2D Arrays in Java

1
2D Arrays in Java

• CSIT Symposium - Atlanta - June 2007
• Sandy Graham - University of Waterloo

2
Overview

• How 2D arrays work in Java
• A natural representation example - the Board
  class
• An interesting assignment idea - manipulating
  greyscale images

3
Features of 2D arrays in Java

• The variable values for all arrays are references
• must allocate space first, and then fill in the
  data
• allows for resizing later
• A 2D array is a 1D array of arrays
• one row of the array is a 1D array
• allows for non-rectangular arrays
• Can query the array for its dimensions
• .length

4
Creating and referencing a 2D Array

• int sample new int4 2
• sample1 0 17
• Note integer values in Java are automatically
  initialized to 0
• in general, values within the array must be
  initialized with an assignment statement

5
Traversing a 2D array

• Suppose you had an array of grades for various
  students
• not all students have the same number of courses
• double transcript new double
• Recall that transcripti is a reference to a 1D
  array of double values
• could be passed as a parameter

6
The Board class

• The Board is an object that displays a graphical,
  rectangular game board which can be interacted
  with and manipulated.
• put pegs and lines on it
• remove pegs and lines from it
• display text messages
• interact by way of clicking
• Used for introducing a variety of Java
  programming topics
• A natural relationship with arrays

7
What can you do with a Board?

• Simple games
• checkers, chess, go, tic-tac-toe, minesweeper,
  ...
• Patterns and simplegraphics
• Elementary animation

8
Possible Programming Topics

• Object construction
• Method calls
• Variables
• Selection and repetition structures
• Overloading
• 1D arrays
• 2D arrays
• more

9
A first picture
10
Creating a Board

• Just like any other real object in Java
• Board b1D new Board(10)
• or
• Board b2D new Board(3,5)

11
Pegs

• We can place pegs on the Board by using the
  putPeg method
• b1D.putPeg("black", 3)
• or
• b2D.putPeg("blue", 2,5)

12
Pegs (continued)

• Pegs are specified by a colour and a row and
  column
• Colours are one of
• black, white, red, orange, yellow, green, cyan,
  pink, blue
• Row and column must be valid for the particular
  board

13
Simple Exercise

• Read in the number of rows and columns and
  produce the following picture (the example uses 5
  rows and 6 columns)

14
Solution

• import java.util.Scanner
• public class Corners
• public static void main(String args)
• Scanner myScanner new Scanner(System.in)
• int rowmyScanner.nextInt()
• int colmyScanner.nextInt()
• Board b new Board(row, col)
• b.putPeg("white", 0, 0)
• b.putPeg("red", 0, col-1)
• b.putPeg("orange", row-1, 0)
• b.putPeg("green", row-1,col-1)

15
Removing Pegs

• Pegs can be removed from the board using
• b1D.removePeg(2)
• or
• b2D.removePeg(4,2)

16
Lines

• Lines can be drawn on the board using
• b2D.drawLine(0,1,3,2)
• and removed using
• b2D.removeLine(0,1,3,2)

17
Input and Output

• The user can interact with the Board using mouse
  clicks
• int getPosition() or
• Coordinate getClick()
• The user can receive text messages on the Board
• void displayMessage(String theMessage)

18
Arrays

• Arrays are difficult to visualize
• Numerical data is not interesting to students
• The Board is an array!!!
• In particular, the state of the Board is known
  only internally to the Board. Moreover, the only
  accessor methods are
• getRows(), getColumns()
• It is not possible to query the Board to find out
  information about the pegs (sort of an MVC
  paradigm)
• provide the .class file of the Board class only

19
Remembering state with arrays

• One dimensional arrays correspond exactly to one
  dimensional Board.
• Two dimensional arrays correspond to a two
  dimensional Board
• Peg information can be stored
• using a String value (representing a colour)
• using an integer value (representing the number
  of times a particular square has been clicked
  upon)
• using a boolean value (representing the presence
  or absence of a peg)
• using a char value (representing an abstract
  value that the peg is modeling)

20
Exercise

• Create a one-dimensional board of size 3, and
  each click on a cell changes the colour of the
  cell from white to red to black to white again.
  The program stops when all pegs are black.

21
Solution

• public class OneD
• public static void main(String args)
• Board b1D new Board(3)
• String colours new String3
• colours0 "white"
• colours1 "red"
• colours2 "black"
• int markers new int3
• markers0 0
• markers1 0
• markers2 0
• b1D.putPeg("white", 0)
• b1D.putPeg("white", 1)
• b1D.putPeg("white", 2)
• while (markers0 3 ! 2 markers1
  3! 2 markers2 3 ! 2)
• int position b1D.getClick().getCol()
• markersposition

22
More advanced projects

• Peg game - one and two dimensions

23
Greyscale images and 2D arrays

• Practice manipulating 2D arrays of integers that
  represent values in a greyscale image
• Sample transformations
• darken
• flip
• rotate
• mirror
• blur
• negative

24
A good project

• Advantages
• many possible array manipulations available
• works with a real life object
• Disadvantages
• lots of hidden code to make it work
• overhead in understanding how the transformations
  code fits with the rest of the project

25
Summary

• Two dimensional arrays in Java are represented
  differently than many other languages
• There are natural two dimensional structures that
  exist that can be modeled
• Visual and interactive projects tend to be
  self-motivating