rinpar Interface

prank_special(3f) - [M_orderpack:RANK:PARTIAL] partially ranks an array in
             ASCENDING order (Insertion Sort)

 Subroutine Prank_Special (INVALS, IRNGT, NORD)

  ${TYPE} (kind=${KIND}), Intent (In) :: INVALS(:)
  Integer, Intent (Out)               :: IRNGT(:)
  Integer, Intent (In)                :: NORD

Where ${TYPE}(kind=${KIND}) may be
o Real(kind=real32)
o Real(kind=real64)
o Integer(kind=int32)
o Character(kind=selected_char_kind("DEFAULT"),len=*)

Partially ranks INVALS(). Returns IRNGT(1:NORD) filled with the
indices of the lowest values in the array INVALS. More technically,
it does a partial ranking of the array INVALS of order NORD.

NORD is restricted to the range 1 to size(IRNGT).

Internally, this version is not optimized for performance, and is
thus not as difficult to read as some other ones.

The subroutine uses an insertion sort, limiting insertion to the
first NORD values. It does not use any work array and is fastest when
NORD is very small (2-5). but worst case behavior can easily happen
(ie. if INVALS initially is inverse sorted). Therefore, In many cases,
the refined Quick-sort method is faster.

 INVALS   array to partially sort
 NORD    number of indices to return, restricted to 1 to size(IRNGT)

 IRNGT   indices of requested number (NORD) of lowest values in INVALS

program demo_prank_special
! partially rank N lowest values in an array
use M_orderpack, only : prank_special
implicit none
character(len=*),parameter :: g='(*(g0,1x))'
integer,allocatable :: INVALS(:)
integer,allocatable :: irngt(:)
integer :: nord
INVALS=[10,5,7,1,4,5,6,8,9,10,1]
nord=5
allocate(irngt(nord))
   write(*,g)'ORIGINAL:',INVALS
   call prank_special(INVALS,irngt,nord)
   write(*,g)'NUMBER OF INDICES TO RETURN:',nord
   write(*,g)'RETURNED INDICES:',irngt
   write(*,g)nord,'SMALLEST VALUES:',INVALS(irngt(:nord))
end program demo_prank_special

ORIGINAL: 10 5 7 1 4 5 6 8 9 10 1
NUMBER OF INDICES TO RETURN: 5
RETURNED INDICES: 4 11 5 2 6
5 SMALLEST VALUES: 1 1 4 5 5

Michel Olagnon - Feb. 2000

John Urban, 2022.04.16

CC0-1.0