Transcript Two –Dimensional Arrays

Two –Dimensional Arrays
Mrs. C. Furman
Java Programming
November 19, 2008
Two Dimensional Arrays
 rows and columns, also called a Matrix
 we must specify the number of row and columns
we will need when we construct the 2DArray.
Similar to Arrays.
0 1 2 3
Example:
int [][] magicSquare = new int [4][4]; 0 0 0 0 0
1 0 0 0 0
2
0 0 0 0
3
0 0 0 0
Accessing Elements in a 2D Array
 Elements are indexed [row][column]
magicSquare [0][0] = 3;
magicSquare [1][3] = 5;
int myNum = magicSquare [1][3];
0
1
2
3
0
3 0
0 0
1
0 0
0 5
2
0 0
0 0
3
0 0 0 0
Initializer List for 2D
int [][]cars = {{10, 7, 12, 10, 4},
{18, 11, 15, 17, 10},
{12, 10, 9, 5, 12},
{16, 6, 13, 8, 3}};
red
brown black white gray
GM
10 7
12 10 4
Ford
18 11 15 17 10
Toyota
12 10 9
BMW
16 6
5
13 8
12
3
Getting Size
 .length gives us the number of rows…
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then when we access a specific row, we
do .length to get number of columns.
int[][] list = {{1,2,3, 4}, {1}, {1, 2, 3}};
int numRows = list.length;//3
int firstCol = list[0].length;//4
int secCol = list[1].length;//1
int thirdCol = list[2].length;//3
Looping Through a 2D Array
Example 3: Write a nested for loop to output all
elements in the 2D array.
for (int row = 0; row < list.length; row++)
{
for (int col = 0; col< list[row].length; col++)
{
System.out.print (list [row][col] + “ “);
}
System.out.println ();
}
Multiplication Table
 Write a loop that will assign a 5 x 5 2D Array to
the multiplication tables of 0 ..4.
The result will look like the below 2D Array
0
1
2
3
4
0 0
0
0
0
0
1 0
1
2
3
4
2 0
2
4
6
8
3 0
3
6
9
12
4 0
4
8
12 16
Picture Encoding
 Bitmap images: each dot or pixel is
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represented separately.
Pictures are 2-D arrays of pixels.
We will be dealing with .jpg (jpeg) files
Each pixel in a picture has a color
The dimension of the picture is measured
in pixels.
JPEG’s
 High quality pictures with smaller storage.
 JPEG is a lossy compression format.
 Lossy compression: it is compressed,
made smaller, but not with 100% of the
quality of the original.
 Typically what gets thrown out is the stuff
that you don’t see or notice anyway, so the
quality is still considered high.
Color
 RGB color model.
 Pixels are encoded with three numbers, 1st
for the amount of red, 2nd for the amount of
green, and 3rd for the amount of blue
 We can make any humanly visible color by
combining different amounts of red, green
and blue.
 Combining all 3 together to get white,
remove all 3 to get black.
 (0,0,0) – black; (255, 255, 255) - white
Some Color examples
 (255, 0, 0) – red
 (100, 0, 0) – darker red
 (0, 100, 0) – dark green
 (0, 0 , 100) – dark blue
 All 3 the same… we get gray…
 (50, 50,50) – dark gray
 (150, 150, 150) – lighter gray