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Foundations of Software DesignFall 2002Marti Hearst

Lecture 13: Queues and Vectors 

 

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Queues

http://www.lib.uconn.edu/DoddCenter/ASC/Wilcox/Students.htm

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Queue

• Container of objects that are inserted and removed according to the principle of– First-in-first-out– FIFO

• Objects can be inserted at any time, but only the least recently inserted can be removed at any time.

• Operations:– Enqueue: put item onto queue– Dequeue: remove item from queue

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A Comparison

• How are queues similar to / different from stacks?

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Queue Running Times

• What is the running time of each operation?• Enqueue

O(1)

• DequeueO(1)

• isEmpty()O(1)

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How are Queues Used?

• Queues are used extensively in– The OS

• For scheduling processes to receive resources

– Computer networking• For keeping storing and sending network packets

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Use of Queues in the OS

http://courses.cs.vt.edu/~csonline/OS/Lessons/Processes/index.html

Processes waitingin a queue

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Use of Queues in Distributed Systems

Animation byRemzi Arpaci-Dusseau

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Sequences, Lists, & Vectors

• There are many ways to implement sequences of items

• In order of abstractness:– Sequence > List > Vector

• Know about Vectors in Java– Can be more intuitive to program with– But can be less efficient– Implements the Enumeration interface

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Java Vector API

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Java Vector API

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Java Vectors

• When you insert an element into a Java Vector, it moves all the elements behind the new one back one position– insertElementAt

• When you delete an element, is moves all the elements behind that one up one position– removeElementAt– removeElement

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Vectors in Java

• How do they differ from arrays?– Can grow the length dynamically– Can insert items into any position– Have a different API (set of method calls)

• What are the running times of the operations?– boolean isEmpty()

• O(1)– Object firstElement()

• O(1)– boolean contains(Object elem)

• O(n)– Object elementAt(int index)

• O(1)

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A Comparison

• How are queues similar to / different from vectors?

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Let’s Write Code!

• Implement a Queue using Vectors• What is the API?

– void enQueue(Object o)– Object deQueue()– int size ()– boolean isEmpty ()– Object front ()

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