The routine may be called by the names m01naf or nagf_sort_realvec_search.
m01naf 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: – LogicalInput
On entry: if valid is set to .TRUE. argument checking will be performed. If valid is set to .FALSE. m01naf will be called without argument checking (which includes checking that array rv is sorted in ascending order) and the routine will return with . See Section 9 for further details.
2: – Real (Kind=nag_wp) arrayInput
On entry: elements
contain real values to be searched.
of rv must be sorted in ascending order.
3: – IntegerInput
On entry: the index of the first element of rv to be searched.
4: – IntegerInput
On entry: the index of the last element of rv to be searched.
5: – Real (Kind=nag_wp)Input
On entry: the sought-after item.
6: – IntegerInput/Output
On entry: ifail must be set to , or to set behaviour on detection of an error; these values have no effect when no error is detected.
A value of causes the printing of an error message and program execution will be halted; otherwise program execution continues. A value of means that an error message is printed while a value of means that it is not.
If halting is not appropriate, the value or is recommended. If message printing is undesirable, then the value is recommended. Otherwise, the value is recommended. When the value or is used it is essential to test the value of ifail on exit.
On exit: unless the routine detects an error or a warning has been flagged (see Section 6).
6Error Indicators and Warnings
If on entry or , explanatory error messages are output on the current error message unit (as defined by x04aaf).
An unexpected error has been triggered by this routine. Please
See Section 7 in the Introduction to the NAG Library FL Interface for further information.
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.
Dynamic memory allocation failed.
See Section 9 in the Introduction to the NAG Library FL Interface for further information.
8Parallelism and Performance
Background information to multithreading can be found in the Multithreading documentation.
m01naf 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 rv is in fact arranged in ascending order. If you wish to search the same array rv many times, you are recommended to set valid to .TRUE. on first call of m01naf and to .FALSE. on subsequent calls, in order to minimize the amount of time spent checking rv, which may be significant if rv is large.
The time taken by m01naf is , where , when .
This example reads a list of double precision numbers and sought-after items and performs the search for these items.