Queues and stacks
Metodický koncept k efektivní podpoře klíčových odborných kompetencí s využitím cizího jazyka
ATCZ62 - CLIL jako výuková strategie na vysoké škole
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Stack – ADT
•stores arbitrary objects
•Insertions and deletions follow the LIFO (Last In First Out) scheme
•Think of a spring-loaded plate dispenser
•Main stack operations
•push(object)
•pop(object)
•Auxiliary stack operations:
•object top() integer size() boolean isEmpty()
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Stack
•Exception
•EmptyStackException –pop and top on empty stack
•Direct applications
•Page-visited history in a Web browser
•Undo sequence in a text editor
•Chain of method calls in the Java Virtual Machine
•Indirect applications
•Auxiliary data structure for algorithms
•Component of other data structures
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Array-based Stack
•A simple way of implementing the Stack ADT uses an array
•We add elements from left to right
•A variable keeps track of the  index of the top element
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S
0
1
2
t
…
Algorithm size()
return t + 1
Algorithm pop()
if isEmpty() then
throw EmptyStackException
 else
t ¬ t - 1
return S[t + 1]

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Array-based Stack
•Performance
•Let n be the number of elements in the stack
•The space used is O(n)
•Each operation runs in time O(1)
•Limitations
•The maximum size of the stack must be defined a priori and cannot be changed
•Trying to push a new element into a full stack causes an implementation-specific exception

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Queue
•The Queue ADT stores arbitrary objects
•Insertions and deletions follow the first-in first-out scheme
•Main queue operations:
•enqueue(object): inserts an element at the end of the queue
•object dequeue(): removes and returns the element at the front of the queue
•Auxiliary queue operations:
•object front(): returns the element at the front without removing it
•integer size(): returns the number of elements stored
•boolean isEmpty(): indicates whether no elements are stored

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Queue
•Exceptions
•Attempting the execution of dequeue or front on an empty queue throws an EmptyQueueException
•Direct applications
•Waiting lists, bureaucracy
•Access to shared resources (e.g., printer)
•Multiprogramming
•Indirect applications
•Auxiliary data structure for algorithms
•Component of other data structures

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Array-based Queue
•Use an array of size N in a circular fashion
•Two variables keep track of the front and rear
•f index of the front element
•r index immediately past the rear element
•Array location r is kept empty
Q
0
1
2
r
f
Normal configuration
Q
0
1
2
f
r
Circular configuration

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Array-based Queue
Algorithm size()
return (N - f + r) mod N
Algorithm isEmpty()
return (f = r)
Algorithm enqueue(o)
if size() = N - 1 then
throw FullQueueException
 else
Q[r] ¬ o
r ¬ (r + 1) mod N
Algorithm dequeue()
if isEmpty() then
throw EmptyQueueException
 else
o ¬ Q[f]
f ¬ (f + 1) mod N
return o