is set to .TRUE. parameter checking will be performed. If VALID
is set to .FALSE. M01NCF will be called without parameter checking, which includes checking that array CH
is sorted in ascending order and the routine will return with
. See Section 8
for further details.
- 2: CH(M2) – CHARACTER(*) arrayInput
: elements M1
contain character data to be searched.
must be sorted in ascending order. The length of each element of CH
must not exceed
. Trailing space characters are ignored.
- 3: M1 – INTEGERInput
: the index of the first element of CH
to be searched.
- 4: M2 – INTEGERInput
: the index of the last element of CH
to be searched.
- 5: ITEM – CHARACTER(*)Input
On entry: the sought-after item. Trailing space characters are ignored.
- 6: IFAIL – INTEGERInput/Output
must be set to
. If you are unfamiliar with this parameter you should refer to Section 3.3
in the Essential Introduction for details.
For environments where it might be inappropriate to halt program execution when an error is detected, the value
is recommended. If the output of error messages is undesirable, then the value
is recommended. Otherwise, if you are not familiar with this parameter, the recommended value is
. When the value is used it is essential to test the value of IFAIL on exit.
unless the routine detects an error or a warning has been flagged (see Section 6
6 Error Indicators and Warnings
If on entry
, 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 .)
On entry, elements M1
are not sorted in ASCII order.
On entry, .
On entry, .
On entry, the length of elements of CH
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
The time taken by M01NCF is , where , when .
This example reads a list of character data and sought-after items and performs the search for these items.
9.1 Program Text
Program Text (m01ncfe.f90)
9.2 Program Data
Program Data (m01ncfe.d)
9.3 Program Results
Program Results (m01ncfe.r)