1. Lecture 8: 2D Arrays and Nested Loops
Yoni Fridman
7/10/01

2. Outline Two-dimensional arrays Creating 2D arrays Using 2D arrays
How 2D arrays are stored
Examples

3. Two-dimensional Arrays
Arrays in Java can have any number of dimensions.
Yesterday we looked at one-dimensional arrays.
Today we’ll look at two-dimensional arrays. 7 2 8 9 5 4 3 1 6

4. Creating 2D Arrays
2D arrays are declared just like 1D arrays, but with two pairs of square brackets:
int[][] myGrid;
Space is allocated for 2D arrays just like for 1D arrays, but again with two pairs of square brackets:
myGrid = new int[rows][cols];
So this declares and allocates a 4 x 9 array, like shown in the last slide, but full of zeros.
int[][] myGrid;
myGrid = new int[4][9]; OR
int[][] myGrid = new int[4][9];

5. Using 2D Arrays
To access the highlighted array element, for example, we write myGrid[2][6].
What will this example do?
columns 1 2 3 4 5 6 7 8
rows 1 2 3 7 2 8 9 5 4 3 1 6 3
myGrid[2][1] = 5;
System.out.println(myGrid[2][0] * myGrid[2][1]);

6. How 2D Arrays Are Stored
It’s intuitive to imagine a 2D array as a table, but Java actually stores it as an array of arrays.
So myGrid.length == 4.
And myGrid[0].length == myGrid[1].length == … == 9.
myGrid 1 2 3 7 2 8 9 5 4 3 1 6 7 2 8 9 5 4 3 1 8 6 2 5 3 9 5 6 8 1 2 6 9 5 4

7. Examples Nested loop example: Displaying a pyramid.
2D array example: Summing a table.

8. Homework
Read: 1.7, 2.3 (middle of p. 34 to middle of p. 36), 2.11, and on-line handout.
Remember: HW3 due tomorrow at the beginning of class.