2D Arrays - Everett Public Schools

Report
BUILDING JAVA
PROGRAMS
CHAPTER 7.5
MULTIDIMENSIONAL ARRAYS
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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]
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Examples
int [][] grid = new int[3][4];
rows
0
1
2
[0][0]
[1][0]
[2][0]
cols
1
2
3
[0][1]
[1][1]
[2][1]
[0][3]
[1][3]
[2][3]
[0][2]
[1][2]
[2][2]
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DECLARATIONS
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
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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]=
2
4
6
8
10
3
6
9
12
15
4
8
12
16
20
grid[2][2]=
grid[1][3]=
grid[3][0]=
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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]=
A
B
C
D
E
V
W
X
Y
Z
H
I
J
K
L
grid[2][2]=
grid[1][3]=
grid[0][5]=
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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;
}
}
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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();
}
}
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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.
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MULTIPLICATION TABLE
int[][]
table
= new
int[5][10];//declaration
int[][]
table
= new
int[5][10];//declaration
= i0;< itable.length;
< 5; i++) { i++) {
for for
(int (int
i = i0;
= j0;< jtable[i].length;
< 10; j++) { j++) {
for for
(int (int
j = j0;
table[i][j]
table[i][j]
= i =* j;i * j;
} }
} }
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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.
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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;
}
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WHAT WE COVERED
•Create, populate and
traverse multidimensional
arrays!
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