CS1020Week 5: 12th Feb 2015

Contents

Sit-in lab #1Common MistakesTake-home Lab #2

Week 52

Sit-in Lab #1

Set A – Message Set B – Wall

Week 5

Advice Understand question thoroughly

What does the output represent? How does each piece of information help you?

Ask your invigilator when in doubtStudy the given input and output files

Design algorithm thoroughly before coding

Don’t use unfamiliar data structures e.g. [], ArrayList, Arraylist?, java.lang.reflect.Array? Sit-in labs are set to be done using topics covered; there is

no need to use advanced topics not yet covered in class

Code incrementally and keep testing

Week 54

Understand Question

This was a sit in lab

How many possible segment sizes?

This was a sit in labThis was a sit in labThis was a sit in labThis was a sit in labThis was a sit in lab

Segments have same size S

OOOO X O X X O X O X OO

How many possible advertisement widths?

OOOO X O X X O X O X OOOOOO X O X X O X O X OOOOOO X O X X O X O X OOOOOO X O X X O X O X OOOOOO X O X X O X O X OO

Adverts have same width W

Week 55

Generalize Problem

This was a sit in lab<WordS > <WordS > <W>Exactly 1 space betw.

words

<W > was…Segment not ≥1 word(s)

<W > was…“Was” split across segments

OOOO X O X X O X O X OO

< ! >X<!>X!X<!>X<!>X<!>

No adjacent doors, doors at ends

< ! >X<!>X …Cannot end at door

< ! >X<!>X …Cannot end at that location

Week 56

Design Algorithm

< W> <W> W <W> <>…Check message.charAt(S-1)

Check message.charAt(S)

Check message.charAt(S+1)

Only need to check for ‘ ’

If middle char is ‘ ’ left and right are letters

If middle char is not ‘ ’

invalid segment size

OOOO X O X X …Check layout.charAt(W-1) == ‘ ’

Check layout.charAt(W) == ‘X’

Check layout.charAt(W+1) == ‘ ’

Only need to check for ‘X’

If middle char is ‘X’ advert ends at immed. left advert starts at immed. Right

If middle char is not ‘X’

advert ends at invalid point

Week 57

Design Algorithm

<WordS > <WordS > <W>

Check charAt(S)

Check charAt(S + S+1)

Check charAt(S + 2(S+1))

As long as < message.length()

< ! >X<!>X!X<!>X<!>X<!>

Check charAt(W)

Check charAt(W + W+1)

Check charAt(W + 2(W+1))

As long as < layout.length()

Week 58

Design Program

while there hasNextLine() // containing 1 message or layout

input = read nextLine()

result = solve(input)

println result

static solve(input : String) : int

create new Message/Layout(input) // instantiate object

possibilities = 0

for (int size/width = 1; size <= input.length(); size++)if object.isPossible…(size/width) then possibilities++

return possibilities

Week 59

Design Program

while there hasNextLine() // containing 1 message or layout

input = read nextLine()

result = solve(input)

println result

static solve(input : String) : int

create new Message/Layout(input) // instantiate object

possibilities = 0

for (int size/width = 1; size <= input.length(); size++)if object.isPossible…(size) then possibilities++

return possibilities

: Message

Message

- _message : String

+ isPossibleSize(pintSize : int) : int

“This was a ”…

isPossibleSize(1) ?? false

Week 510

Write Skeleton

static solve(input : String) : int

pre-cond:input not NULL or empty, must be valid msg/layout

isPossible…(size/width : int) : boolean // instance method

pre-cond:1 <= given size/width <= message/layout length()

post-cond: // same as description in this casereturns true if message can be broken down into segments of

given size (adverts can have given width), false otherwise

Week 511

Write Skeleton

Pre-condition: What must be true before method executes?

Parameter valuese.g. 1 <= width <= layout length

Values of some other attributes in this object/class(method that removes student) e.g. Student data already filled

State of some other part of the systeme.g. Connection open, other program waiting for message

Week 512

Write Skeleton

Post-condition: What must be true after method executes?

(Scanner’s nextLine() method) e.g. Return value

returns the next token preceding the first “\n” character What happens to some other attribute/object

One “\n” character from the stream is discarded

State of some other part of the system(close() method) e.g. The Scanner and the underlying stream is closed. Subsequent calls to next…() methods will result in IllegalStateException being thrown.

Week 513

Algo Code

Check charAt(S)

Check charAt(S + S+1)

Check charAt(S + 2(S+1))

/*…*/

public boolean isPossible…(int size/width) {

for (int doorIdx = width; doorIdx < _layout.length();

doorIdx += width +1)if (this._layout.charAt(width) != ‘ ’) return false;

return true;

}

Week 514

Test

javac FileName.javacompile source file into bytecode

java DriverClassNamemanual test run

java DriverClassName < input.intest run

java DriverClassName < input.in > output.actualsave output to a file

diff output.actual output.outcompare your output with the expected output

Week 515

Refactor

Cleanup indentation Ensure comments are meaningful and generally accurate Cleanup obviously redundant logic

(Pretend you are another coder) Cleanup comments Check pre-/post-conditions Check method names and variable names

Follow Java naming convention Meaningful/descriptive

Week 516

Challenge Yourself

After Sit-in-Lab ask yourself how you could have coded better ask yourself how you could have improved your

algorithm revise what you are unsure of try the other problem set under test conditions

Since we have learnt arrays and are learning Collections: Use int[] primitive 1-D array to improve solve()’s efficiency Use ArrayList<Integer> to improve solve()’s efficiency

Week 517

(Lab TAs: Please customise this section on the common mistakes your class made in Sit-in Lab #1, or reminders for your class.)

Common Mistakes

Week 518

Take-home Lab #2

Exercise 1 – KWIC Index System

Week 5

Week 320

Key Word In Context index system Objective is to implement KWIC index system

with some operations: Circular shift a string Capitalize the first character of keyword Order all the title in alphabetical order

Solution: Implement the given two classes: KWIC.class and

KWICIndexSystem.class Using ArrayList. You should know how to

add/delete/search an element in an ArrayList.

KWIC Index

Class: KWICIndexSystem

Scanner sc

sc.next() vs. sc.nextLine()

Scan numOfWordToIgnore && numOfTitle

Add them to respective ArrayList<String>

Week 521

KWIC Index

Class: KWIC

ArrayList<String>

Modularity Every single method in the KWIC class is IMPORTANT!

Week 522

KWIC Index

API

Use ArrayList<E> class (requirement) Check out the API!

http://docs.oracle.com/javase/7/docs/api/java/util/ArrayList.html

Use Collections class and its sort method Check out the API!

http://docs.oracle.com/javase/7/docs/api/java/util/Collections.html

Week 523

KWIC Index

Take-home Lab #2

Exercise 2 – Simple Social Network

Week 5

Week 325

Simple Social Network Objective is to implement a simple social

network with some operations: Register a new user (R userName) Set a user follows another user (F userA userB) Set a user unfollow another user (U userA userB) Find mutual follow users (M)

Solution: Implement the given two classes: SocialNetwork.class

and User.class Using Array. You should know how to add/delete/search

an element in an array.

Social N

etwork

Week 326

Understand Question Things to take note:

Assume user name is unique; no users with same name will be registered.

R user1 user1 will not be provided in the input Multiple users can be registered in one line operation:

R user1 user2 user3 user4 .... A user cannot follows himself/herself. You must deal with

the case when F userA userA provided in the input.

Social N

etwork

Class: User

Create Object: User Data attributes : String name, User[ ] following, … Constructor: public User(String Name)

Initialize your data attributes Methods:

public void setFollow(User user) public void setUnfollow(User user)

Create your own helper methods: eg. public boolean isFollowing(User user) //check is this user

following an given user; eg. public String getName() //get the name of this user ….

Week 527

Social N

etwork

Class: SocialNetwork

SocialNetwork class: how to use User class! Data attributes: User[ ] allUsers, int numOfUsers Constructor: public SocialNetwork()

Initialize your data attributes Register a new user: public void registerUser(String name)

Create an object of User class with given name. Set user A follows user B: public void setFollow(User A, User B)

Use setFollow method in User class. Set user A unfollow user B: public void setUnfollow(User A, User B)

Use setUnfollow method in User class. Find mutual follow users

Can use isFollowing method in User class.

Week 528

Social N

etwork

Array vs. ArrayList<E>

Fixed length vs. Dynamic length

Access by index vs. Access by iterator

Complexity (time and space)

…

Week 529

Social N

etwork

Take-home Lab #2

Exercise 3 – Gold Hunters

Week 5

Objective is to output how much gold there is in each cell on the map given the position of mines.

Solution Store input map in a 2D array Compute amount of gold on the map Output map showing amount of gold in each non-mine

cell

Gold HuntersG

old Hunters

Week 531

Integer Array (Store both input and output) Only manipulate 1 array Different states of a cell need to be represented by carefully

chosen values, e.g. cannot use values 0 to 8 to represent mine cells (why?)

Character Array (Store input) + Integer Array (Store output) Store the input as is. Encode different states of a cell in the character array. Integer

array only stores the gold amount in each cell. Need to deal with 2 arrays.

Either way is fine.

Store input map in 2D arrayG

old Hunters

Week 532

Each mine “adds” 1 gold to every neighboring cell around it.

Maximum area of effect is 3x3 square around the mine (excluding its own cell).

Compute amount of gold on the map (1/2)

+1(R-1,C-1)

+1(R-1,C)

+1(R-1,C+1)

+1(R,C-1)

*(R,C)

+1(R,C+1)

+1(R+1,C-1)

+1(R+1,C-1)

+1(R+1,C+1)

location of mine

Gold H

unters

Week 533

Use double for-loop to go through array and locate each mine on the map

If mine located at index (R,C) Use double for-loop to go through indices of the possible

cells within the 3x3 area of effect

If an index is valid (not outside boundary of array), increment amount of gold in that cell by 1

Compute amount of gold on the map (2/2)

for (int i=R-1; i <= R+1; i++) for (int j=C-1; j <= C+1; j++) ...

Gold H

unters

Week 534

Double for-loop to go through the integer array and print out the gold in each cell

Print ‘*’ if cell is a mine cell.

Output gold on mapG

old Hunters

Week 535

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