Structured Problem Solving

1
Structured Problem Solving

• Data Structures 2
• Operations on arrays

2
Quick question

• Essentials1 is Beer
• Essentials4 does not exist

3
Another question

• Array1..5 of integer

4
Final question

• Array 1..151..7 of integer

5
Programming with arrays

• We use arrays in programs and carry out
  operations on them
• Assignment
• Retrieval
• Initialisation
• Searching
• Sorting

6
Operations on arrays assignment

• Scores4 29

7
Operations on arrays retrieval

• What would be the output of
• System.out.println(Scores3Scores10)

62
8
Example of retrieval

• Write a fragment of Java that would display the
  third name in the array.
• System.out.println(PlayOffTeams3)

9
Example of retrieval

• Write a fragment of Java that would display all
  the names in the array.

10
Example of retrieval

• for( i 1 ilt4 i)
• System.out.println(PlayOffTeamsi)

11
Searching and sorting arrays
12
Java Array

• int X new int99999
• int i
• for (i0 ilt 99999 i)
• Xi i2
• X0 gets 0
• X1 gets 2
• X2 gets 4 etc

13
Searching an array

• Example
• How does hole in the wall find your account from
  your credit card details ?
• Need to search through the array of bank records

14
Different kinds of search available

• Linear Search
• Binary search

15
Linear Search

• Searching for a bank record
• 1,000 bank records
• Fastest find in -
• 1 comparison
• Slowest find in -
• 1,000 comparisons
• Average find in
• 500 comparisons

16

• Search for a name in a telephone directory
• Is a linear search appropriate ?
• No !
• A better solution in this case is a binary search

17
The Binary Search

• Search for Evans in the array of names which have
  been sorted into alphabetical order

Figure 12
18

• 1 Ball
• 2 Davies
• 3 Evans
• 4 Galt
• 5 Hurst
• 6 Martin
• 7 Mason
• 8 Moore
• 9 Perkins
• 10 Stephens

See pages 37, 38 in booklet
19

• 1 Ball
• 2 Davies
• 3 Evans
• 4 Galt
• 5 Hurst
• 6 Martin
• 7 Mason
• 8 Moore
• 9 Perkins
• 10 Stephens

See pages 37, 38 in booklet
20

• 1 Ball
• 2 Davies
• 3 Evans
• 4 Galt
• 5 Hurst
• 6 Martin
• 7 Mason
• 8 Moore
• 9 Perkins
• 10 Stephens

See pages 37, 38 in booklet
21
Sorting

• We need to sort to
• be able to use the fast binary search
• produce reports in required order

22
Sorting

• Many different ways of carrying out a sort on an
  array.
• We shall examine EXCHANGE SORT (Also known as the
  BUBBLE SORT)
• We shall look at others

23
Exchange Sort

• Simple
• Slow

24
See pages 41, 42 in booklet

• 1 Davies
• 2 Martin
• 3 Perkins
• 4 Evans
• 5 Mason
• 6 Ball
• 7 Stephens
• 8 Moore
• 9 Hurst
• 10 Galt

Swap
25
See pages 41, 42 in booklet

• 1 Davies
• 2 Martin
• 3 Evans
• 4 Perkins
• 5 Mason
• 6 Ball
• 7 Stephens
• 8 Moore
• 9 Hurst
• 10 Galt

Swap
26
See pages 41, 42 in booklet

• 1 Davies
• 2 Martin
• 3 Evans
• 4 Mason
• 5 Perkins
• 6 Ball
• 7 Stephens
• 8 Moore
• 9 Hurst
• 10 Galt

Swap
27
See pages 41, 42 in booklet

• 1 Davies
• 2 Martin
• 3 Evans
• 4 Mason
• 5 Ball
• 6 Perkins
• 7 Stephens
• 8 Moore
• 9 Hurst
• 10 Galt

Swap
28

• 1 Davies
• 2 Martin
• 3 Evans
• 4 Mason
• 5 Ball
• 6 Perkins
• 7 Moore
• 8 Stephens
• 9 Hurst
• 10 Galt

See pages 41, 42 in booklet
Swap
29

• 1 Davies
• 2 Martin
• 3 Evans
• 4 Mason
• 5 Ball
• 6 Perkins
• 7 Moore
• 8 Hurst
• 9 Stephens
• 10 Galt

See pages 41, 42 in booklet
Swap
30
See pages 41, 42 in booklet

• 1 Davies
• 2 Martin
• 3 Evans
• 4 Mason
• 5 Ball
• 6 Perkins
• 7 Moore
• 8 Hurst
• 9 Galt
• 10 Stephens

At the end of this first pass, Stephens is in the
correct position alphabetically.
This is repeated for a second pass starting at
position 1 and again and again until the array is
completely sorted.
31
Java Exchange Sort
Consider an array of 10,000 surnames, called
names, indexed from 0 to 9,999. for (i1
ilt9999 i) for (j0 jlt 9999 j) if
(namesj gt namesj1) temp
namesj namesj namesj1 namesj1
temp
32

• Please refer to page 43 of the booklet plus the
  exercise posted on the web site and on the H
  drive.

33
Sorting Demos

• http//www.cs.ubc.ca/spider/harrison/Java/sorting-
  demo.html

• http//www.cs.hope.edu/alganim/animator/Animator.
  html