1. Building Java Programs Chapter 7.5
Multidimensional Arrays

2. two-dimensional array
Allow for the creation of table-like data structures with a row and column format. The first subscript is a row index in a table while the second subscript is a column index in a table. Declaration type[][] name = new type [row size][col size] Examples int [][] grid = new int[3][4];
cols 1 2 3
[0][0]
[0][1]
[0][2]
[0][3]
rows 1
[1][0]
[1][1]
[1][2]
[1][3] 2
[2][3]
[2][0]
[2][1]
[2][2]

3. declarations Think of a 2D Array as An Array of Arrays
int[][]table = new int[3][4]; int[][] table = { {0,0,0,0}, {0,0,0,0}, {0,0,0,0} };
Think of a 2D Array as
An Array of Arrays

4. Referencing elements of 2D array
The array below is a 3 x 5 array of ints
Declaration:
Int [][] grid = new int[3][5];
grid[0][0]=
grid[2][2]=
grid[1][3]=
grid[3][0]=  2 4  6  8
10   3 9 
 12
15   4
12 
 16
20 

5. Referencing elements of 2D array
The array below is a 3 x 5 array of ints
Declaration:
char [][] grid = new char[3][5];
grid[0][0]=
grid[2][2]=
grid[1][3]=
grid[0][5]= A B C D E V W X Y Z H I J K L

6. Referencing elements of 2d Array
int[][] grid = new int[3][5]; for (int row = 0; row < 3; row++){ for (int col = 0; col < 5; col++){ grid[row][col] = row + col; } }

7. Passing Two-Dimensional Arrays to Methods
public void printTable (int[][] table){ for (int row = 0; row < table.length; row++){ for (int col = 0; col < table[row].length; col++){ System.out.printf(“%4d”, table[row][col]); } System.out.println(); } }

8. Lets try some code
Write a piece of code that constructs a two-dimensional array of integers named table with 5 rows and 10 columns. Fill the array with a multiplication table, so that array element [i][j] contains the value i * j. Use nested for loops to build the array.

9. Multiplication table
int[][] table = new int[5][10];//declaration for (int i = 0; i < 5; i++) { for (int j = 0; j < 10; j++) { table[i][j] = i * j; }
int[][] table = new int[5][10];//declaration
for (int i = 0; i < table.length; i++) {
for (int j = 0; j < table[i].length; j++) {
table[i][j] = i * j; }

10. More Code Matrix add
Write a method named matrixAdd that accepts a pair of two-dimensional arrays of integers as parameters, treats the arrays as 2D matrices and adds them, returning the result. The sum of two matrices A and B is a matrix C where for every row i and column j, Cij = Aij + Bij. You may assume that the arrays passed as parameters have the same dimensions.

11. public static int[][] matrixAdd(int[][]a, int[][]b){ int rows = a
public static int[][] matrixAdd(int[][]a, int[][]b){ int rows = a.length; int cols = 0; if (rows>0){ cols=a[0].length; } int[][]sum= new int[rows][cols]; //declare sum matrix for(int r=0;r<a.length;r++){ for(int c=0;c<a[r].length;c++){ sum[r][c]=a[r][c] +b[r][c]; } return sum;

12. Create, populate and traverse multidimensional arrays!
What we covered
Create, populate and traverse multidimensional arrays!