Linked Structures

Traverse and Print

Walk from the head pointer to null, visiting each node exactly once without random indexing.

Algorithm

The replay labels nodes by value, such as node(20), and never exposes object identity or memory addresses. This Fortran DSA implementation uses the same small chain as the rest of the DSA track.

cursor A cursor reference names the node currently being visited.
null stop Traversal ends when the cursor reaches the null marker.

Basic Implementation

basic.f90
Replay: real traced execution (multi-file project)
program linked_list_lesson
    implicit none
    type :: list_node
        integer :: value
        type(list_node), pointer :: next => null()
    end type list_node
    type(list_node), pointer :: head, cursor, new_head
    type(list_node), pointer :: n1, n2, n3, n4

    allocate(n1); n1%value = 10
    allocate(n2); n2%value = 20
    allocate(n3); n3%value = 30
    n1%next => n2; n2%next => n3; n3%next => null()
    head => n1
    call print_chain(head)
contains
    subroutine print_chain(start)
        type(list_node), pointer :: start
        type(list_node), pointer :: cur
        cur => start
        do while (associated(cur))
            write(*, '(I0,A)', advance='no') cur%value, ' -> '
            cur => cur%next
        end do
        write(*, '(A)') 'null'
    end subroutine print_chain
end program linked_list_lesson

  1. head ← 10 -> 20 -> 30 -> null

    1program linked_list_lesson2    implicit none
    values this step10 -> 20 -> 30 -> nullhead

  2. output ← 10

    20cur => start21do while (associated(cur))22    write(*, '(I0,A)', advance='no') cur%value, ' -> '
    values this step10outputnode(10)cursor

  3. output ← 10 -> 20

    20cur => start21do while (associated(cur))22    write(*, '(I0,A)', advance='no') cur%value, ' -> '
    values this step10 -> 20outputnode(20)cursor

  4. output ← 10 -> 20 -> 30 -> null

    20cur => start21do while (associated(cur))22    write(*, '(I0,A)', advance='no') cur%value, ' -> '
    values this step10 -> 20 -> 30 -> nulloutputnode(30)cursor

  5. stdout ← 10 -> 20 -> 30 -> null

    14    head => n115    call print_chain(head)16contains
    values this step10 -> 20 -> 30 -> nullstdout10 -> 20 -> 30 -> nulloutput

Complexity

  • Time: O(n)
  • Space: O(1)

Implementation notes

  • Keep the explicit node and pointer/reference operations; array shortcuts hide the linked-list state this lesson is meant to replay.
  • The final output prints the chain in a deterministic a -> b -> null form for cross-language comparison.