cse 2221 - stackweb.cse.ohio-state.edu/software/2221/web-sw1/extras/slides/32.stack.pdfstack • the...

Stack

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Stack

• The Stack component family allows you to manipulate strings of entries of any (arbitrary) type in LIFO (last-in-first-out) order– A kind of “dual” to Queue– Remember, "first" and "last" here refer to the

temporal order in which entries are put into the string and taken out of it, not about the order in the string when it is written down

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Interfaces and Classes

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Stack

Stack1L Stack2

implements implements

StackKernel

extends

Standard

extends

Interfaces and Classes

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Stack

Stack1L Stack2

implements implements

StackKernel

extends

Standard

extendsStandard has contracts

for three methods:clear

newInstancetransferFrom

Interfaces and Classes

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Stack

Stack1L Stack2

implements implements

StackKernel

extends

Standard

extends

StackKernel has contracts for three

methods:pushpop

length

Interfaces and Classes

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Stack

Stack1L Stack2

implements implements

StackKernel

extends

Standard

extends

Stackhas a contract for three

other methods:top

replaceTopflip

Mathematical Model

• The value of a Stack variable is modeled as a string of entries of type T

• Formally:type Stack is modeled bystring of T

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No-argument Constructor

• Ensures:this = < >

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Example

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Code State

Stack<Integer> si =new Stack1L<>();

Example

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Code State

Stack<Integer> si =new Stack1L<>();

si = < >

push

void push(T x)

• Adds x at the top (left end) of this.• Aliases: reference x• Updates: this• Ensures:this = <x> * #this

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Example

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Code Statesi = < 3, 70 >k = 49

si.push(k);

Example

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Code Statesi = < 3, 70 >k = 49

si.push(k);

si = < 49, 3, 70 >k = 49

Example

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Code Statesi = < 3, 70 >k = 49

si.push(k);

si = < 49, 3, 70 >k = 49

Note the alias created here, which you cannot see in the tracing table; you should be able to draw the appropriate diagram showing it.

pop

T pop()

• Removes and returns the entry at the top (left end) of this.

• Updates: this• Requires:this /= < >

• Ensures:#this = <pop> * this

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Example

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Code Statesi = < 49, 3, 70 >z = –584

z = si.pop();

Example

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Code Statesi = < 49, 3, 70 >z = –584

z = si.pop();

si = < 3, 70 >z = 49

length

int length()

• Reports the length of this.• Ensures:length = |this|

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top

T top()

• Returns the entry at the the top (left end) of this.

• Aliases: reference returned by top• Requires:this /= < >

• Ensures:<top> is prefix of this

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Example

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Code Statesi = < 49, 3, 70 >k = –58

k = si.top();

Example

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Code Statesi = < 49, 3, 70 >k = –58

k = si.top();

si = < 49, 3, 70 >k = 49

Example

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Code Statesi = < 49, 3, 70 >k = –58

k = si.top();

si = < 49, 3, 70 >k = 49

Note the alias created here, which you cannot see in the tracing table; you should be able to draw the appropriate diagram showing it.

replaceTop

T replaceTop(T x)

• Replaces the top of this with x, and returns the old top.• Aliases: reference x• Updates: this• Requires:

this /= < >

• Ensures:<replaceTop> is prefix of #this andthis = <x> * #this[1, |#this|)

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Example

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Code Statesi = < 49, 70 >k = –58j = 16

k = si.replaceTop(j);

Example

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Code Statesi = < 49, 70 >k = –58j = 16

k = si.replaceTop(j);

si = < 16, 70 >k = 49j = 16

Example

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Code Statesi = < 49, 70 >k = –58j = 16

k = si.replaceTop(j);

si = < 16, 70 >k = 49j = 16

Note the alias created here, which you cannot see in the tracing table; you should be able to draw the appropriate diagram showing it.

Another Example

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Code Statesi = < 49, 70 >j = 16

j = si.replaceTop(j);

Another Example

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Code Statesi = < 49, 70 >j = 16

j = si.replaceTop(j);

si = < 16, 70 >j = 49

Another Example

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Code Statesi = < 49, 70 >j = 16

j = si.replaceTop(j);

si = < 16, 70 >j = 49

This use of the method avoids creating an alias: it

swaps j with the entry previously at the top.

flip

void flip()

• Reverses (“flips”) this.• Updates: this• Ensures:this = rev(#this)

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Example

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Code State

s1 = < 18, 6, 74 >

s1.flip();

Example

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Code State

s1 = < 18, 6, 74 >

s1.flip();

s1 = < 74, 6, 18 >

Resources

• OSU CSE Components API: Stack– http://cse.osu.edu/software/common/doc/

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