Example: Linked Lists

Pointer components may ``point to'' objects of any intrinsic type, any derived type previously defined or to the type being currently defined. Note that it is not possible to have a target type which has yet to be defined.

It is therefore possible to construct linked lists using derived types and pointer components. For example, objects of the type Node contain a value, an id number, a pointer to the previous node and one to the next node in the double linked list.

module CircularList
implicit none
type node
  integer :: id 
  real :: value
  type(node), pointer :: prev, next
end type node
!  
contains 
!
   function BuildCircularList(n)
     implicit none
     integer, intent(in) :: n 
     type(node), pointer :: BuildCircularList
     integer :: i, ierr
     type(node), pointer :: head, temp, curr 
     !
     allocate(temp,stat=ierr) 
     if (ierr.ne.0) then 
       print*, "allocation error"; return 
     end if 
     head%id = 1; head%value = 0.0 
     !
     curr => head 
     do i=2,n 
        allocate(temp,stat=ierr) 
        if (ierr.ne.0) then 
           print*, "allocation error"; return 
        end if 
        temp%id = i; temp%value = 0.0 
        curr%next => temp; temp%prev => curr; curr => temp
     end do 
     ! the next 2 lines circularly ``close'' the list
     curr%next => head; head%prev => curr 
     BuildCircularList => head 
  end function BuildCircularList
!  
end module CircularList

program test 
use CircularList
implicit none
  type(node), pointer :: f, b 
  f => BuildCircularList(3) 
  b => f 
  do i=1,7 
    print*, f%id, f%value, b%id, b%value
    f => f%next; b => b%prev
  end do 
end program test