Linked lists 1

Mr. Forkan Karim Mazumder
Faculty
Dept. of Computer Science and
Engineering

Linked Lists
One way to store data is in arrays.
But arrays have certain disadvantages
such as it is relatively expensive to
insert and delete elements in an
array. Also, since an array usually
occupies a block of memory space,
one cannot simply

double or triple the size of an array
when additional space is required.

Another way of storing a list in
memory is to have each element in
the list contain a field, called a link or
pointer, which contains the address
of the next element in the list.

Thus successive elements in the list
need not occupy adjacent space in
memory. This will make it easier to
insert and delete elements in the list.
So for inserting, deleting, searching
data, it is better to use list with
pointers rather than array. This type
of data structure is called linked list.

A linked list, or one-way list, is a
linear collection of data elements,
called nodes, where the linear order
is given by means of pointers. That
is, each node is divided into two
parts: the first part contains the
information of the element, and the
second part, called the link field or
next pointer field, contains the

address of the next node in the list.
Figure below is a schematic diagram
of a linked list with 6 nodes. Each
node is pictured with two parts. The
left part represents the information
part of the node, which may contain
an entire record of data items (e.g.
NAME, ADDRESS . . .).

The right part represents the next
pointer field of the node, and there
is an arrow drawn from it to the next
node in the list. This follows the
usual practice of drawing an arrow
from a field to a node when the
address of the node appears in the
given field. The pointer of the last
node contains a special value, called

the null pointer, which is any invalid
address.

(In actual practice, 0 or a negative
number is used for the null pointer.)
The null pointer, denoted by x in the
diagram, signals the end of the list.
The linked list also contains a list
pointer variable-called START or
NAME-which contains the address of
the first node in the list; hence there
is an arrow drawn from START to

the first node. Clearly, we need only
this address in START to trace
through the list. A special case is the
list that has no nodes. Such a list is
called the null list or empty list and
is denoted by the null pointer in the
variable START.

EXAMPLE
A hospital ward contains 12 beds, of
which 9 are occupied as shown in Fig
below. Suppose we want an
alphabetical listing of the patients.
This listing may be given by the
pointer field, called Next in the
figure. We use the variable START to
point to the first patient.

Hence START contains 5, since the
first patient, Adams, occupies bed 5.
Also, Adams's pointer is equal to 3,
since Dean, the next patient,
occupies bed 3; Dean's pointer is 11,
since Fields, the next patient,
occupies bed 11; and so on. The
entry for the last patient (Samuels)
contains the null pointer, denoted by

O. (Some arrows have been drawn to
indicate the listing of the first few
patients.)

REPRESENTATION OF LINKED LISTS IN
MEMORY
Let LIST be a linked list. Then LIST
will be maintained in memory,
unless otherwise specified or
implied, as follows. First of all, LIST
requires two linear arrays-we will
call them here INFO and LINK-such
that INFO[K] and LINK[K] contain,

respectively, the information part
and the next pointer field of a node
of LIST. As noted above, LIST also
requires a variable name-such as
START-which contains the location of
the beginning of the list, and a next
pointer denoted by NULL-which
indicates the end of the list.

Since the subscripts of the arrays
INFO and LINK will usually be
positive, we will choose NULL = 0,
unless otherwise stated.

EXAMPLE:
Figure pictures a linked list in
memory where each node of the list
contains a single character. We can
obtain the actual list of
characters, or, in other words, the
string, as follows:
START = 9, so INFO[9] = N is the first
character.

LINK[9] = 3, so INFO[3] = 0 is the
second character.
LINK[3] = 6, so INFO[6] = 0 (blank) is
the third character.
LINK[6] = 11, so INFO[11] = E is the
fourth character.
LINK[11] = 7, so INFO[7]= X is the
fifth character.

LINK[7] = 10, so INFO 0] = I is the
sixth character.
LlNK[10] = 4, so INFO] = T is the
seventh character.
LlNK[4] = 0, the NULL value, so the
list has ended.
In other words, NO EXIT is the
character string.

We require two tests. First we have
to check to see whether we have
reached the end of the list; i.e., first
we check to see whether
PTR= NULL
If not, then we check to see whether
INFO[PTR] = ITEM

The two tests cannot be performed
at the same time, since INFO[PTR] is
not defined when PTR = NULL.

TRAVERSING A LINKED LIST
Our traversing algorithm uses
a pointer variable PTR which points
to the node that is currently being
processed. Accordingly, LINK[PTR]
points to the next node to be
processed. The assignment
PTR := LINK[PTR]

moves the pointer to the next node
in the list.

Algorithm :
(Traversing a Linked List) Let LIST be a
linked list in memory. This algorithm
traverses LIST, applying an operation
PROCESS to each element of LIST.
The variable PTR points to the node
currently being processed.

1. Set PTR := START. [Initializes pointer
PTR].
2.Repeat Steps 3 and 4 while PTR #NULL.
3.Apply PROCESS to INFO[PTR].
4.Set PTR:=LINK[PTR]. [PTR now points
to the next node.]
[End of Step 2 loop.]
5.Exit.

The following procedure presents
about how traversing a linked list.
They are similar to the above
algorithm.

Procedure : PRINT(INFO, LINK,
START)
This procedure prints the
information at each node of the list.
1. Set PTR := START.
2. Repeat Steps 3 and 4 while PTR #
NULL:
3. Write: INFO[PTR].

4. Set PTR := LINK[PTR]. [Updates
pointer.]
5. Return.
This procedure finds elements in a
linked list.

Procedure: COUNT (INFO, LINK,
START, NUM)
1. Set NUM := 0. [Initializes
counter.]
2. Set PTR := START. Initializes
pointer.]
3. Repeat Steps 4 and 5 while PTR #
NULL.

4. Set NUM := NUM+1.
[Increases NUM by 1]
5. Set PTR:=LINK[PTR]. [Updates
pointer.]
6. Return.

Linked lists 1

More Related Content

What's hot

Similar to Linked lists 1

Linked lists 1