CS1020
Week 5: 12th Feb 2015
Contents

Sit-in lab #1

Common Mistakes

Take-home Lab #2
2
Week 5
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 doubt
Study 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
4
Week 5
Understand Question
This was a sit in lab
OOOO X O X X O X O X OO
How many possible
segment sizes?
How many possible
advertisement widths?
This
This
This
This
This
was
was
was
was
was
a
a
a
a
a
sit
sit
sit
sit
sit
in
in
in
in
in
lab
lab
lab
lab
lab
Segments have same size
S
5
OOOO
OOOO
OOOO
OOOO
OOOO
X
X
X
X
X
O
O
O
O
O
X
X
X
X
X
X
X
X
X
X
O
O
O
O
O
X
X
X
X
X
O
O
O
O
O
X
X
X
X
X
OO
OO
OO
OO
OO
Adverts have same width W
Week 5
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
6
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 5
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
7
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 5
Design Algorithm
<WordS > <WordS > <W>
Check charAt(S)
Check charAt(S + S+1)
Check charAt(S + 2(S+1))
< ! >X<!>X!X<!>X<!>X<!>
Check charAt(W)
Check charAt(W + W+1)
Check charAt(W + 2(W+1))
As long as <
message.length()
As long as < layout.length()
8
Week 5
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
9
Week 5
Design Program
while there hasNextLine() // containing 1 message or layout
Message
input = read nextLine()
- _message : String
: Message
result = solve(input)
+ isPossibleSize(
“This was a ”…
println result
pintSize : int) : int
isPossibleSize(1)
??
false
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
10
Week 5
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 case
returns true if message can be broken down into segments of
given size (adverts can have given width), false otherwise
11
Week 5
Write Skeleton
Pre-condition: What must be true before method
executes?

Parameter values
e.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 system
e.g. Connection open, other program waiting for
message
12
Week 5
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.
13
Week 5
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;
}
14
Week 5
Test
javac FileName.java
compile source file into bytecode
java DriverClassName
manual test run
java DriverClassName < input.in
test run
java DriverClassName < input.in > output.actual
save output to a file
diff output.actual output.out
compare your output with the expected output
15
Week 5
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
16
Week 5
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
17
Week 5
Common Mistakes

(Lab TAs: Please customise this section on the
common mistakes your class made in Sit-in Lab
#1, or reminders for your class.)
18
Week 5
Take-home Lab #2
Exercise 1 – KWIC Index System
Week 5

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:


20
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.
Week 3
KWIC Index
Key Word In Context index system

Scanner sc

sc.next() vs. sc.nextLine()

Scan numOfWordToIgnore && numOfTitle

Add them to respective ArrayList<String>
21
KWIC Index
Class: KWICIndexSystem
Week 5
KWIC Index
Class: KWIC

ArrayList<String>

Modularity

22
Every single method in the KWIC class is IMPORTANT!
Week 5
KWIC Index
API

Use ArrayList<E> class (requirement)

Check out the API!
http://docs.oracle.com/javase/7/docs/api/java/util/ArrayList.h
tml

Use Collections class and its sort method

Check out the API!
http://docs.oracle.com/javase/7/docs/api/java/util/Collections
.html
23
Week 5
Take-home Lab #2
Exercise 2 – Simple Social Network
Week 5

Objective is to implement a simple social network with
some operations:





Register a new user
Set a user follows another user
Set a user unfollow another user
Find mutual follow users
(R userName)
(F userA userB)
(U userA userB)
(M)
Solution:


25
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.
Week 3
Social Network
Simple Social Network

Things to take note:



26
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.
Week 3
Social Network
Understand Question
Social Network
Class: User

Create Object: User


Data attributes : String name, User[ ] following, …
Constructor: public User(String Name)


Methods:



public void setFollow(User user)
public void setUnfollow(User user)
Create your own helper methods:



27
Initialize your data attributes
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 5
Social Network
Class: SocialNetwork

SocialNetwork class: how to use User class!


Data attributes: User[ ] allUsers, int numOfUsers
Constructor: public SocialNetwork()


Register a new user: public void registerUser(String name)


Use setFollow method in User class.
Set user A unfollow user B: public void setUnfollow(User A, User B)


Create an object of User class with given name.
Set user A follows user B: public void setFollow(User A, User B)


Initialize your data attributes
Use setUnfollow method in User class.
Find mutual follow users

28
Can use isFollowing method in User class.
Week 5

Fixed length vs. Dynamic length

Access by index vs. Access by iterator

Complexity (time and space)

…
29
Social Network
Array vs. ArrayList<E>
Week 5
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



31
Gold Hunters
Gold Hunters
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
Week 5
Gold Hunters
Store input map in 2D array

Integer Array (Store both input and output)



Character Array (Store input) + Integer Array (Store
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?)
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.
32
Week 5


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).
33
+1
+1
+1
(R-1,C-1)
(R-1,C)
(R-1,C+1)
+1
*
+1
(R,C-1)
(R,C)
(R,C+1)
+1
+1
+1
(R+1,C-1)
(R+1,C-1)
(R+1,C+1)
Gold Hunters
Compute amount of gold on the map
(1/2)
location of
mine
Week 5


Use double for-loop to go through array and locate
each mine on the map
If mine located at index (R,C)

Gold Hunters
Compute amount of gold on the map
(2/2)
Use double for-loop to go through indices of the possible
cells within the 3x3 area of effect
for (int i=R-1; i <= R+1; i++)
for (int j=C-1; j <= C+1; j++)
...

34
If an index is valid (not outside boundary of array),
increment amount of gold in that cell by 1
Week 5


Double for-loop to go through the integer array and
print out the gold in each cell
Print ‘*’ if cell is a mine cell.
35
Gold Hunters
Output gold on map
Week 5
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