Paragrafo 10.5

Java - Fondamenti di programmazione
C. Thomas Wu
Copyright © 2009 - The McGraw-Hill Companies srl
P a r a g r a f o 10.5
presentazione
10.5
Q
La rubrica (Address Book)
uesto paragrafo tratta gli array bidimensionali, utili per rappresentare dati in
forma tabellare. Ogni elemento in un array bidimensionale è individuato da
due indici, uno individua la riga, l’altro la colonna. In realtà, Java non offre
una struttura esplicita per realizzare array bidimensionali, che vengono di fatto implementati come array di array.
PARAGRAFO 10.5
Java - Fondamenti di programmazione
C. Thomas Wu
Copyright © 2009 - The McGraw-Hill Companies srl
576
Chapter 10
Arrays and Collections
10.5 Two-Dimensional Arrays
twodimensional
array
A table organized in rows and columns is a very effective means for communicating
many different types of information. Figure 10.13 shows sample data displayed in a
tabular format. In Java, we represent tables as two-dimensional arrays. The arrays
we have discussed so far are one-dimensional arrays because they have only one
index. In this section, we describe how two-dimensional arrays are used in Java.
Let’s begin with an example. Consider the following table with four rows and
five columns. The table contains the hourly rate of programmers based on their skill
level. The rows (horizontal) represent the grade levels, and the columns (vertical)
Distance Table (in miles)
Los
Angeles
San
Francisco
San Jose
San Diego
Monterey
Los Angeles
—
600
500
150
450
San Francisco
600
—
100
750
150
San Jose
500
100
—
650
50
San Diego
150
750
650
—
600
Monterey
450
150
50
600
—
Multiplication Table
4
5
6
1
2
3
1
1
2
3
4
5
6
7
8
9
2
2
4
6
8
10
12
14
16
18
3
3
6
9
12
15
18
21
24
27
4
4
8
12
16
20
24
28
32
36
5
5
10
15
20
25
30
35
40
45
6
6
12
18
24
30
36
42
48
54
7
7
14
21
28
35
42
49
56
63
8
8
16
24
32
40
48
56
64
72
9
9
18
27
36
45
54
63
72
81
7
8
Tuition Table
Day
Students
Boarding
Students
Grades 1–6
$16,000.00
$28,000.00
Grades 7–8
$19,000.00
$31,000.00
Grades 9–12
$22,500.00
$34,500.00
Figure 10.13 Examples of information represented as tables.
9
Java - Fondamenti di programmazione
C. Thomas Wu
Copyright © 2009 - The McGraw-Hill Companies srl
10.5 Two-Dimensional Arrays
577
represent the steps within a grade level. Reading the table, we know a programmer
with skill grade level 2, step 1 earns $36.50 per hour.
Grade
0
1
2
3
0
1
Step
2
3
4
10.50
20.50
34.00
50.00
12.00
22.25
36.50
60.00
14.50
24.00
38.00
70.00
16.75
26.25
40.35
80.00
18.00
28.00
43.00
99.99
We declare the pay scale table as
double[][] payScaleTable;
or
double payScaleTable[][];
and create the array as
payScaleTable = new double[4][5];
The payScaleTable array is a two-dimensional array because two indices—
one for the row and another for the column—are used to refer to an array element.
For example, to refer to the element at the second column (column 1) of the third
row (row 2), we say
payScaleTable[2][1]
Figure 10.14 illustrates how the two indices are used to access an array element of
a two-dimensional array.
Row # Column #
payScaleTable[ 2 ][ 1 ]
0
1
2
0
1
2
36.50
3
Figure 10.14 Accessing an element of a two-dimensional array.
3
4
Java - Fondamenti di programmazione
C. Thomas Wu
Copyright © 2009 - The McGraw-Hill Companies srl
578
Chapter 10
Arrays and Collections
Let’s go over some examples to see how the elements of two-dimensional
arrays are manipulated. This code finds the average pay of the grade 2 programmers.
double average, sum = 0.0;
for (int j = 0; j < 5; j++) {
sum += payScaleTable[2][j];
}
average = sum / 5;
The next example prints out the pay difference between the lowest and highest
steps for each grade level.
double difference;
for (int i = 0; i < 4; i++) {
difference = payScaleTable[i][4] - payScaleTable[i][0];
System.out.println("Pay difference at Grade Level " +
i + " is " + difference);
}
This code adds $1.50 to every skill level.
for (int i = 0; i < 4; i++) {
for (int j = 0; j < 5; j++) {
payScaleTable[i][j] += 1.50;
}
}
In the previous examples, we used literal constants such as 5 and 4 to keep them
simple. For real programs, we need to write a loop that will work for two-dimensional
arrays of any size, not just with the one with four rows and five columns. We can use
the length field of an array to write such a loop. Using the length field, we can
rewrite the third example as
for (int i = 0; i < payScaleTable.length; i++) {
for (int j = 0; j < payScaleTable[i].length; j++) {
payScaleTable[i][j] += 1.50;
}
}
Do you notice a subtle difference in the code? Let’s examine the difference
between the expressions
payScaleTable.length
and
payScaleTable[i].length
Java - Fondamenti di programmazione
C. Thomas Wu
Copyright © 2009 - The McGraw-Hill Companies srl
10.5 Two-Dimensional Arrays
579
First, there is actually no explicit structure called two-dimensional array in Java.
We only have one-dimensional arrays in Java. However, we can have an array of
arrays, and this is how the conceptual two-dimensional array is implemented in
Java. The sample array creation
payScaleTable = new double[4][5];
is really a shorthand for
payScaleTable = new double[4][ ];
payScaleTable[0]
payScaleTable[1]
payScaleTable[2]
payScaleTable[3]
=
=
=
=
new
new
new
new
double[5];
double[5];
double[5];
double[5];
which is equivalent to
payScaleTable = new double[4][ ];
for (int i = 0; i < 4; i++) {
payScaleTable[i] = new double[5];
}
Figure 10.15 shows the effect of executing the five statements. The expression
payScaleTable.length
refers to the length of the payScaleTable array itself.
payScaleTable
0
1
payScaleTable.length == 4
2
3
And the expression
payScaleTable[1].length
refers to the length of an array stored at row 1 of payScaleTable.
payScaleTable
0
0
1
2
3
4
1
2
3
payScaleTable[1].length == 5
Java - Fondamenti di programmazione
C. Thomas Wu
Copyright © 2009 - The McGraw-Hill Companies srl
580
Chapter 10
Arrays and Collections
Executing...
1
Will result in...
payScaleTable
0
1
payScaleTable = new double[4][ ];
2
3
2
payScaleTable
0
1
2
3
4
0
1
2
3
4
0
1
2
3
4
0
1
2
3
4
0
1
2
3
4
0
1
2
3
4
0
1
2
3
4
0
1
2
3
4
0
1
2
3
4
0
1
2
3
4
0
1
payScaleTable[0] = new double[5];
2
3
3
payScaleTable
0
1
payScaleTable[1] = new double[5];
2
3
4
payScaleTable
0
1
payScaleTable[2] = new double[5];
2
3
5
payScaleTable
0
1
payScaleTable[3] = new double[5];
2
3
Figure 10.15 Executing the statements on the left in sequence will create the array of arrays shown on
the right.
Java - Fondamenti di programmazione
C. Thomas Wu
Copyright © 2009 - The McGraw-Hill Companies srl
10.5 Two-Dimensional Arrays
581
We call an array that is part of another a subarray. The payScaleTable has four
subarrays of the same length. Since we allocate the subarrays individually, we can
create subarrays of different lengths. The following code creates a triangular array
whose subarray triangularArray[i] has length i.
triangularArray = new double[4][ ];
for (int i = 0; i < 4; i++)
triangularArray[i] = new double[i+1];
The resulting triangularArray looks like this:
triangularArray
0
0
0
1
0
1
2
0
1
2
1
2
3
3
An array of arrays can be initialized at the time of declaration. The following
declaration initializes the payScaleTable array:
double[][] payScaleTable
= { {10.50, 12.00,
{20.50, 22.25,
{34.00, 36.50,
{50.00, 60.00,
14.50,
24.00,
38.00,
70.00,
16.75,
26.25,
40.35,
80.00,
18.00},
28.00},
43.00},
99.99} };
Here’s the complete sample program:
/*
Chapter 10 Sample Program: Sample program for processing
2-D array of double.
File: Ch10PayScaleTable.java
*/
class Ch10PayScaleTable {
public static void main (String[] args) {
double[][] payScaleTable
= { {10.50,
{20.50,
{34.00,
{50.00,
12.00,
22.25,
36.50,
60.00,
14.50,
24.00,
38.00,
70.00,
16.75,
26.25,
40.35,
80.00,
18.00},
28.00},
43.00},
99.99} };
Java - Fondamenti di programmazione
C. Thomas Wu
Copyright © 2009 - The McGraw-Hill Companies srl
582
Chapter 10
Arrays and Collections
//Find the average pay of level 2 employees
double sum = 0.0, average;
for (int j = 0; j < 5; j++) {
sum += payScaleTable[2][j];
}
average = sum / 5;
System.out.println(" Average of Level 2 Employees: " + average );
System.out.println("\n");
//Display the pay difference at each grade level
double difference;
for (int i = 0; i < 4; i++) {
difference = payScaleTable[i][4] - payScaleTable[i][0];
System.out.println("Pay difference at Grade Level " +
i + " is " + difference);
}
//Print out the pay scale table
System.out.println("\n");
for (int i = 0; i < payScaleTable.length; i++) {
for (int j = 0; j < payScaleTable[i].length; j++) {
System.out.print( payScaleTable[i][j] + "
" );
}
System.out.println("");
}
//Increase the pay by 1.50 for every level/step
//and display the resulting table
System.out.println("\n");
for (int i = 0; i < payScaleTable.length; i++) {
for (int j = 0; j < payScaleTable[i].length; j++) {
payScaleTable[i][j] += 1.50;
System.out.print(payScaleTable[i][j] + "
}
System.out.println("");
}
}
}
");
Java - Fondamenti di programmazione
C. Thomas Wu
Copyright © 2009 - The McGraw-Hill Companies srl
10.6 Lists and Maps
583
We can nest for-each loops to process a two-dimensional array. Remember
that the two-dimensional array is structurally an array of arrays (as illustrated in
Figure 10.15), and the nested for-each loops will make this fact explict. To print out
the pay scale table, for example, we write
for (double[] row : payScaleTable) {
for (double pay : row) {
System.out.print(pay + "
");
}
System.out.println("");
}
The outer loop iterates over the rows in the payScaleTable two-dimensional array. Each
row is one-dimensional array of double, so the type is declared as double[]. And the
inner loop iterates over the elements in each row. Notice that we cannot rewrite the
other loop statements in the Ch10PayScaleTable program by using the for-each loop.
There is no limit to the number of dimensions an array can have. We can
declare three-dimensional, four-dimensional, and higher-dimensional arrays. However, arrays with a dimension higher than 2 are not frequently used in objectoriented languages. For example, data that were represented as a three-dimensional
array in a non-object-oriented language can be represented more naturally as a onedimensional array of objects with each object containing an array or some other
form of data structure (see Exercise 12 on page 615).
1. Write a code fragment to compute the average pay of the pays stored in the
payScaleTable array.
2. Write a code fragment that finds the largest integer in this two-dimensional array.
int[][] table = new int[10][10];
3. What is an output from this code?
int[][] table = new int[10][5];
System.out.println(table.length);
System.out.println(table[4].length);
10.6 Lists and Maps
array overflow
Once an array is created, its capacity cannot be changed. For example, if we create
an array of 20 elements, then we are limited to store at most 20 elements in using
this array. If we need to add elements, then we have to create a new array. (Note: We
will learn how to do this in Section 10.7.) We call the condition in which an array
does not have any unused position left to add another element an array overflow.