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3.3 Abstract Data Types
3.3.1 What is an Abstract Data Type?
An abstract data type ADT is a collection of data elements provided with a set of operations that are defined on the data elements. Stacks, queues and binary trees are
some examples of ADT. In this section, you will be learning about stacks and queues.
3.3.2 Stacks
A stack is a set of data elements wherein manipulation of the elements is allowed only at the top of the stack. It is a linearly ordered collection of data on which the discipline of
“last in, first out” LIFO is imposed. Stacks are useful for a variety of applications such as pattern recognition and conversion among infix, postfix and prefix notations.
The two operations associated with stacks are the push and pop operations. Push simply means inserting at the top of the stack whereas pop refers to removing the element at
the top of the stack. To understand how the stack works, imagine how you would add or remove a plate from a stack of plates. Your instinct would tell you to add or remove only
at the top of the stack because otherwise, there is a danger of causing the stack of plates to fall.
Here is a simple illustration of how a stack looks like. n-1
... 6
5 Jayz
top
4 KC
3 Jojo
2 Toto
1 Kyla
DMX bottom
Table 3: Stack illustration
The stack is full if the top reaches the cell labeled n-1. If the top is equal to -1, this indicates that the stack is empty.
3.3.3 Queues
The queue is another example of an ADT. It is a linearly ordered collection of elements which follow the discipline of “first-in, first-out”. Its applications include job scheduling in
operating system, topological sorting and graph traversal.
Enqueue and dequeue are the operations associated with queues. Enqueue refers to inserting at the end of the queue whereas dequeue means removing front element of the
queue. To remember how a queue works, think of the queue’s literal meaning – a line. Imagine yourself lining up to get the chance meeting your favorite star up close. If a new
person would like to join the line, this person would have to go to the end of the line. Otherwise, there would probably be some fight. This is how enqueue works. Who gets to
meet hisher favorite star first among those who are waiting in the line? It should have
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been the first person in the line. This person gets to leave the line first. Relate this to how dequeue works.
Here is a simple illustration of how a queue looks like. 1
2 3
4 5
6 7
8 9
... n-1
Eve Jayz
KC Jojo
Toto Kyla
DMX
front end
← Insert
→ Delete
Table 4: Queue illustration
The queue is empty if end is less than front. Meanwhile, it is full when the end is equal to n-1.
3.3.4 Sequential and Linked Representation
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