CS200: Queues. Prichard Ch. 8. CS200 - Queues 1

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1 CS200: Queues Prichard Ch. 8 CS200 - Queues 1

2 Queues First In First Out (FIFO) structure Imagine a checkout line So removing and adding are done from opposite ends of structure add to tail (back), remove from head (front) Used in operating systems (e.g. print queue). CS200 -Queues 2

3 Possible Queue Operations enqueue(in newitem: QueueItemType) Add new item at the back of a queue dequeue(): QueueItemType Retrieve and remove the item at the front of a queue peek(): QueueItemType Retrieve item from the front of the queue. Retrieve the item that was added earliest. isempty():boolean createqueue() CS200 - Queues 3

4 Reference-Based Implementation 1 A linked list with two external references A reference to the front A reference to the back 60 Item. Next At which end do we enqueue / dequeue? Reference of the First Node WHY?. Reference of the Last Node CS200 - Stacks 4

5 Reference-Based Implementation 2 A circular linked list with one external reference lastnode references the back of the queue lastnode.getnext() references the front Last Node: node reference CS200 -Queues 5

6 Inserting an item into nonempty queue 1. newnode.next = lastnode.next; 2. lastnode.next = newnode; 3. lastnode = newnode; Last Node. New node CS200 -Queues 6

7 Inserting a New Item Insert a new item into the empty queue 60. Last Node. CS200 -Queues 7

8 Insert new item into the queue public void enqueue (Object newitem) { Node newnode = new Node(newItem); if (isempty()) { A. Empty queue newnode.next = newnode; // point to yourself } else { B. items in queue newnode.next = lastnode.next; // last points to first lastnode.next = newnode; } } lastnode = newnode; // we are now last CS200 -Queues 8

next; if (firstnode == lastnode) { lastnode = null; } else{ lastnode.

9 Removing an item from queue public Object dequeue() throws QueueException { if (!isempty()) { Node firstnode = lastnode.next; if (firstnode == lastnode) { lastnode = null; } else{ lastnode.next = firstnode.next; } return firstnode.item; } else { exception handling } Why? } CS200 -Queues 9

10 Removing an Item What happens to this node? First Node. Last Node Node firstnode = lastnode.next; if (firstnode == lastnode) lastnode = null; else lastnode.next = firstnode.next; return firstnode.item; CS200 -Queues 10

11 Naïve Array-Based Implementation Drift wastes space How do we initialize front and back? (Hint: what does a queue with a single element look like? what does an empty queue look like? ) CS200 -Queues 11

12 Solving Drift: Circular implementation of a queue MAX_QUEUE-1 0 FRONT 6 a e 1 5 o i BACK CS200 -Queues 12

13 Solving Drift: Delete MAX_QUEUE a e 1 5 o i BACK CS200 -Queues 13

14 Solving Drift: Delete MAX_QUEUE e 1 5 o i BACK CS200 -Queues 14

15 Solving Drift Insert u MAX_QUEUE When either front or back advances past MAX_QUEUE-1, it wraps around (to 0: using % MAX_QUEUE) 1 5 u o i 2 FRONT 4 3 CS200 -Queues 15

16 Queue with Single Item back and front are pointing at the same slot. MAX_QUEUE u CS200 -Queues 16

17 Empty Queue: remove Single Item Remove last item. front passed back. MAX_QUEUE u CS200 -Queues 17

18 Insert the last item back catches up to front when the queue becomes full. MAX_QUEUE-1 0 BACK 6 f n a e 1 Problem? Solution? 5 When the queue is FULL, front is one slot ahead of back as well. b u o 4 3 CS200 -Queues i 2 Maintain size: 0:empty max_queue: full 18

19 Wrapping the values for front and back Initializing front = 0 back = MAX_QUEUE-1 count = 0 Adding back = (back+1) % MAX_QUEUE; items[back] = newitem; ++count; Deleting deleteitem = items[front]; front = (front +1) % MAX_QUEUE; --count; CS200 -Queues 19

20 enqueue with Array public void enqueue(object newitem) throws QueueException { if (!isfull()) { back = (back+1) % (MAX_QUEUE); items[back] = newitem; ++count; } } else throw new QueueException(your_message); CS200 -Queues 20

21 dequeue() public Object dequeue() throws QueueException { } if (!isempty()) { Object queuefront = items[front]; front = (front+1) % (MAX_QUEUE); --count; return queuefront; } else throw new QueueException (your_message); CS200 -Queues 21

22 Implementation with (Array)List You can implement operation dequeue() as the list operation remove(0). peek() as get(0) enqueue() as add(newitem) // at tail Front of queue Back of queue a e i o Position in the list CS200 -Queues 22

23 Questions What is an advantage of the circular array implementation over linked list? A. Faster to enqueue B. Uses less memory C. Can more easily fix and enforce a maximum size D. Fewer allocations CS200 -Queues 23

24 Expressions: infix to postfix conversion Prichard: 7.4 Let s do some * 4 2 * (3-4) 1 - (2 + 3 * 4) / 5 observations? CS200 -Queues 24

25 Expressions: infix to postfix conversion * * + 2 * * (3-4) (2 + 3 * 4) / * + 5 / - 1. operand order does not change 2. operators come after second operand and obey associativity and precedence rules 3. ( ) converts the inner expression to an independent postfix expression CS200 -Queues 25

26 infix to postfix implementation Use a queue to create the resulting postfix expression the operands get immediately enqueued Use a stack to store the operators operators get pushed on the stack when to pop and enqueue? let s play CS200 -Queues 26

27 2 + 3 * 4 stack queue action * 4 enqueue + 3 * 4 2 push 3 * enqueue * push * enqueue * pop; enqueue * pop; enqueue * + CS200 -Queues 27

28 2 * stack queue action 2 * enqueue * push * 2 enqueue + 4 * 2 3 push? NO!! Because * has higher precedence than + and so binds to * 2 3 pop; enqueue * push * enqueue * 4 pop; enqueue 2 3 * 4 + CS200 -Queues 28

29 stack queue action enqueue push enqueue push? NO!! Because of left associativity binds to pop; enqueue push enqueue pop; enqueue CS200 -Queues 29

30 2 ( ) stack queue action 2 ( ) enqueue - ( ) 2 push ( ) - 2 enqueue or push? the expression inside the ( ) makes its own independent postfix, so we push the ( then use the stack as before until we see a ) then we pop all the operators off the stack and enqueue them, until we see a ( and delete the ( ( ) - 2 enqueue ( + 4 ) push + ( 4 ) enqueue continued next page CS200 -Queues 30

31 2 ( ) stack queue action + ( 4 ) enqueue + ( ) pop, enqueue until (, delete ( pop, enqueue until stack empty CS200 -Queues 31

32 in2post algorithm when encountering operand: enqueue open: push close: Do it for: 1-(2+3*4)/5 pop and enqueue operators, until open on stack pop open operator: if stack empty or top is open push, else pop and enqueue operators with greater or equal precedence, until operator with lower precedence on stack, or open on stack, or stack empty end of input: pop and enqueue all operators until stack empty CS200 -Queues 32

33 What about unary operators? e.g. not in logic expressions such as: not true and false not ( true or false ) not not true not has higher priority than and, true and not false is true and (not false) and has higher priority than or not is right associative not not true is not ( not true ) CS200 -Queues 33

34 not true and false not true and false stack queue action push true and false not enqueue and false not true not higher priority pop, enqueue not and false true not push false and true not enqueue and true not false pop, enqueue true not false and CS200 -Queues 34

35 not(true or false) not (true or false) stack queue action push (true or false) not push ( true or false) not enqueue ( or false ) not true push or ( false ) not true enqueue CS200 -Queues 35

36 not(true or false) continued stack queue action or ( false ) not true enqueue or ( pop, enqueue ) not true false until ( not true false or pop, enqueue true false or not CS200 -Queues 36

37 not not true stack queue action not not true push not true not push or enqueue? push! not is right associative, its operand is ahead of it not true not enqueue not not true pop and enqueue true not not CS200 -Queues 37

38 in2post algorithm when encountering operand: enqueue open: push close: pop and enqueue operators, until open on stack pop open end of input: pop and enqueue all operators until stack empty CS200 -Queues 38

39 in2post continued when encountering and, or: if stack empty or top is open push, else pop and enqueue operators with greater or equal precedence, until operator with lower precedence on stack, or open on stack, or stack empty not: push do it for not (not true or false) CS200 -Queues 39

1/22/13. Queue? CS 200 Algorithms and Data Structures. Implementing Queue Comparison implementations. Photo by David Jump 9. Colorado State University

1/22/13. Queue? CS 200 Algorithms and Data Structures. Implementing Queue Comparison implementations. Photo by David Jump 9. Colorado State University //3 Part Queues CS Algorithms and Data Structures Outline Queue? Implementing Queue Comparison implementations 8 Photo by David Jump 9 //3 "Grill the Buffs" event Queue A queue is like a line of people