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C Library Functions  - prank (3)

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NAME

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

CONTENTS

Synopsis
Description
Options
Examples
Author
Maintainer
License

SYNOPSIS

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)

DESCRIPTION

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.

OPTIONS

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

EXAMPLES

Sample program:

   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

Results:

    random array now sorted

AUTHOR

Michel Olagnon - Feb. 2000

MAINTAINER

John Urban, 2022.04.16

LICENSE

CC0-1.0

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Nemo Release 3.1

prank (3)

July 22, 2023

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