Algorithms and Data Structures

N.Wirth. Algorithms and Data Structures. Oberon version 137
q
8
27
p
13
27 21 13 8 21
Fig. 4.8. Insertion before p^.
The trick evidently consists of actually inserting a new component after p^ and thereafter interchanging the
values of the new element and p^.
Next, we consider the process of list deletion. Deleting the successor of a p^ is straightforward. This is
shown here in combination with the reinsertion of the deleted element at the head of another list (designated
by q). Figure 4.9 illustrates the situation and shows that it constitutes a cyclic exchange of three pointers.
r := p.next; p.next := r.next; r.next := q; q := r
q
q
p
Fig. 4.9. Deletion and re-insertion
The removal of a designated element itself (instead of its successor) is more difficult, because we
encounter the same problem as with insertion: tracing backward to the denoted element's predecessor is
impossible. But deleting the successor after moving its value forward is a relatively obvious and simple
solution. It can be applied whenever p^ has a successor, i.e., is not the last element on the list. However, it
must be assured that there exist no other variables pointing to the now deleted element.
We now turn to the fundamental operation of list traversal. Let us assume that an operation P(x) has to
be performed for every element of the list whose first element is p^. This task is expressible as follows:
WHILE list designated by p is not empty DO
perform operation P;
proceed to the successor
END
In detail, this operation is described by the following statement:
WHILE p # NIL DO
P(p); p := p.next