ch13.htm

Java_by_Example,初级经典例子哦,珍藏版本

Array.<BR>

</B>

</BLOCKQUOTE>

<BLOCKQUOTE>

<PRE>

table[0][0] = 0;

table[1][0] = 1;

table[2][0] = 2;

table[3][0] = 3;

table[0][1] = 4;

table[1][1] = 5;

table[2][1] = 6;

table[3][1] = 7;

table[0][2] = 8;

table[1][2] = 9;

table[2][2] = 10;

table[3][2] = 11;

table[0][3] = 12;

table[1][3] = 13;

table[2][3] = 14;

table[3][3] = 15;

</PRE>

</BLOCKQUOTE>

<HR>

<P>

You refer to a value stored in a two-dimensional array by using

subscripts for both the column and row in which the value you

want is stored. For example, to retrieve the value 11 from the

<TT>table[][]</TT> array shown in Figure 13.4, you use a line

like this:

<BLOCKQUOTE>

<PRE>

int value = table[3][2];

</PRE>

</BLOCKQUOTE>

<P>

A quick way to initialize a two-dimensional array is to use nested

<TT>for</TT> loops, as shown in Listing 13.7.

<HR>

<BLOCKQUOTE>

<B>Listing 13.7&nbsp;&nbsp;LST13_11.TXT: Using Loops to Initialize

a Two-Dimensional Array.<BR>

</B>

</BLOCKQUOTE>

<BLOCKQUOTE>

<PRE>

for (int x=0; x&lt;3; ++x)

{

    for (int y=0; y&lt;3; ++y)

    {

        table[x][y] = 5;

    }

}

</PRE>

</BLOCKQUOTE>

<HR>

<P>

If you've never seen nested loops before, you're about to discover

how handy they can be. In the case of Listing 13.7, the outside

loop (the <TT>x</TT> loop) starts first, setting <TT>x</TT> to

0. But the body of the loop is another loop. So the inside loop

(the <TT>y</TT> loop) starts, setting <TT>y</TT> to 0, which brings

the program to the line that initializes an element of the array.

Because <TT>x</TT> and <TT>y</TT> both equal 0, the array element

<TT>table[0][0]</TT> gets set to 5. Then the inside loop sets

<TT>y</TT> to 1, which means <TT>table[0][1]</TT> gets set to

5. When the inner loop finishes, the program branches back to

the outer loop, setting <TT>x</TT> to 1. The inner loop repeats

again, only this time with <TT>x</TT> equal to 1 and <TT>y</TT>

going from 0 to 2. Finally, when both loops finish, the entire

array is initialized.

<P>

Of course, to create the array shown in Figure 13.4 with loops,

you have to be a little more tricky, as shown in Listing 13.8.

Work through each loop to see how the array gets initialized.

<HR>

<BLOCKQUOTE>

<B>Listing 13.8&nbsp;&nbsp;LST13_8.TXT: Initializing the Array

Elements to Different Values.<BR>

</B>

</BLOCKQUOTE>

<BLOCKQUOTE>

<PRE>

for (int x=0; x&lt;3; ++x)

{

    for (int y=0; y&lt;3; ++y)

    {

        table[x][y] = x + y * 4;

    }

}

</PRE>

</BLOCKQUOTE>

<HR>

<H3><A NAME="ExampleCreatingaTwoDimensionalArray">

Example: Creating a Two-Dimensional Array</A></H3>

<P>

Suppose that you need a table-like array that can hold 80 integers

in eight columns and 10 rows. First, you'd declare the array like

this:

<BLOCKQUOTE>

<PRE>

int numbers[][];

</PRE>

</BLOCKQUOTE>

<P>

After declaring the array, you need to create it in memory, like

this:

<BLOCKQUOTE>

<PRE>

numbers = new int[8][10];

</PRE>

</BLOCKQUOTE>

<P>

The last step is to initialize the array, probably using nested

<TT>for</TT> loops:

<BLOCKQUOTE>

<PRE>

for (int x=0; x&lt;8; ++x)

    for (int y=0; y&lt;10; ++y)

        numbers[x][y] = 0;

</PRE>

</BLOCKQUOTE>

<P>

These lines initialize the <TT>numbers[][]</TT> array to all zeroes.

<H2><A NAME="ExampleUsingTwoDimensionalArraysinanApplet"><FONT SIZE=5 COLOR=#Ff0000>

Example: Using Two-Dimensional Arrays in an Applet</FONT></A></H2>

<P>

To be sure you understand how arrays work, you'll put a two-dimensional

array to work in a program called Applet18. The Applet18 applet

creates and initializes a two-dimensional array with six columns

and eight rows. (Try to imagine the elements of this array as

the rows and columns of a spreadsheet.) The program then prints

the contents of the array in the Applet's display area, so you

can see that the array truly holds the values to which it was

initialized. Listing 13.9 is the program, whereas Figure 13.5

shows the applet running under the Appletviewer application.

<P>

<A HREF="f13-5.gif"><B> Figure 13.5 : </B><I>This is Applet18 running under Appletviewer.



</I></A><P>

<HR>

<BLOCKQUOTE>

<B>Listing 13.9&nbsp;&nbsp;Applet18.java: Using a Two-Dimensional

Array.<BR>

</B>

</BLOCKQUOTE>

<BLOCKQUOTE>

<PRE>

import java.awt.*;

import java.applet.*;



public class Applet18 extends Applet

{

    int table[][];



    public void init()

    {

        table = new int[6][8];



        for (int x=0; x&lt;6; ++x)

            for (int y=0; y&lt;8; ++y)

                table[x][y] = x+y*6;

    }



    public void paint(Graphics g)

    {

        for (int x=0; x&lt;6; ++x)

            for (int y=0; y&lt;8; ++y)

            {

                String s = String.valueOf(table[x][y]);

                g.drawString(s, 50+x*25, 50+y*15);

            }

    }

}

</PRE>

</BLOCKQUOTE>

<HR>

<P>

<IMG ALIGN=RIGHT SRC="pseudo.gif" HEIGHT=94 WIDTH=94 BORDER=1>

<BLOCKQUOTE>

Tell Java that the program uses classes in the <TT>awt</TT> package.

<BR>

Tell Java that the program uses classes in the <TT>applet</TT>

package.<BR>

Derive the <TT>Applet18</TT> class from Java's <TT>Applet</TT>

class.<BR>

    Declare a two-dimensional integer array.<BR>

    Override the <TT>Applet</TT> class's <TT>init()</TT> method.

<BR>

        Create an array with six columns and eight rows.<BR>

        Loop from 0 to 5.<BR>

            Loop from 0 to 7.<BR>

                Initialize the currently indexed array element.

<BR>

    Override the <TT>Applet</TT> class's <TT>paint()</TT> method.

<BR>

        Loop from 0 to 5.<BR>

            Loop from 0 to 7.<BR>

                Convert the array element to a string.<BR>

                Display the array element's value.

</BLOCKQUOTE>

<P>

Notice in <TT>init()</TT> and <TT>paint()</TT> how the nested

loops don't have curly braces like the example shown in Listing

13.8. This is because when you have only one statement in a program

block, the curly braces are optional. In Applet18's <TT>init()</TT>

method, the outside loop contains only one statement, which is

the inner <TT>for</TT> loop. The inner <TT>for</TT> loop also

contains only a single statement, which is the line that initializes

the currently indexed element of the array. In the <TT>paint()</TT>

method, the outer loop contains only one statement, which is the

inner <TT>for</TT> loop. However, the inner loop contains two

statements, so the curly braces are required in order to mark

off that program block.

<H2><A NAME="Summary"><FONT SIZE=5 COLOR=#Ff0000>

Summary</FONT></A></H2>

<P>

Arrays are a powerful data structure that enable you to store

many related values using the same variable name. A one-dimensional

array is a lot like a list of values that you can access by telling

Java the appropriate subscript (or index). But because array subscripts

always start at 0, the subscript is always one less than the number

of the associated element. You can also create multidimensional

arrays (or, to be more precise, arrays of arrays). A two-dimensional

array is organized much like a table. To access the elements of

a two-dimensional array, you need two subscripts. The first subscript

identifies the column of the table and the second identifies the

row.

<H2><A NAME="ReviewQuestions"><FONT SIZE=5 COLOR=#Ff0000>

Review Questions</FONT></A></H2>

<OL>

<LI>What is an array?

<LI>Why are arrays easier to use than a bunch of related variables?

<LI>What is an array subscript? How is a subscript like an index?

<LI>What is a two-dimensional array?

<LI>If you had an array of 50 integers, what is the largest valid

subscript?

<LI>What happens if you try to access a nonexistent array element?

<LI>Describe why a <TT>for</TT> loop is appropriate for accessing

an array?

<LI>How would you use <TT>for</TT> loops to initialize a two-dimensional

array?

</OL>

<H2><A NAME="ReviewExercises"><FONT SIZE=5 COLOR=#Ff0000>

Review Exercises</FONT></A></H2>

<OL>

<LI>Declare an array that can hold 50 integers.

<LI>Write the code that creates the array you declared in exercise

1.

<LI>Write a <TT>for</TT> loop that initializes the array to the

values 50 through 99.

<LI>Write the Java code to declare and create a two-dimensional

array with 10 columns and 15 rows.

<LI>Write nested <TT>for</TT> loops that initialize the array

from exercise 4 to the values 0 through 149.

<LI>Write the Java code needed to display, in table form, the

values in the array from exercise 5.

<LI>Modify Applet17 so that it stores and displays not only the

bowlers' scores, but also the bowlers' names. Create three <TT>TextField</TT>

objects to enable the user to enter names and three for entering

the scores. Name the program <TT>ScoreApplet.java</TT>. Figure

13.6 shows what the final applet should look like at startup.

(You can find the solution to this programming problem in the

CHAP13 folder of this book's CD-ROM.)<BR>

<A HREF="f13-6.gif"><B> Figure 13.6 : </B><I>This is the ScoreApplet applet running under Appletviewer.



</I></A><P>

</OL>

<HR>











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