PHP 5/MySQL Programming- P26 potx
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<h1>Counting Backwards</h1>
<?
for ($i = 10; $i > 0; $i—){
print “$i <br>\n”;
} // end for loop
?>
</body>
</html>
If you understand how for loops work, the changes all make sense. I’m counting
backwards this time, so
$i begins with a large value (in this case 10). The condi-
tion for continuing the loop is now
$i > 0, which means the loop continues as
long as
$i is greater than 0. The loop ends as soon as $i is 0 or less.
Note that rather than adding a value to
$i, this time I decrement by 1 each time
through the loop. If you’re counting backwards, be very careful that the sentry
variable has a mechanism for getting smaller. Otherwise the loop never ends.
Recall that
$i++ adds 1 to $i; $i— subtracts 1 from $i.
Using a while Loop
PHP, like most languages, provides another kind of looping structure even more
flexible than the
for loop. You can use the while loop when you know how many
times something will happen. Figure 4.6 shows how a
while loop can work much
like a
for loop.
Repeating Code with a while Loop
The code for the while.php program is much like the for loop example, but you
can see that the
while loop is a little bit simpler:
<html>
<head>
<title>
A simple While Loop
</title>
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</head>
<body>
<h1>A simple while loop</h1>
<?
$i = 1;
while ($i <= 10){
print “$i <br>\n”;
$i++;
} // end while
?>
</body>
</html>
The while loop requires only one parameter, which is a condition. The loop con-
tinues as long as the condition is evaluated as
TRUE. As soon as the condition is
evaluated as
FALSE, the loop exits.
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FIGURE 4.6
Although this
program’s output
looks a lot like the
basic for loop,
it uses a different
construct to
achieve the
same result.
This particular program starts by initializing the variable $i, then checking to
see if it’s greater than or equal to
10 in the while statement. Inside the loop body,
the program prints the current value of
$i and increments $i.
Recognizing Endless Loops
The flexibility of the while construct gives it power, but with that power comes
potential for problems.
while loops are easy to build, but a loop that works
improperly can cause a lot of trouble. It’s possible that the code in the loop will
never execute at all. Even worse, you might have some sort of logical error that
causes the loop to continue indefinitely. As an example, look at the
badWhile.php
code:
<html>
<head>
<title>
A bad While Loop
</title>
</head>
<body>
<h1>A bad while loop</h1>
<?
$i = 1;
while ($i <= 10){
print “$i <br>\n”;
$j++;
} // end while
?>
</body>
</html>
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The badWhile.php program shows what happens when you have an endless
loop in your code. If you run this program, it may temporarily slow down your Web
server. Be sure your server is configured to stop a PHP process when the user
presses the stop button on the browser. (This is a default setting on most PHP
installations.)
The badWhile.php program has a subtle but deadly error. Look carefully at the
source code and see if you can spot it. The code is just like the first
while pro-
gram, except instead of incrementing
$i, I incremented $j. The variable $j has
nothing to do with
$i and $i never changes. The loop keeps going on forever,
because it cannot end until
$i is greater than or equal to 10, which never hap-
pens. This program is an example of the classic
endless loop.
Every programmer
alive has written them accidentally, and you will too.
Usually the culprit of an endless loop is a sloppy variable name, spelling, or
capitalization. If you use a variable like $myCounter as the sentry variable
but then increment $MyCounter, PHP tracks two entirely different variables. Your
program won’t work correctly. This is another reason to be consistent
on your variable naming and capitalization conventions.
Building a Well-Behaved Loop
Fortunately, you have guidelines for building a loop that behaves as you wish.
Even better, you’ve already learned most of the important ideas, because these
fundamental concepts are built into the
for loop’s structure. When you write a
while loop, you are responsible for these three things:
• Creating a sentry variable
• Building a condition
• Ensuring the loop can exit
I discuss each of these ideas in the following sections.
Creating and Initializing a Sentry Variable
If your loop is based on a variable’s value (there are alternatives), make sure you
do these three things:
• Identify that variable.
• Ensure that variable has appropriate scope.
• Make sure that variable has a reasonable starting value.
TRICK
TRAP
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You might also check that value to ensure the loop runs at least one time (at least
if that’s your intent). Creating a variable is much like the initialization stage of a
for construct.
Building a Condition to Continue the Loop
Your condition usually compares a variable and a value. Make sure you have a
condition that can be met and be broken. The hard part is ensuring that the pro-
gram gets out of the loop at the correct time. This condition is much like the con-
dition in the
for loop.
Ensuring the Loop Can Exit
There must be some trigger that changes the sentry variable so the loop can exit.
This code must exist inside the code body. Be sure it is possible for the sentry vari-
able to achieve the value necessary to exit the loop by making the condition
false.
Working with Basic Arrays
Programming is about the combination of control structures (like loops) and
data structures (like variables). You know the very powerful looping structures.
Now it’s time to look at a data structure that works naturally with loops.
Arrays
are special variables made to hold lists of information. PHP makes it quite
easy to work with arrays. Look at Figure 4.7, whose
basicArray.php program
demonstrates two arrays.
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FIGURE 4.7
The information
displayed on this
page is stored in
two array variables.
