! This file: http://ftp.aset.psu.edu/pub/ger/fortran/hdk/TestSortAM.f90
!
! An implementation of QuickSort in functional programming style
! by Arjen Markus, as posted at comp.lang.fortran on 13 March 2006

program sortdata
 real, dimension(1000) :: data

 call random_number( data )

 write(*,*) 'First 20: '
 write(*,'(5f10.4)') data(1:20)
 write(*,*) 'Last 20: '
 write(*,'(5f10.4)') data(1000-21:1000)

 data = qsort_reals( data )
 write(*,*) 'Sorted:'
 write(*,*) 'First 20: '
 write(*,'(5f10.4)') data(1:20)
 write(*,*) 'Last 20: '
 write(*,'(5f10.4)') data(1000-21:1000)

contains
recursive function qsort_reals( data ) result( sorted )
 real, dimension(:), intent(in) :: data
 real, dimension(1:size(data))  :: sorted

 if ( size(data) > 1 ) then
     sorted = (/ qsort_reals( pack( data(2:), data(2:) > data(1) ) ), &
                 data(1), &
                 qsort_reals( pack( data(2:), data(2:) <= data(1) ) ) /)
 else
     sorted = data
 endif
end function
end program