CS255 Syllabus

How Linked List are Stored inside the Computer

One of the few data structures that will become very clear in assembler is the linked list because you will need to manipulate the low level addresses to traverse the entire list
A computer program needs information to do its work (solve some problem)
A linked list is one of the many data structures that Computer Science has developed to manage/organize information
The linked list is a dynamic structure, i.e., the number of elements in a linked list can increase and decrease in time
(In contrast, the array data structure is static, number of elements stays constant)

Structure of a linked list element

A linked list element always consists of 2 parts...
A data part consisting of one or more variables that hold the actual information you want to stored
A linkage part consisting of one or more reference variables that hold addresses of the neighboring linked list elements

Symbolically:

    Linked list element:

    +-----------------------+
    |			    |
    |			    | <---- data part (variables that
    |			    |	    hold actually information
    |			    |	    that you want to store
    +-----------------------+
    |			    | <---- linkage part (reference variable
    |			    |	    containing address of neighboring
    +-----------------------+	    list elements

Structure of a linked list

A linked list consists of zero or more linked list elements chained together by their linkage parts.
The special reference value null indicates "no more elements"
The data parts of the elements chained would contain the actual information stored

Symbolically:

    A simple linked list:

    +--------+    -->+--------+    -->+--------+
    | info1  |   /   | info2  |   /   | info3  |
    |        |  /    |        |  /    |        |
    +--------+ /     +--------+ /     +--------+
    | ref1   |-      | ref2   |-      | null   |
    +--------+       +--------+       +--------+

The linked list elements are linked together by using references which are addresses

Each linked list element is located somewhere in memory, and thus, each linked list element has a starting memory address

The linkage reference variables contains the address of the next linked list element

Example: the above linked list of 3 element would be linked together as follows:

 We have to make some assumptions to do the example.

 The assumptions are:
	First element of linked is is located at address 8000
	Second element of linked is is located at address 10000
	Third element of linked is is located at address 9000

 Memory:
        |               |
        +---------------+
  8000  |  info1        |  Linked list element 1
        +---------------+
        | ref1=10000    |-------------------------------------+
        +---------------+                                     |
        |               |                                     |
        +---------------+                                     |
	  .......                                             |
	  .......                                             |
        +---------------+                                     |
  9000  |  info3        |  Linked list element 3 <----+       |
        +---------------+                             |       |
        | ref3=0 (null) |                             |       |
        +---------------+                             |       |
	  .......                                     |       |
	  .......                                     |       |
        +---------------+                             |       |
 10000  |  info2        |  Linked list element 2      | <-----+
        +---------------+                             |
        | ref2=9000     |-----------------------------+
        +---------------+

Notice that although I drew arrows from one linked list element to the next one, these arrows are just imaginary ones represented by the address value contained by the "ref" variables
(A reference in Computer Science is the same as a memory address !)

To access the entire linked list (i.e., to access all elemenets in the linked list), you (only) need to know the location (address) of the first list element

That is why you need an extra variable usually called head that contains a reference (address) of the first element in the linked list:

Symbolically:

    The user view of a linked list:

   head
    +--------+     +--------+    -->+--------+    -->+--------+
    |        |---->| info1  |   /   | info2  |   /   | info3  |
    +--------+     |        |  /    |        |  /    |        |
                   +--------+ /     +--------+ /     +--------+
                   | ref1   |-      | ref2   |-      | null   |
                   +--------+       +--------+       +--------+

This is what happens in reality:

 Memory:
        +---------------+
  head: |  8000         |-------------------------------+
        +---------------+                               |
        |               |                               |
	  .......                                       |
	  .......                                       |
        +---------------+                               |
  8000  |  info1        |  Linked list element 1  <-----+
        +---------------+
        | ref1=10000    |-------------------------------------+
        +---------------+                                     |
        |               |                                     |
        +---------------+                                     |
	  .......                                             |
	  .......                                             |
        +---------------+                                     |
  9000  |  info3        |  Linked list element 3 <----+       |
        +---------------+                             |       |
        | ref3=0 (null) |                             |       |
        +---------------+                             |       |
	  .......                                     |       |
	  .......                                     |       |
        +---------------+                             |       |
 10000  |  info2        |  Linked list element 2      | <-----+
        +---------------+                             |
        | ref2=9000     |-----------------------------+
        +---------------+