program ordinavett
!
!  legge un file di coppie di numeri e le ordina
!  a x crescente
!  i vettori (allocati dinamicamente) passano ai sottoprogrammi in forma presunta
!

real, allocatable,dimension(:) :: x,y
integer :: n,i

INTERFACE
subroutine readdati (x,y,n)
implicit none

integer, intent(OUT) :: n
real, allocatable,dimension(:) :: x,y
end subroutine readdati

subroutine ordina (x,y,n)
implicit none

integer, intent(IN) :: n
real, dimension(:),intent(OUT):: x,y
end subroutine ordina
END INTERFACE

call readdati(x,y,n)
call ordina(x,y,n)

do i=1,n
   write(6,'(2f8.2)')x(i),y(i)
enddo

end program ordinavett

subroutine readdati (x,y,n)
implicit none

integer, intent(OUT) :: n
real, allocatable,dimension(:) :: x,y
real ::xx,yy

integer :: iend,i
character (len=40) :: nome

1 Write(6,*)'Nome File?'
read(5,'(a)')nome
open (unit=50,status='old',file=nome,err=1)

! questa parte serve per contare i record 

do i=1,1000000
   read(50,*,iostat=iend)xx,yy
   if(iend /= 0)exit
enddo
if(iend > 0)then
   write(6,*)'Errore di lettura :',iend
   stop
endif

n=i-1
write(6,*) 'Lette ',n,' righe'

rewind(50)

! alloco i vettori necessari

allocate(x(n))
allocate(y(n))
!  leggo i dati nei vettori allocati

do i=1,n
   read(50,*)x(i),y(i)
enddo

end subroutine readdati



subroutine ordina (x,y,n)
implicit none

integer, intent(IN) :: n
real, dimension(:),intent(OUT):: x,y

real::t
integer::i,j

write(6,'(i2,10f5.1)')0,(x(i),i=1,n)

do j = n-1, 1 , -1
   do i=1,j
      if(x(i) > x(i+1)) then
         call scambio(x(i),x(i+1))
         call scambio(y(i),y(i+1))
      endif
   enddo

write(6,'(i2,10f5.1)')j,(x(i),i=1,n)

enddo


end subroutine ordina


subroutine scambio(v1,v2)

implicit none
real :: v1,v2,t

t=v1
v1=v2
v2=t

end subroutine scambio