I is for - Duke University€¦ · •Demo: implements Iterable 10/3/2018 Compsci 201, Fall 2018,...

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Compsci 201Linked Lists + Hashing

Midterm DetailsOwen Astrachan [email protected] 3, 2018

10/3/2018 Compsci 201, Fall 2018, Linked Lists + Hashing 1

I is for …

• Interface• LinkedList implements List

• Inheritance• LinkedList extends AbstractSequentialList

10/3/2018 Compsci 201, Fall 2018, Linked Lists + Hashing

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H is for …

• Hashing• What better way to have a bucket list?

• HackDuke• Oct 13/14, hackduke.org

• Social good

9/22/17 Compsci 201, Fall 2017, Analysis+Markov

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PFtWBFB

• Interfaces and Inheritance• In the context of ArrayList and LinkedList• In code we use in assignments and class• Toward Markov Part 2

• Linked lists as implementation technique• Why nodes are the basis for interview questions

• Midterm and performance explained


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Some Catch-up and Review• What is the purpose of x.hashCode() ?• Convert x to a number for hashing• Must make sure it’s valid index, e.g., |% size|

• Multiple objects can have same .hashCode• Use .equals to find that which you seek• If a.equals(b) then a.hashCode() == b.hashCode()

• Converse is NOT true


DIY Set with ArrayList

• https://github.com/astrachano/diyad-hashing

• Benchmarking with synthetic data


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y = 0.7591x2 + 0.1958x - 1.1903R² = 0.9868

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ArraySet

Array Set P oly. (Arra ySe t)

# elts, array, hash10000, 0.732, 0.00920000, 2.273, 0.00330000, 5.371, 0.00340000, 10.701, 0.00450000, 19.573, 0.00660000, 23.968, 0.00770000, 41.555, 0.01180000, 46.163, 0.01190000, 67.684, 0.013100000,73.093, 0.015

Just Say No.. When you can

O(n2)


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SimpleHashSet Constructor

ArrayList<ArrayList<Type>> myElements;

• What is each element in the list myElements?

• What is myElements.get(k).size()


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https://github.com/astrachano/diyad-hashing

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SimpleHashSet.add()


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ArrayList, Why?

WOTO Reviewed

http://bit.ly/201fall18-sept28-1• What can we learn from responses• What is the big idea with hashing?

• Why are buckets with ArrayLists good?• Easy to implement

• Why are buckets with ArrayLists bad?• Remove is O(n) but have to shift once found• Add is O(1), but 1 + 2 + 4 + 2n = 2x2n-1• Adding 2n elements is O(2n), but …


Summary of O-notation for ArrayList

• ArrayList allocates array[] storage for k items• When full? Allocate 2*k, copy over, continue• Total storage? 1 + 2 + 4 + … + 2k

• Note: 1 + 2 + 4 + 8 + 16 = 31• In general: 2x2k-1 or 2k+1 – 1

• If N = 2k, then sum is O(N) since 2x2k is 2N• Coefficients do not matter• Amortized analysis: some .add calls? $$$$$


Luis von Ahn• Duke 2000, Math• Duke Honorary Degree 2017• CEO Duolingo• Macarthur Award, 2006• MIT-Lemelson Prize, 2018


“It’s amazing how motivating it is to sit with somebody and say, ‘What you’re doing is really important.’ I use that a lot.”

http://bit.ly/201fall18-sept28-1

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Java Interface Iterable

• Classes can only extend one class: inheritance

• Inherit method implementations• and state, e.g., instance variables, but cannot

access private, only protected or public

• Limiting: single inheritance

• Classes can implement many interfaces

• Must supply all methods from each interface• Iterable interface: use for-each loop


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Existing ArraySet class

• Cannot use for-each loop, Java complains

• Can only iterate over an array or an instance of java.lang.Iterable


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How to implement Iterable

• Must have a method named iterator• Method must return an Iterator object• Interface with .hasNext() and .next()

• This is easy for ArraySet since ArrayList is an Iterable• Simply return its Iterator

• Demo: implements Iterable<Type>


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SimpleHashSet implements Iterable

• Make a new class, don’t change one that works• Must return an Iterator, that’s a class/object• Nested aka inner class• Track: bucket # and index # in the bucket

• Software Engineering and Design• Open/Closed principle for code and classes• Open for extension, closed to modification• https://stackify.com/solid-design-open-closed-principle/


https://stackify.com/solid-design-open-closed-principle/

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Details and Iterable


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• What Bucket? ArrayList?

• ArrayList of ArrayList<Type>

• What Index in ArrayList?

• This is an index into the “current” ArrayList

Implementing Iterable/Iterator


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• https://github.com/astrachano/diyad-hashing

• Think of this as read-only in 201 • Extend for inheritance, implement for interface

Extend SimpleHashSet<Type>• What does new class get as a result of extension?• Implementation of several set methods• Access to protected instance variables

• When you call x.method(..) what happens?• Find method in class for x• Find method in super-class for x• Object x can make explicit call on super(..)

even when there is an @Override


Implement Iterable<Type>

• Must supply iterator() method that returns

• Iterator<Type> object which means …• Has .next() and .hasNext() methods

• We can “mix-in” more than one interface, something to think about in software design courses• Extend once, implement many


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https://github.com/astrachano/diyad-hashing

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Barbara Liskov

Turing Award Winner in 2008 for contributions to practical and theoretical foundations of

programming language and system design, especially related to data abstraction, fault tolerance, and distributed computing.

The advice I give people in general is that you should figure out what you like to do, and what you can do well—and the two are not all that dissimilar, because you don’t typically like doing something if you don’t do it well. … So you should instead watch—be aware of what you’re doing, and what the opportunities are, and step into what seems right, and see where it takes you.

What is a java.util.List in Java?

• Interface for collection of elements• Add, remove, traverse, …• What can a list do to itself?• What can we do to a list?

• Why more than one kind of list: Array and Linked?• Useful in different applications• How do we analyze differences?• How do we use them in code?


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What's the Difference Here?

• How does find-a-track work? Fast forward?


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Review: list.remove(0)


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• What is “faster”? LinkedList or ArrayList

y = -4E-05x + 0.0009

y = 0.0064x2 - 0.0156x + 0.0238R² = 0.9984

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RemoveFirst

l in ke d arra y Line ar (l in ke d) P oly. (a rray )

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From array to linked list

• How do we implement ArrayList and LinkedList?• Low-level implementation details• We use an array[] for ArrayList• We use a Node for LinkedList

• Why are we using many nodes and not an array?• Same number of nodes as Strings• Not amortized analysis, but exactly N

• Remove without shifting


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What’s in a Node?

• Some information

• Place to snap another node

• In Java we’ll see • String reference: info• Node reference: next


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Visualizing/Understanding Nodes

• https://github.com/astrachano/classcode201fall18

• LowLevelLinkDemo.java• Note: this not needed below


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Adding New Nodes• To add to the end of a linked list• Need reference to first node• only through first node can we access entire list

• Need reference to last node• To add a new last node

• Often need initialization code• First node anchors list• Must do before loop• Loop will add over and over to end


https://github.com/astrachano/classcode201fall18

https://github.com/astrachano/classcode201fall18/blob/master/src/LowLevelLinkDemo.java

10/4/18

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Adding nodes to end

• Loops: Initialize, loop, finalize

• Loop invariant: last always references last node• Note initialization and update in the loop


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Visualizing Code

• Using Java Tutor: https://goo.gl/EufQp6

• See first and last: both Node variables


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Adding first node to linked list

• Repeatedly add first element, initially null

• New first node points at ”old” first node• First references/points to new first node• Can use first = new Node(vg[k],first)


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Array Traversal

• Visiting (printing) every value in an array

• Initialize index, print w/index, increment index• Elements of array are adjacent in memory


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https://goo.gl/EufQp6

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List Traversal

• Visiting (printing) every value in an array

• Start with first node, print .info, advance .next• Done when current node is null


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WOTO

http://bit.ly/201fall18-oct3-1

https://goo.gl/YfXsKr for visualization


LinkHashStringSet

• We’ll look more at this Friday and Beyond


35 10/3/2018 Compsci 201, Fall 2018, Linked Lists + Hashing

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http://bit.ly/201fall18-oct3-1

https://goo.gl/YfXsKr

10/4/18

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Midterm

• Mean 80.4%, Median 82.9%

• Interpreting grades• Historically what this means …• 70% on midterms, on track for B or above (50)


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Historical Trends Midterm I

• Spring 2018: Median 74%, Mean 70%

• Fall 2017: Median 85%, Mean 83%• Fall 2016: Median 81%, Mean 77%

• Final Grades In Spring 2018• 53% A- above, 82% B, 92% B-, 96% C-

• Final Grades In Fall 2017• 66% A- above, 91% B, 94% B-, 97% C-


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Learning Interpretation• Constructors, methods, this, and more (Q2)• Class as a whole at 71%, 9 total points

• What is purpose of .toString and .equals (Q4B)• Class as a whole 74%, .toString(), 2 points

• How does hashing work, performance (Q4D)• One point was a huge stretch, 3/360, 4 points


Work Coming

• APTs with low-level linked lists

• Will review examples: Friday

• APT Quiz review• How to develop solutions: Friday

• Markov Part 2• After fall break, see calendar for details


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I is for - Duke University€¦ · •Demo: implements Iterable 10/3/2018 Compsci 201, Fall 2018,...

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