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,
$0$ is returned. Second, if a value equal to the sought-after item is not found, the index of the immediate lower value is returned.
If on entry
${\mathbf{ifail}}=0$ or
$-1$, explanatory error messages are output on the current error message unit (as defined by
x04aaf).
Not applicable.
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.
This example reads a list of double precision numbers and sought-after items and performs the search for these items.