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Questions before we Start

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Piet Mondrian Art 'Turtle' Graphics. VectorExercise. 4. 2-dimensional boolean array ... Piet Mondrian is a famous 20th century painter. So called 'Modern Art' ... – PowerPoint PPT presentation

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Title: Questions before we Start

1
Questions before we Start?

Any questions on HW?
HW 5--3 classes (PayrollApp (client), Payroll,
Employee). Gave you 90 of PayrollApp.

2
Week 06 - a

Nested Loops
Two-Dimensional Arrays
Computer Graphics
Vectors

3
Sample Programs Referenced(code located in
course folder)

java arrays.BoolArrayLab
Multiplication Table
Magic Triangle
Eight Queens
Sudoku (handout)

Piet Mondrian Art
"Turtle" Graphics
VectorExercise

4
2-dimensional boolean array(AnimatedIllustrations
)

java arrays.BoolArrayLab

5
Nested for Loops

Nested loops frequently used to process
two-dimensional arrays
Often body of inner loop is where the main
computation is done
Example
for (i 0 i lt m i)
before inner loop
for (j 0 j lt n j)
body of inner loop
after inner loop

6
Nested Loops
7
Multiplication Table Problem

A Java program to produce a multiplication table
Let the user specify the largest number
See Multiplication Table Project

8
Dependent for Loops

Sometimes the extent of the inner nested loop
will depend on the index value of the outer loop
What is the result of the following code?
for (i 0 i lt 3 i)
System.out.print("i " i " j ")
for (j 0 j lt i j)
System.out.print(" " j)
// end of "j" loop
out.println()
// end of "i" loop

9
Nested Loop Contained inOther Statements

What would be printed by the following?
"Volunteer" to the Board
for (int i 1 i lt 10 i)
if (i 2 0) // i even
for (int j 1 j lt i/2 j)
System.out.print()
else // i odd
for (int k 1 k lt 5 i/2 k)
System.out.print()
System.out.println()

10
Output

11
Magic Triangle

Input a word (say GISMO), print out in a
triangle
G
I I
S S S
M M M M
O O O O O
Need spaces to decrease before printing each row
Print a space between letters
Try running the project (enter ABRACADABRA)

12
Magic Triangle Code

public static void main()
String word JOptionPane.showInputDialog(
"Please enter a magic word.")
int size word.length()
for (int row1 rowltsize row)
char ch word.charAt(row-1)
for (int colrow colltsize col)
System.out.print(" ")
for (int col1 colltrow col)
System.out.print(ch" ")
System.out.println()
// end of "row" loop
// end of main program

13
Two-Dimensional Arrays

Used to store tabular data
Declaration and allocation similar to one
dimensional arrays
Need to specify both the number of rows and
columns during allocation
Example
final int COLS 6, ROWS 5
double someTable
someTable new doubleROWSCOLS
or
double someTable new doubleROWSCOLS

14
Two-Dimensional Arrays

Sample

Other examples?
Several HW scores for list of Students
52 weeks of Pay for 150 Employees

15
Arrays of Arrays

Writing
someTable new doubleROWSCOLS
is shorthand for
someTable new doubleROWS
for (int i 0 i lt ROWS i)
someTablei new doubleCOLS

16
PopulatingTwo-Dimensional Arrays

final int ROWS 4, COLS 3
String someTable new StringROWSCOLS
int r, c
// reads strings needed to fill
// someTable one row at a time
for (r 0 r lt ROWS r)
for (c 0 c lt COLS c)
someTablerc JOptionPane.showInputDialog(
"")
What value would you enter 1st? 2nd? 3rd? 6th?
How many values would we have to read from the
keyboard?
Later in the term we will see how to read data
from a file

17
Initializing Two-Dimensional Arrays

double energyTable
18.9, 19.4, 34.2, 3.9, 5.7, 0.3,
19.1, 19.3, 33.6, 3.0, 6.2, 0.2,
18.8, 19.6, 32.9, 3.1, 6.6, 0.2,
18.9, 20.3, 33.5, 2.8, 6.7, 0.2,
19.6, 20.8, 33.8, 3.1, 6.5, 0.2

18
Computing Row Totals
19
Computing Row Totals

double rowTotals new doubleROWS
for (r 0 r lt ROWS r)
// compute total for row r
rowTotalsr 0.0
for (c 0 c lt COLS c)
rowTotalsr rowTotalsr
someTablerc
How would you compute column totals?

20
Two-Dimensional Arrays and length

someTable.length is number of rows in someTable
someTablei.length is number of columns in row i
of someTable
In Java rows can be of different lengths!
Example
int someTable new doubleROWS
for (int i 0 i lt someTable.length i)
someTablei new inti 1
out.println("Length of row "i" is "
someTablei.length)
Draw a diagram

21
Chess Queen Attack ProblemBlueJ
22
Queen Attack Client

public static void main(String args)
QueenAttack board new QueenAttack()
int row Integer.parseInt(JOptionPane.showInput
Dialog(
"Row 1-8 on which to place
queen?"))
int col Integer.parseInt(JOptionPane.showInput
Dialog(
"Column 1-8 on which to place
queen?"))
board.placeQueen(row,col)
board.display() // could display as text or
graphics
// end of main method

23
QueenAttack class(top-down approach)

public class QueenAttack
private char gameBoard new char88
public void placeQueen(int row, int col)
System.out.println("placeQueen on row "row
" col "col)
// end of placeQueen method
public void display()
System.out.println("display gameBoard")
// end of display method
// end of EightQueens class

24
placeQueen Method

public void placeQueen(int row, int col)
for (int r1 rlt8 r)
for (int c1 clt8 c)
if (rrow ccol
Math.abs(r-row) Math.abs(c-col))
gameBoardr-1c-1 'x'
gameBoardrow-1col-1 'Q'
// end of placeQueen method

25
display Method

public void display()
for (int row 1 row lt 8 row)
for (int col 1 col lt 8 col)
System.out.print(" "gameBoardrow-1col-1
)
System.out.println()
// end of row loop
// end of display method

26
Volunteer to the Board...

Solve the Sudoku Puzzle

first
27
Java BREAK
28
Computer Graphics

Computer Graphics is the study of methods of
representing and manipulating images
A two-dimensional array can be used to represent
the image to be displayed
This array is called a frame it has one entry
for each pixel giving its color
Applets have this Graphics display area created
for them
The number of pixels in the frame is the
resolution the display device

29
Computer Art

Piet Mondrian is a famous 20th century painter
So called "Modern Art"
Several paintings consisted of abstract black
lines on a white canvas with red, blue or yellow
rectangles
Mondrian Project is running on Course Schedule
web page
Click for new painting
Paintings are for sale proceeds will benefit the
"CS Professors' Pizza Fund" ?

30
Turtle Graphics(related to Lab 8)

Interactive Applet
Again, the drawing box is being used as a
2-dimensional array.
Turtle can move up/down, left/right
7-pixel steps in the direction it is heading
Turtle Code at end of today's lecture notes
Talk about it more once we have covered "Button"
class

31
Traverse a Maze

Classic 2-dimensional program
See CSc-105 Home Page (with Maze)
More on Mazes after we cover Recursion

32
Arrays vs. Vectors

Arrays
Used to store collections
Must allocate a fixed number of cells
Can store either primitive values or objects
All elements must be same data type
Difficult to insert/remove values in the middle
Must move other elements manually
Dont behave like classes

Vectors
Used to store collections
Can grow or shrink as needed (dynamic)
Can only store objects
Elements may be of different data types
Provide methods to add, retrieve, insert, or
remove elements in the middle of a vector
Regular Java class - part of the java.util library

33
Declaration and Allocation

Declaration syntax
Vector name
Note the data type of the objects that will be
stored in the vector is not specified. (All are
assumed to be of type Object.)
Allocation syntax
name new Vector()
Note no size needed (can be specified)
Declaration and allocation
Vector name new Vector()

34
Methods that Insert Objects into a Vector

addElement(Object obj)
Appends the object obj to the end of the vector
and increases its size by 1
insertElementAt(Object obj, int i)
Inserts the object obj at index i and shifts all
the elements above location i up by 1. Also
increases the size by 1
setElementAt(Object obj, int i)
Replaces the object at location i with the object
obj
NOTE All of the above methods are of type void

35
Exercise"Volunteer to Board"

Draw a diagram showing the contents of memory
after each of the following statements are
executed
Vector x new Vector()
x.addElement(Mary)
x.addElement(Tom)
x.insertElementAt(Ann, 0)
x.setElementAt(Fred, 1)

36
Accessing Elements in a Vector

Method elementAt(int i)
Returns an Object type reference to the element
at index i of the vector
Example of use
Object y x.elementAt(2)
Why Object? What is Object?
Experiment with Vector Exercises project

37
Other Methods of theVector Class

int size()
Returns the number of elements in the vector
NOTE not length()!
void removeElementAt(int i)
Removes element at index i
Moves the rest of the elements down one
Decreases the size by 1
String toString()
Returns a String representation of the vector
enclosed in brackets
Uses the toString methods of the individual
elements
Puts commas between

38
Exercise

Trace the following code
for (int i 0 i lt x.size() i)
Object y x.elementAt(i)
System.out.println(y.toString())
Is toString needed? Could we just print y?
We know that the elements are Strings can we
store the result in a String
Review and try in Vector Exercises project

39
The Need for Casting

Suppose we want to store the reference returned
by elementAt to an object of the correct
(specific) type
E.g. String y x.elementAt(i)
Compiler error ? - why?
We can use casting
Specify the data type in parentheses
E.g. String y (String) x.elementAt(i)
Again, try in Vector Exercises project

40
Wrapper Classes

Vectors can store objects but not primitives
What if we want to store integers in a vector?
Wrapper classes encapsulate primitive values in
objects
Java wrapper classes
Boolean
Character
Integer
Double
Have used the class method Integer.parseInt()

41
Methods of the Integer Class

Integer(int n) constructor
Integer a new Integer(3)
Integer(String s) constructor
Integer b new Integer(3)
int intValue() returns the integer wrapped in
the object
int c a.intValue() // parseInt for String
Boolean equals(Object obj) returns true if the
"wrapped" integers have the same value
a.equals(b)

42
Using Wrapper Classeswith Vectors