refpar Interface

prank_basic(3f) - [M_orderpack:RANK:PARTIAL] partially ranks an array
             (Quick-Sort)

 Subroutine Prank_Basic (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=*)

creates index IRNGT() which partially ranks input array INVALS(),
up to order NORD.

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

Internally this routine uses a pivoting strategy such as the one used
in finding the median based on the Quick-Sort algorithm. It uses a
temporary array, where it stores the partially ranked indices of the
values. It iterates until it can bring the number of values lower
than the pivot to exactly NORD, and then uses an Insertion-Sort to
rank this set, since it is supposedly small.

 INVALS      array to partially rank
 IRNGT      array to hold indices of ranked elements
 NORD       number of elements to rank

program demo_prank
! create index to lowest N values in input array in ascending order
use,intrinsic :: iso_fortran_env, only : int32, real32, real64
use M_orderpack, only : prank_basic
implicit none
real(kind=real32) :: valsr(2000)
integer           :: indx(2000)
integer           :: i
real,allocatable  :: results(:)
   ! create some random data
   call random_seed()
   call random_number(valsr)
   valsr=valsr*1000000.0-500000.0
   ! get 300 lowest values sorted
   call prank_basic(valsr,indx,300)
   !
   results=valsr(indx(:300))
   ! check if sorted
   do i=1,300-1
      if (results(i+1).lt.results(i))then
         write(*,*)'ERROR: not sorted'
         stop 1
      endif
   enddo
   write(*,*)'random array now sorted'
end program demo_prank

 random array now sorted

Michel Olagnon - Feb. 2000

John Urban, 2022.04.16

CC0-1.0