Multi dimensional arrays

Thus far, you have used one-dimensional arrays to model linear
collections of elements. You can use a two-dimensional array to
represent a matrix or a table. For example, the following table that
describes the distances between the cities can be represented using
a two-dimensional array.
2
Motivations
Chicago
Boston
New York
Atlanta
Miami
Dallas
Houston
Distance Table (in miles)
Chicago Boston New York Atlanta Miami Dallas Houston
0 983 787 714 1375 967 1087
983 0 214 1102 1763 1723 1842
787 214 0 888 1549 1548 1627
714 1102 888 0 661 781 810
1375 1763 1549 661 0 1426 1187
967 1723 1548 781 1426 0 239
1087 1842 1627 810 1187 239 0
1723 1548 781 1426 0 239

3
Objectives
To give examples of representing data using two-dimensional arrays
To declare variables for two-dimensional arrays, create arrays, and
access array elements in a two-dimensional array using row and column
indexes
To program common operations for two-dimensional arrays (displaying
arrays, summing all elements, finding min and max elements, and
random shuffling)

Declare/Create Two-dimensional Arrays
4
// Declare array ref var
dataType[][] refVar;
// Create array and assign its reference to variable
refVar = new dataType[10][10];
// Combine declaration and creation in one statement
dataType[][] refVar = new dataType[10][10];
// Alternative syntax
dataType refVar[][] = new dataType[10][10];

5
Declaring Variables of Two-dimensional
Arrays and Creating
Two-dimensional Arrays
int[][] matrix = new int[10][10];
or
int matrix[][] = new int[10][10];
matrix[0][0] = 3;
for (int i = 0; i matrix.length; i++)
for (int j = 0; j matrix[i].length; j++)
matrix[i][j] = (int)(Math.random() * 1000);
double[][] x;

Two-dimensional Array Illustration
0 1 2
6
0 1 2 3 4
0
0 1 2 3 4
7
1
2
3
4
0
1
2
3
4
matrix[2][1] = 7;
matrix = new int[5][5];
3
7
0
1
2
int[][] array = {
{1, 2, 3},
{4, 5, 6},
{7, 8, 9},
{10, 11, 12}
};
1
2
3
4
5
6
8
9
10
11
12
array.length? 4
array[0].length? 3
matrix.length? 5
matrix[0].length? 5

Declaring, Creating, and Initializing Using
7
Shorthand Notations
You can also use an array initializer to declare, create and
initialize a two-dimensional array. For example,
int[][] array = new int[4][3];
array[0][0] = 1; array[0][1] = 2; array[0][2] = 3;
int[][] array = {
{1, 2, 3},
{4, 5, 6},
{7, 8, 9},
{10, 11, 12}
};
Same as

8
Lengths of Two-dimensional
Arrays
x
x[0]
x[1]
x[2]
x[0][0] x[0][1] x[0][2] x[0][3]
x[1][0] x[1][1] x[1][2] x[1][3]
x[2][0] x[2][1] x[2][2] x[2][3]
x.length is 3
x[0].length is 4
x[1].length is 4
x[2].length is 4
int[][] x = new int[3][4];

9
Lengths of Two-dimensional
Arrays, cont.
int[][] array = {
{1, 2, 3},
{4, 5, 6},
{7, 8, 9},
{10, 11, 12}
};
array.length
array[0].length
array[1].length
array[2].length
array[3].length
array[4].length ArrayIndexOutOfBoundsException

10
Ragged Arrays
Each row in a two-dimensional array is itself an array. So,
the rows can have different lengths. Such an array is
known as a ragged array. For example,
int[][] matrix = {
{1, 2, 3, 4, 5},
matrix.length is 5
{2, 3, 4, 5},
matrix[0].length is 5
{3, 4, 5},
{4, 5},
{5}
};

11
Ragged Arrays, cont.
1 2 3 4 5
int[][] triangleArray = {
{1, 2, 3, 4, 5},
{2, 3, 4, 5},
{3, 4, 5},
{4, 5},
{5}
};
1 2 3 4
1 2 3
1 2
1 2

Processing Two-Dimensional Arrays
See the examples in the text.
1. (Initializing arrays with input values)
2. (Printing arrays)
3. (Summing all elements)
4. (Summing all elements by column)
5. (Which row has the largest sum)
6. (Finding the smallest index of the largest element)
7. (Random shuffling)
12

Initializing arrays with input values
java.util.Scanner input = new Scanner(System.in);
System.out.println(Enter + matrix.length + rows and +
matrix[0].length + columns: );
for (int row = 0; row matrix.length; row++) {
for (int column = 0; column matrix[row].length; column++) {
matrix[row][column] = input.nextInt();
}
}
13

Initializing arrays with random values
matrix[row][column] = (int)(Math.random() * 100);
}
}
14

15
Printing arrays
System.out.print(matrix[row][column] + );
}
System.out.println();
}

16
Summing all elements
int total = 0;
total += matrix[row][column];
}
}

17
Summing elements by column
for (int column = 0; column matrix[0].length; column++) {
int total = 0;
for (int row = 0; row matrix.length; row++)
total += matrix[row][column];
System.out.println(Sum for column + column + is
+ total);
}

18
Multidimensional Arrays
Occasionally, you will need to represent n-dimensional
data structures. In Java, you can create n-dimensional
arrays for any integer n.
The way to declare two-dimensional array variables and
create two-dimensional arrays can be generalized to
declare n-dimensional array variables and create n-dimensional
arrays for n = 3. For example, the following
syntax declares a three-dimensional array variable scores,
creates an array, and assigns its reference to scores.
double[][][] scores = new double[10][5][2];

19
Problem: Calculating Total Scores
Objective: write a program that calculates the total score for
students in a class. Suppose the scores are stored in a three-dimensional
array named scores. The first index in scores refers to
a student, the second refers to an exam, and the third refers to the
part of the exam. Suppose there are 7 students, 5 exams, and each
exam has two parts--the multiple-choice part and the programming
part. So, scores[i][j][0] represents the score on the multiple-choice
part for the i’s student on the j’s exam. Your program displays the
total score for each student.
TToottaallSSccoorree RRuunn

Multi dimensional arrays

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