|
|
Free Open Book
Sams Teach Yourself JavaScript in 24 Hours |
|
Using for LoopsLoops are useful any time you need a section of code to execute more than once. The for keyword is the first tool to consider for creating loops. A for loop typically uses a variable (called a counter or an index) to keep track of how many times the loop has executed, and it stops when the counter reaches a certain number. A basic for statement looks like this: for (var = 1; var < 10; var++) {There are three parameters to the for loop, separated by semicolons:
After the three parameters are specified, a left brace ({) is used to signal the beginning of a block. A right brace (}) is used at the end of the block. All the statements between the braces will be executed with each iteration of the loop. The parameters for a for loop may sound a bit confusing, but once you're used to it, you'll use for loops frequently. Here is a simple example of this type of loop: for (i=0; i<10; i++) {
document.write("This is line " + i + "<br>");
}These statements define a loop that uses the variable i, initializes it with the value of zero, and loops as long as the value of i is less than 10. The increment expression, i++, adds one to the value of i with each iteration of the loop. Because this happens at the end of the loop, the output will list the numbers zero through nine. When a loop includes only a single statement between the braces, as in this example, you can omit the braces if you want. The following statement defines the same loop without braces: for (i=0; i<10; i++)
document.write("This is line " + i + "<br>");Did you Know? It's a good style convention to use braces with all loops whether they contain one statement or many. This makes it easy to add statements to the loop later without causing syntax errors. The loop in this example contains a document.write statement that will be repeatedly executed. To see just what this loop does, you can add it to a <script> section of an HTML document as shown in Listing 7.3. Listing 7.3. A Loop Using the for Keyword
This example displays a message with the loop's counter during each iteration. The output of Listing 7.3 is shown in Figure 7.2. Figure 7.2. The results of the for loop example.
Notice that the loop was only executed nine times. This is because the conditional is i<10. When the counter (i) is incremented to 10, the expression is no longer true. If you need the loop to count to 10, you can change the conditional; either i<=10 or i<11 will work fine. By the Way You might notice that the variable name i is often used as the counter in loops. This is a programming tradition that began with an ancient language called Forth. There's no need for you to follow this tradition, but it is a good idea to use one consistent variable for counters. (To learn more about Forth, see the Forth Interest Group's website at www.forth.org.) The structure of the for loop in JavaScript is based on Java, which in turn is based on C. Although it is traditionally used to count from one number to another, you can use just about any statement for the initialization, condition, and increment. However, there's usually a better way to do other types of loops with the while keyword, described in the next section.
|
Main Menu |
| 500 Juegos Gratis | 500 Giochi Gratis | 500 Jeux Gratuits | 500 Jogos Gratis | 500 Kostenlose Spiele |
