rnkpar Interface

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

 Subroutine Prank (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)

Partially ranks array INVALS(), returning indices of the requested
number of elements (NORD) in index array IRNGT(), where NORD is the
order ( aka. the number of sorted elements requested).

This routine is refined for speed, and uses a pivoting strategy such
as the one of finding the median based on the quick-sort algorithm,
but we skew the pivot choice to try to bring it to NORD as fast as
possible. It uses two temporary arrays, where it stores the indices of
the values smaller than the pivot (ILOWT), and the indices of values
larger than the pivot that we might still need later on (IHIGT). It
iterates until it can bring the number of values in ILOWT to exactly
NORD, and then uses an insertion sort to rank this set, since it is
supposedly small.

 INVALS      array to rank the elements of
 IRNGT      returned ranks
 NORD       number of rank values to return

program demo_prank
! partially rank array
use,intrinsic :: iso_fortran_env, only : int32, real32, real64
use M_orderpack, only : prank
implicit none
integer,parameter :: ivals=300
real(kind=real32) :: valsr(2000)
real(kind=real32) :: out(ivals)
integer           :: indx(2000)
integer           :: i
   call random_seed()
   call random_number(valsr)
   valsr=valsr*1000000.0-500000.0
   call prank(valsr,indx,ivals)
   out=valsr(indx(:ivals))
   do i=1,ivals-1
      if (out(i+1).lt.out(i))then
         write(*,*)'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