CS1020
Week 8: 12th March 2015
Contents

Sit-in lab #2

Common Mistakes

Take-home Lab #3
Week 8
Sit-in Lab #2
Set A – Construct
Set B – Evaluation
Week 8
Objectives
• Collections
– Use array of objects, and java.util.Arraylist
– Length vs size vs capacity
– Handle insertion, sequential searching efficiently
• Problem solving
– Design algorithm without changing structure, behaviour
– Think of objects in terms of: the data they have, and the
services they provide
Week 8
4
Understand Question
10
30
buildUnit(
20
✓
40
50
,
)
Week 8
5
Design Classes (1/2)
Sequential
searching
Week 8
6
Design Classes (Set A: 2/2)
: Construct
2
size == 2
capacity == 2
: Building
size == 1
capacity == 3
1
+ size() 2
: ArrayList
<Building>
: Building
For efficiency, store (logical) size!
Week 8
7
Design Classes (Set B: 2/2)
: Evaluate
2
A
B
size == 2
capacity == 2
: Category
1
+ size() 2
: ArrayList
<Category>
size == 1
capacity == 3
C
: Category
D
E
For efficiency, store (logical) size!
Week 8
8
Insertion (Set A: 1/2)
How to add Building in sequence?
How to add Unit in sequence?
: Construct
2
: Building
1
1
+ size() 2
: Building
: ArrayList
<Building>
Week 8
9
Insertion (Set B: 1/2)
How to add Category in sequence?
How to add Offense in sequence?
: Evaluate
2
A
B
: Category
1
+ size() 2
: ArrayList
<Category>
1
C
: Category
D
:Category
Week 8
E
10
Insertion (Set A: 2/2)
How to add Unit efficiently?
a) _arr[size++] = givenUnit;
c) Unit[] temp = new Unit[
_arr.length + 1];
b) int currIdx = 0;
// copy _arr’s units to temp
for (Unit curr : _arr) {
temp[_arr.length] = givenUnit;
if (curr == null)
_arr = temp;
_arr[currIdx] = givenUnit;
currIdx++;
d) All of the above
}
e) None of the above
2
: Building
Week 8
11
Insertion (Set B: 2/2)
How to add Offence efficiently?
a) _arr[size++] = givenOffence; c) Offence[] temp = new Offense[
_arr.length + 1];
b) int currIdx = 0;
// copy _arr’s units to temp
for (Offence curr : _arr) {
temp[_arr.length] =
if (curr == null)
givenOffence;
_arr[currIdx] = givenUnit;
_arr = temp;
currIdx++;
d) All of the above
}
e) None of the above
2
A
B
: Building
Week 8
12
Searching (Set A: 1/5)
How to prevent redundant searches?
buildUnit(
,
)
: Construct
2
: Building
3
+ size() 2
: Building
: ArrayList
<Building>
Week 8
13
Searching (Set B: 1/5)
How to prevent redundant searches?
recordOffence( category, offence)
: Construct
2
A
B
C
D
: Category
3
+ size() 2
E
: Category
: ArrayList
<Category>
Week 8
14
Searching (Set A: 2/5)
buildUnit(
,
)
Find the first matching building
Find the first matching unit within
: Construct
2
: Building
3
+ size() 2
: ArrayList
<Building>
: Building
Abstraction: I don’t care how Building finds the 1st matching Unit
Week 8
15
Searching (Set B: 2/5)
recordOffence( category, offence)
Find the first matching category
Find the first matching offence within
: Evaluate
2
: Category
3
+ size() 2
: ArrayList
<Category>
: Category
Abstraction: I don’t care how Category finds the 1st matching Offence
Week 8
16
Searching (Set A: 3/5)
buildUnit(
: Construct
2
: Building
3
+ size() 2
: ArrayList
<Building>
: Building
,
)
Unit selected = null; // why initialize?
/* Find the first matching building */
For each Building currBldg : _buildings
If currBldg.hasName(bldgName)
selected = currBldg.(
find first matching unit within
)()
… Attempt to build unit
What’s wrong with this?
Week 8
17
Searching (Set B: 3/5)
recordOffence( category, offence)
: Evaluate
+ size() 2
: ArrayList
<Category>
Offence selected = null; // why initialize?
/* Find the first matching category */
2
For each Category currCat : _categories
: Category
If currCat.hasName(offenceName)
selected = currCategory.(
3
find first matching unit within
)()
: Category
… Attempt to build unit
What’s wrong with this?
Week 8
18
Searching (Set A: 4/5)
findUnitByName( blueCar )
For each Unit currUnit : _units
If currUnit.getName().equals( blueCar )
return currUnit
return null
What’s wrong with this?
2
: Building
Week 8
19
Searching (Set B: 4/5)
findOffenseByName( offenceA )
For each Offence currOffence : _offenses
If currOffence.getName().equals( offenceA )
return currOffence
return null
What’s wrong with this?
2
A
B
: Building
Week 8
20
Searching (Set A: 5/5)
/* Find the first matching unit */
findUnitByName( blueCar )
Unit match = null
For each Unit currUnit : _units
If currUnit.getName().equals( blueCar )
match = currUnit
return match
What’s wrong with this?
3
: Building
Week 8
21
Searching (Set B: 5/5)
/* Find the first matching offense */
findUnitByName( offenseA )
Offence match = null
For each Offense currOffsense : _offenses
If currOffense.getName().equals( offenseA )
match = currOffense
return match
What’s wrong with this?
3
A
B
C
: Building
Week 8
22
Common Mistakes (1/2)
• Inefficient implementation of array-based collection
– Re-creating array on insertion
– Finding the first null element as the location to insert to
– Searching continues after first match is found
• Incorrect searching in array-based collection
– Searching past the logical “end” of the array
• Results in NullPointerException or undesired behaviour
– Using equals() method to compare each Building object in
the ArrayList<Building> with building NAME, a String
Week 8
23
Common Mistakes (2/2)
• Fail to visualize how problem should be solved
– “I must create getters/accessors for this to work…”
– Failing to view classes as templates of objects that store
data and provide services
• “Don’t know what this method is for”
• “Don’t know which method to place this functionality in”
• “Don’t know which method to invoke”
(When writing real software, changing what a public method
does will likely affect other programs)
Week 8
24
Next Step (1/3)
• Understand relationship between:
–
–
–
–
Collection
List
RandomAccess
ArrayList/Vector
• Simulate your own ArrayList to understand how
java.util.ArrayList works! See next page…
(You should have done this during recess week)
Week 8
25
Next Step (2/3)
• Implement an array-based Collection supporting
– Efficient insertion, proportional to 1 operation
– Sequential search and removal, proportional to N
operations
when we have N elements in the array
• Implement an array-based List supporting
– Insertion into a given index in the middle of the list
– Removing from a given index in the middle of the list
Week 8
26
Next Step (3/3)
• Implement your own LinkedList<E> and ListNode<E>
from scratch
• Think:
– Why use LinkedList when there’s ArrayList?
– Why use ArrayList when there’s array?
– Which operations are more efficient in LinkedList, and
which are more efficient in ArrayList?
Week 8
27
Take-home Lab #3
Exercise 1 – Flip The List
28
Week 8
Simple Algorithm (1/11)
Starting index to
be flipped
A
B
C
D
E
Keep a pointer to link the last index
Week 8
29
Simple Algorithm (2/11)
Pointer 1
A
Pointer 2
B
C
Week 8
Pointer 3
D
E
30
Simple Algorithm (3/11)
Pointer 1
A
B
Pointer 2
C
Pointer 3
D
E
NULL
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31
Simple Algorithm (4/11)
Pointer 1
A
Pointer 2
B
C
Pointer 3
D
E
NULL
Week 8
32
Simple Algorithm (5/11)
Pointer 1
A
B
Pointer 2
C
Pointer 3
D
E
NULL
Week 8
33
Simple Algorithm (6/11)
Pointer 1
A
B
Pointer 2
C
Pointer 3
D
E
NULL
Week 8
34
Simple Algorithm (7/11)
Pointer 1
A
B
Pointer 3
Pointer 2
C
D
E
NULL
Week 8
35
Simple Algorithm (8/11)
Pointer 1
A
B
Pointer 3
Pointer 2
C
D
E
NULL
Week 8
36
Simple Algorithm (9/11)
Pointer 2
Pointer 1
A
B
C
D
E
NULL
Week 8
37
Simple Algorithm (10/11)
Pointer 2
Pointer 1
A
B
C
D
E
NULL
Week 8
38
Simple Algorithm (11/11)
Pointer 2
Pointer 1
A
B
C
D
E
NULL
Week 8
39
Exception
Start
Index
A
Start
Index
B
C
Week 8
D
E
40
Take-home Lab #3
Exercise 2 – Kallang Wave
41
Week 8
Kallang Wave

Simulate a wave throughout an entire stadium




Multiple rows are not simulated
Simulation runs from left to right
Members of the audience are represented by ASCII
characters
Different characters represent different states, not
individuals
42
Week 8
Audience
43
Week 8
Audience as ASCII
||,,,
|,,,|
,,,||
,,||,
,||,,
44
Week 8
Circular Linked List


Stadium is round  need a “round” linked list
Must use circular linked list



Either from skeleton or self-made
Given CircularLinkedList class is incomplete
Bonus marks for efficient use

45
Will need modification of data structure
Week 8
END OF FILE
46
Week 8