• We have studied linked list in C++: click here

• The techqinues to manipulate linked lists is unchanged in F90

• The only change is how to express the manipulations

• We will only discuss insert and delete at the start of the list

• I use the same list structure as in the examples of C++:

  MODULE ListModule TYPE ListElem REAL :: value; TYPE(ListElem), POINTER :: next; END TYPE ListElem END MODULE

• F90 does not have a NULL value...

• To assign a NULL value to a POINTER variable use the intrinsic function NULLIFY()

  Example:

  TYPE(ListElem), POINTER :: head NULLIFY(head) ! head = NULL

  ---

• To test if a POINTER variable is NULL use the intrinsic function ASSOCIATED

  Example:

  TYPE(ListElem), POINTER :: head ... if ( .NOT. ASSOCIATED( head ) THEN ! if ( head != NULL ) ... else ... end if

• The empty list is always represented as follows:

• This can be accomplished by the following statement:

  MODULE ListModule TYPE ListElem REAL :: value; TYPE(ListElem), POINTER :: next; END TYPE ListElem END MODULE PROGRAM list USE ListModule TYPE(ListElem), POINTER :: head NULLIFY(head) END PROGRAM

• Recall that the ALLOCATE (intrinsic) function is used to allocate memory for dynamic data structures

• To allocate memory of a linked list element :

  TYPE(ListElem), POINTER :: p ALLOCATE(p) !! C++: "p = new ListElem();"

• The following diagrams show insert algorithm in F90
  1. This is the initial EMPTY list

     TYPE(ListElem), POINTER :: head TYPE(ListElem), POINTER :: elem NULLIFY(head) ! head = NULL

     Result:

     ---

  2. Make a new list element

     ALLOCATE( elem ) elem%value = 1500

     Result:

     ---

  3. Link new element at start of list:

     elem%next => head // Make new list element points // to the start of "old list" head => elem // Make "head" points to // the **new** starting object

     Result:

• Input:

  head : start of a linked list elem: start of a linked list

• Output:

  head of the new linked list

  ---

• Insert at head function:

  FUNCTION InsertList(head, elem) USE ListModule IMPLICIT NONE type( ListElem ), pointer :: head, elem type( ListElem ), pointer :: InsertList elem%next => head InsertList => elem END FUNCTION

• Input:

  head : start of a linked list

• Output:

  head of the new linked list

  ---

• Delete List function:

  FUNCTION DeleteList(head) USE ListModule IMPLICIT NONE type( ListElem ), pointer :: head type( ListElem ), pointer :: DeleteList type( ListElem ), pointer :: h IF ( associated( head ) ) THEN h = head ! Use to deallocate head => head%next ! 2nd elem is now head deallocate(h) ! Deallocate old first elem END IF DeleteList => head END FUNCTION

• Example program:

  PROGRAM LinkedList USE ListModule INTERFACE SUBROUTINE PrintList(head) USE ListModule type( ListElem ), pointer :: head END SUBROUTINE FUNCTION InsertList(head, elem) USE ListModule type( ListElem ), pointer :: head, elem type( ListElem ), pointer :: InsertList END FUNCTION FUNCTION DeleteList(head) USE ListModule type( ListElem ), pointer :: head type( ListElem ), pointer :: DeleteList END FUNCTION END INTERFACE ! --------------------------------------------- ! Start of main program ! --------------------------------------------- type( ListElem ), pointer :: head type( ListElem ), pointer :: newElem integer, parameter :: N = 4 NULLIFY( head ) ! Empty list ! --------------------------------------------- ! Add the N elements ! --------------------------------------------- DO i = 1, N ALLOCATE( newElem ) CALL random_number( newElem%f ) head => InsertList(head, newElem) CALL PrintList(head) END DO head => DeleteList(head) END PROGRAM LinkedList

  ---

• Example Program: (Demo above code)                                                
  • Prog file: click here

PS: Nothing has changed in the theory. You should be able to figure out inserting at different places yourself...