HTML and CSS Reference
In-Depth Information
In this loop, the while loop continues as long as the value of the i variable remains
less than 4. Each time through the command block, the loop writes the value of i into a
table cell and then increases the counter by 1.
Like for loops, while loops can be nested within one another. The following code
demonstrates how to create the 3 × 2 table shown earlier in Figure 12-13 using nested
while loops:
var rowNum = 1;
while (rowNum < 4) {
var colNum = 1;
while (colNum < 3) {
document.write(“<td>” + rowNum + “,” + colNum + “</td>”);
Again, the initial values of the counter variables are set before the while loops are run
and are updated within the command blocks.
Because for loops and while loops share many of the same characteristics, which
one you choose for a given application is often a matter of personal preference. In
general, for loops are used whenever you have a counter variable and while loops are
used for conditions that don't easily lend themselves to using counters. For example, you
could construct a while loop that would run only when the current time falls within a
specified time interval.
Use a for loop when your
loop contains a counter
variable. Use a while
loop for a more general
stopping condition.
Exploring the do / while Loop
In the for and while loops, the test to determine whether to continue the loop is made
before the command block is run. JavaScript also supports a program loop called do /
while that tests the condition to continue the loop right after the latest command block
is run. The structure of the do / while loop is as follows:
do {
while ( continue );
For example, the following code is used to create the table shown earlier in Figure 12-12
as a do / while loop:
var i = 0;
do {
document.write(“<td>” + i + “</td>”);
while (i < 4);
The do / while loop is usually used when the program loop should run at least once
before testing for the stopping condition.
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