Stack II

Data StructuresData Structures

The Stack: DefinitionThe Stack: Definition

A stack is an ordered collection of items intowhich new items may be inserted and fromwhich items may be deleted calledthe TOP of the stackA stack is an ordered collection of items intowhich new items may be inserted and fromwhich items may be deleted at one end calledthe TOP of the stack

What is a StackWhat is a Stack

A common data structure incomputing.A common data structure incomputing.

Data items are "popped" and"pushed" (retrieved and stored)from the top of the stack.Data items are "popped" and"pushed" (retrieved and stored)from the top of the stack.

Stacks normally have a maximumsize. It is an error to push itemsonto a full stack, or pop items offan empty stack.Stacks normally have a maximumsize. It is an error to push itemsonto a full stack, or pop items offan empty stack.

LIFO: Last In First OutLIFO: Last In First Out

Stack featuresStack features

ORDERING: maintains order when elements added(new elements are added to the end by default)ORDERING: maintains order when elements added(new elements are added to the end by default)

DUPLICATES: yes (allowed)DUPLICATES: yes (allowed)

OPERATIONS:OPERATIONS:

add element to end of list ('push')add element to end of list ('push')

remove element from end of list ('pop')remove element from end of list ('pop')

isEmpty()isEmpty()

isFull()isFull()

Top()Top()

Stacks in computer scienceStacks in computer science

the stack is one of the most important datastructures in all of computer sciencethe stack is one of the most important datastructures in all of computer science

compilers use stacks to evaluate expressionscompilers use stacks to evaluate expressions

stacks are great for reversing things, matching uprelated pairs of things, and backtrackingalgorithmsstacks are great for reversing things, matching uprelated pairs of things, and backtrackingalgorithms

stack programming problems:stack programming problems:

reverse letters in a string, reverse words in a line, orreverse a list of numbersreverse letters in a string, reverse words in a line, orreverse a list of numbers

examine a file to see if its braces { } and otheroperators matchexamine a file to see if its braces { } and otheroperators match

convert infix expressions to postfix or prefixconvert infix expressions to postfix or prefix

Another implementation of StackAnother implementation of Stack

Stacks can be declared by using structurescontaining two objectsStacks can be declared by using structurescontaining two objects

#define STACKSIZE 100#define STACKSIZE 100

struct stack {struct stack {

int top;int top;

int items[STACKSIZE];int items[STACKSIZE];

};};

struct stack s;struct stack s;

empty()empty()

If (s.top==-1)If (s.top==-1)

Stack is emptyStack is empty

ElseElse

stack is not emptystack is not empty

int empty(struct stack *ps)int empty(struct stack *ps)

{{

if (ps->top == -1)if (ps->top == -1)

return(TRUE);return(TRUE);

elseelse

return(FALSE);return(FALSE);

}}

if (empty(&s)) if (empty(&s))

stack is empty stack is empty

elseelse

stack is not empty stack is not empty

Push()Push()

void push(struct stack *ps, int x)void push(struct stack *ps, int x)

{ ps->items[++(ps->top)] = x; { ps->items[++(ps->top)] = x;

}}

void push(struct stack *ps, int x)void push(struct stack *ps, int x)

{if (ps->top==STACKSIZE-1) {if (ps->top==STACKSIZE-1)

{{

cout<<“stack overflow”;cout<<“stack overflow”;

exit();exit();

}}

ps->items[++(ps->top)] = x; ps->items[++(ps->top)] = x;

return;return;

}}

POP( )POP( )

int pop(struct stack *ps)int pop(struct stack *ps)

{{

if (empty(ps))if (empty(ps))

{{

cout<<stack underflow;cout<<stack underflow;

exit(1);exit(1);

} return (ps->items[ps->top--]);} return (ps->items[ps->top--]);

}}

To use the pop function, the programmer can declare int xand writeTo use the pop function, the programmer can declare int xand write

x=pop(&s);x=pop(&s);