Week 06 (A)

Week 06 (A)

1/13

In This Lecture ...

• Varieties of linked lists: Stacks and Queues (Sec.10.2)

Coming Up ...

• Abstract Data Types (Sec.10.5)

Nerds You Should Know #5

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... Nerds You Should Know #5

3/13

Alan Turing

Founder of Computer Science 1930's Maths/Physics at Cambridge 1936 The Turing Machine (framework for computability theory) 1940-45 Code breaker (cracked Enigma code at Bletchley Park) 1946-50 Designed early computer Papers on neural nets, programming, chess computers 1950 Posed the "Turing Test" for AI 1954 Suicide by poisoned apple

Biography:   "Alan Turing: The Enigma"   by   Andrew Hodges

Varieties of Linked List

4/13

By varying insertion/deletion strategies

• can create different styles of lists
• useful for a variety of purposes

• stacks: last-in, first-out (LIFO)
  (can be used for parsing, expression evaluation, etc.)
• queues: first-in, first-out (FIFO)
  (can be used for discrete event simulation, etc.)

Stacks

5/13

Insert and delete are called push and pop

• push(S,V) ... places value V on top of stack S
• pop(S) ... remove and return top element from S

Stack s = emptyStack();

push(s,10); push(s,15); push(s,7);
x = pop(s); push(s,12); y = pop(s); 

// x == 7  &&  y == 12

Example animation

... Stacks

6/13

Applications for stacks:

• page-visited history in a Web browser
• undo sequence in a text editor
• checking for balanced brackets
• postfix calculator (next slide)
• chain of function calls in a program
• are examples of LAST IN, FIRST OUT (LIFO)

Exercise: Reverse Polish Notation

7/13

Normal arithmetic uses infix notation, e.g. 1 + 2

Infix places operator between operands.

An alternative notation puts operator last, e.g. 1 2 +

This postfix style is called reverse polish notation (RPN).

Advantages: no parentheses needed to disambiguate.

... Exercise: Reverse Polish Notation

8/13

Implement a reverse polish calculator.

Example:

1 2 +                  (1 + 2)
= 3
3 2 *                  (3 * 2)
= 6
4 3 + 6 *              ((4 + 3) * 6)
= 42
1 2 3 4 + * +          ((3 + 4) * 2) + 1)
= 15
1 2 + 5 * 7 - 4 /      ((((1 + 2) * 5) - 7) / 4)
= 2

... Exercise: Reverse Polish Notation

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Main loop:

• read a line of input, split into tokens (splitLine())
• scan tokens, evaluate as RPN to compute result
• print result, and repeat

• if next token is number: push onto stack
• if next token is operator: pop, pop, apply, push

Exercise: Stack Data Type

10/13

Implement a Stack (of ints) data type using linked lists:

typedef struct stackNode { ... } StackNode;
typedef struct stackRep { ... } StackRep;
typedef StackRep *Stack;

Stack emptyStack();   // make an empty Stack
int height(Stack s);  // # values in Stack
int isEmpty(Stack s); // is Stack empty?
push(Stack s, int v); // add value on top 
int pop(Stack s);     // remove top value

Queues

11/13

Insert and delete are called enqueue and dequeue

• enqueue(Q,V) ... places value V at the tail of Q
• dequeue(Q) ... remove and return element from head of Q

Queue q = emptyQueue();

enqueue(q,10); enqueue(q,15); enqueue(q,7);
x = dequeue(q); enqueue(q,12); y = dequeue(q);

// x == 10  &&  y == 15

Example animation

... Queues

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Applications for queues:

• the checkout at a supermarket
• people queueing to go onto a bus
• objects flowing through a pipe (where they cannot overtake each other)
• chat messages
• web page requests arriving at a web server
• printing jobs arriving at a printer
• are examples of FIRST IN, FIRST OUT (FIFO)

Exercise: Queue Data Type

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Implement a Queue data type using linked lists:

typedef struct queueNode { ... } QueueNode;
typedef struct queueRep { ... } QueueRep;
typedef QueueRep *Queue;

Queue emptyQueue();      // make an empty Queue
int length(Queue q);     // # values in Queue
int isEmpty(Queue q);    // is Queue empty?
enqueue(Queue q, int v); // add value at tail 
int dequeue(Queue s);    // remove head value

Produced: 24 Aug 2016