Chapter 3 Arrays in Java

1
Chapter 3 Arrays : Motivations
Often you will have to store a large number of values during
the execution of a program. Suppose, for instance, that you
need to read one hundred numbers, compute their average, and
find out how many numbers are above the average. Your
program first reads the numbers and computes their average,
and then compares each number with the average to determine
whether it is above the average. The numbers must all be stored
in variables in order to accomplish this task. You have to
declare one hundred variables and repeatedly write almost
identical code one hundred times. From the standpoint of
practicality, it is impossible to write a program this way. So,
how do you solve this problem?

2
Introducing Arrays
Array is a data structure that represents a collection of the
same types of data.
5.6
4.5
3.3
13.2
4
34.33
34
45.45
99.993
11123
double[] myList = new double[10];
myList reference
myList[0]
myList[1]
myList[2]
myList[3]
myList[4]
myList[5]
myList[6]
myList[7]
myList[8]
myList[9]
Element value
Array reference
variable
Array element at
index 5

3
Declaring Array Variables
datatype[] arrayRefVar;
Example:
double[] myList;
datatype arrayRefVar[];
// This style is allowed, but not preferred
Example:
double myList[];

4
Creating Arrays
arrayRefVar = new datatype[arraySize];
Example:
myList = new double[10];
myList[0] references the first element in the array.
myList[9] references the last element in the array.

5
Declaring and Creating
in One Step
datatype[] arrayRefVar = new
datatype[arraySize];
datatype arrayRefVar[] = new
datatype[arraySize];
double myList[] = new double[10];

6
The Length of an Array
Once an array is created, its size is fixed. It cannot be
changed. You can find its size using
arrayRefVar.length
For example,
myList.length returns 10

7
Indexed Variables
The array elements are accessed through the index. The
array indices are 0-based, i.e., it starts from 0 to
arrayRefVar.length-1. In the example in Figure 3.1,
myList holds ten double values and the indices are
from 0 to 9.
Each element in the array is represented using the
following syntax, known as an indexed variable:
arrayRefVar[index];

8
Using Indexed Variables
After an array is created, an indexed variable can
be used in the same way as a regular variable.
For example, the following code adds the value
in myList[0] and myList[1] to myList[2].
myList[2] = myList[0] + myList[1];

9
Array Initializers
Declaring, creating, initializing in one step:
double[] myList = {1.9, 2.9, 3.4, 3.5};
This shorthand syntax must be in one
statement.

10
Declaring, creating, initializing
Using the Shorthand Notation
double[] myList = {1.9, 2.9, 3.4, 3.5};
This shorthand notation is equivalent to the
following statements:
myList[0] = 1.9;
myList[1] = 2.9;
myList[2] = 3.4;
myList[3] = 3.5;

11
CAUTION
Using the shorthand notation, you
have to declare, create, and initialize
the array all in one statement.
Splitting it would cause a syntax
error. For example, the following is
wrong:
double[] myList;
myList = {1.9, 2.9, 3.4, 3.5};

12
Trace Program with Arrays
public class Test {
public static void main(String[] args) {
int[] values = new int[5];
for (int i = 1; i < 5; i++) {
values[i] = i + values[i-1];
}
values[0] = values[1] + values[4];
}
}
Declare array variable values, create an
array, and assign its reference to values
After the array is created
0
1
2
3
4
0
0
0
0
0
animation

13
public class Test {
for (int i = 1; i < 5; i++) {
values[i] =i + values[i-1];
}
}
}
i becomes 1
0
1
2
3
4
0
0
0
0
0
animation

14
public class Test {
for (int i = 1; i < 5; i++) {
}
}
}
i (=1) is less than 5
0
1
2
3
4
0
0
0
0
0
animation

15
public class Test {
for (int i = 1; i < 5; i++) {
}
}
}
After this line is executed, value[1] is 1
After the first iteration
0
1
2
3
4
0
1
0
0
0
animation

16
public class Test {
for (int i = 1; i < 5; i++) {
}
}
}
After i++, i becomes 2
animation
0
1
2
3
4
0
1
0
0
0

17
public class Test {
for (int i = 1; i < 5; i++) {
values[i] = i+ values[i-1];
}
}
}
i (= 2) is less than 5
animation
0
1
2
3
4
0
1
0
0
0

18
public class Test {
for (int i = 1; i < 5; i++) {
}
}
}
After this line is executed,
values[2] is 3 (2 + 1)
After the second iteration
0
1
2
3
4
0
1
3
0
0
animation

19
public class Test {
for (int i = 1; i < 5; i++) {
}
}
}
After this, i becomes 3.
0
1
2
3
4
0
1
3
0
0
animation

20
public class Test {
for (int i = 1; i < 5; i++) {
}
}
}
i (=3) is still less than 5.
0
1
2
3
4
0
1
3
0
0
animation

21
public class Test {
for (int i = 1; i < 5; i++) {
}
}
}
After this line, values[3] becomes 6 (3 + 3)
After the third iteration
0
1
2
3
4
0
1
3
6
0
animation

22
public class Test {
for (int i = 1; i < 5; i++) {
}
}
}
After this, i becomes 4
0
1
2
3
4
0
1
3
6
0
animation

23
public class Test {
for (int i = 1; i < 5; i++) {
}
}
}
i (=4) is still less than 5
0
1
2
3
4
0
1
3
6
0
animation

24
public class Test {
for (int i = 1; i < 5; i++) {
}
}
}
After this, values[4] becomes 10 (4 + 6)
After the fourth iteration
0
1
2
3
4
0
1
3
6
10
animation

25
public class Test {
public static void main(String[] args)
{
for (int i = 1; i < 5; i++) {
}
}
}
After i++, i becomes 5
animation
0
1
2
3
4
0
1
3
6
10

26
public class Test {
for (int i = 1; i < 5; i++) {
}
}
}
i ( =5) < 5 is false. Exit the loop
animation
0
1
2
3
4
0
1
3
6
10

27
public class Test {
for (int i = 1; i < 5; i++) {
}
}
}
After this line, values[0] is 11 (1 + 10)
0
1
2
3
4
11
1
3
6
10
animation

28
Example: Analyzing Array Elements
Objective:
The program receives 6 numbers from the user,
Finds the largest number
And counts the occurrence of the largest number
entered.
Suppose you entered 3, 5, 2, 5, 5, and 5,
the largest number is 5
and its occurrence count is 4. TestArrayTestArray

29
Problem: Assigning Grades
Objective: read student scores (int), get the best
score, and then assign grades based on the
following scheme:
– Grade is A if score is >= best–10;
– Grade is B if score is >= best–20;
– Grade is C if score is >= best–30;
– Grade is D if score is >= best–40;
– Grade is F otherwise.
AssignGradeAssignGrade

30
Two-dimensional Arrays
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
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.

31
Declare/Create Two-dimensional Arrays
// 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];

32
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;

33
Two-dimensional Array Illustration
0 1 2 3 4
0
7
0 1 2 3 4
1
2
3
4
0
1
2
3
4
matrix[2][1] = 7;matrix = new int[5][5];
3
7
0 1 2
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

34
Declaring, Creating, and Initializing Using
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

35
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];

36
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

37
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},
{2, 3, 4, 5},
{3, 4, 5},
{4, 5},
{5}
};
matrix.length is 5
matrix[0].length is 5

38
Ragged Arrays, cont.
1 2 3 4 5int[][] 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

39
Problem: Grading Multiple-
Choice Test
Objective: write a
program that grades
multiple-choice test.A B A C C D E E A D
D B A B C A E E A D
E D D A C B E E A D
C B A E D C E E A D
A B D C C D E E A D
B B E C C D E E A D
B B A C C D E E A D
E B E C C D E E A D
0 1 2 3 4 5 6 7 8 9
Student 0
Student 1
Student 2
Student 3
Student 4
Student 5
Student 6
Student 7
Students’ Answers to the Questions:
D B D C C D A E A D
0 1 2 3 4 5 6 7 8 9
Key
Key to the Questions:
GradeExamGradeExam

40
Problem: Finding Two Points
Nearest to Each Other
FindNearestPointsFindNearestPoints
(1, 1)
(-1, -1)
(-1, 3)
(2, 0.5)
(3, 3)
(4, 2)
(2, -1)
(4, -0.5)
-1 3
-1 -1
1 1
2 0.5
2 -1
3 3
4 2
4 -0.5
x y
0
1
2
3
4
5
6
7

41
Case Study: Sudoku
5 3 7
6 1 9 5
9 8 6
8 6 3
4 8 3 1
7 2 6
6
4 1 9 5
8 7 9
5 3 4 6 7 8 9 1 2
6 7 2 1 9 5 3 4 8
1 9 8 3 4 2 5 6 7
8 5 9 7 6 1 4 2 3
4 2 6 8 5 3 7 9 1
7 1 3 9 2 4 8 5 6
9 6 1 5 3 7 2 8 4
2 8 7 4 1 9 6 3 5
3 4 5 2 8 6 1 7 9
The objective is to fill the grid (see Figure 3.2(a)) so that every row,
every column, and every 3×3 box contain the numbers 1 to 9, as
shown in Figure 3.2(b).

42
Case Study: Sudoku
SudokuSudoku
int[][] grid =
{{5, 3, 0, 0, 7, 0, 0, 0, 0},
{6, 0, 0, 1, 9, 5, 0, 0, 0},
{0, 9, 8, 0, 0, 0, 0, 6, 0},
{8, 0, 0, 0, 6, 0, 0, 0, 3},
{4, 0, 0, 8, 0, 3, 0, 0, 1},
{7, 0, 0, 0, 2, 0, 0, 0, 6},
{0, 6, 0, 0, 0, 0, 2, 8, 0},
{0, 0, 0, 4, 1, 9, 0, 0, 5},
{0, 0, 0, 0, 8, 0, 0, 7, 9}
};
int[][] freeCellList =
{{0, 2}, {0, 3}, {0, 5}, {0, 6}, {0, 7}, {0, 8},
{1, 1}, {1, 2}, {1, 6}, {1, 7}, {1, 8},
{2, 0}, {2, 3}, {2, 4}, {2, 5}, {2, 6}, {2, 8},
{3, 1}, {3, 2}, {3, 3}, {3, 5}, {3, 6}, {3, 7},
{4, 1}, {4, 2}, {4, 4}, {4, 6}, {4, 7},
{5, 1}, {5, 2}, {5, 3}, {5, 5}, {5, 6}, {5, 7},
{6, 0}, {6, 2}, {6, 3}, {6, 4}, {6, 5}, {6, 8},
{7, 0}, {7, 1}, {7, 2}, {7, 6}, {7, 7},
{8, 0}, {8, 1}, {8, 2}, {8, 3}, {8, 5}, {8, 6}
};

Chapter 3 Arrays in Java

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Chapter 3 Arrays in Java