quick sort in Fortran

quicksort.f

! quicksort.f -*-f90-*-
! Author: t-nissie, some tweaks by 1AdAstra1
! License: GPLv3
! Gist: https://gist.github.com/t-nissie/479f0f16966925fa29ea
!!
recursive subroutine quicksort(a)
  implicit none
  real :: a(:)
  real x, t
  integer :: first = 1, last
  integer i, j

  last = size(a, 1)
  x = a( (first+last) / 2 )
  i = first
  j = last
  
  do
     do while (a(i) < x)
        i=i+1
     end do
     do while (x < a(j))
        j=j-1
     end do
     if (i >= j) exit
     t = a(i);  a(i) = a(j);  a(j) = t
     i=i+1
     j=j-1
  end do
  
  if (first < i - 1) call quicksort(a(first : i - 1))
  if (j + 1 < last)  call quicksort(a(j + 1 : last))
end subroutine quicksort

quicksort.makefile

#-*-Makefile-*- for quicksort
##

## gfortran (GNU Fortran in GCC)
FC=gfortran
FFLAGS=-g -Wall -ffree-form -O2

### ifort (Intel Fortran)
#FC=ifort
#FFLAGS=-g -free -warn all

all: sortreal8
	./$<

sortreal8: sortreal8.o quicksort.o
	$(FC) $(FFLAGS) -o $@ $^

clean:
	rm -f core *.o *.mod sortreal8

sortreal8.f

! sortreal8.f -*-f90-*-
! Author: t-nissie
!!
program sortreal8
  implicit none
  real ::  a(10)
  a( 1) =  1.0d0
  a( 2) =  1.0d0
  a( 3) =  5.0d0
  a( 4) =  3.0d0
  a( 5) = -1.0d0
  a( 6) = 11.0d0
  a( 7) =  1.0d1
  a( 8) = 31.0d0
  a( 9) = -5.0d0
  a(10) =  1.1d0
  call quicksort(a)
  write(6,'(f10.5)') a
end program sortreal8
!Local variables:
!  compile-command: "gfortran -g -Wall -ffree-form -O2 -c sortreal8.f && gfortran -g -Wall -ffree-form -O2 -c quicksort.f && gfortran -g -Wall -ffree-form -O2 -o sortreal8 sortreal8.o quicksort.o && ./sortreal8"
!End:

👏

This is great, thank you!