program bubble_sort

 implicit none       

 integer, allocatable  :: a(:)
 integer               :: i,n

 do
   write(*,*) 'Array length'
   read(*,*) n
   if (n.gt.0) exit
 enddo

 allocate(a(n))

 write(*,*) 'Initial array'
 read(*,*) a

 call bubble(a,n)

 write(*,*)'Sorted array'
 write(*,*) a 

 deallocate(a)

 stop

end program bubble_sort


subroutine bubble(a,n)

 implicit none       

 integer, intent(in)    :: n
 integer, intent(inout) :: a(n)
 integer                :: i,j

 do i=1,n
    do j=2,n-(i-1)
       if (a(j-1).gt.a(j)) then
          call scambia(a(j-1),a(j))
       endif    
    enddo
 enddo

 return

end subroutine bubble


subroutine scambia(x,y)

 implicit none

 integer, intent(inout)    :: x,y

 integer                   :: tmp

 tmp=x
 x=y
 y=tmp

 return

end subroutine scambia