PHP 101 (part 3) - Chocolate Fudge And Time Machines - Doing It By The Numbers (
Page 4 of 9 )
Both the "while" and "do-while"
loops continue to iterate for so long as the specified conditional expression
remains true. But there often arises a need to execute a certain set of statements
a specific number of times - for example, printing a series of thirteen sequential
numbers, or repeating a particular set of <TD>> cells five times. In such cases,
clever programmers reach for the "for" loop...
The "for" loop typically looks like this:
for (initial value of counter; condition; update counter)
{
do this!
}
Looks like gibberish? Well, hang in there for a minute...the "counter" here is
a PHP variable that is initialized to a numeric value, and keeps track of the
number of times the loop is executed. Before each execution of the loop, the "condition"
is tested - if it evaluates to true, the loop will execute once more and the counter
will be appropriately incremented; if it evaluates to false, the loop will be
broken and the lines following it will be executed instead.
And here's a simple example that demonstrates how this loop can be used:
<html>
<head>
<basefont face="Arial">
</head>
<body>
<center>Turning
The Tables, PHP-Style!</center>
<br>
<?
// define the number
$number
= 7;
// use a for loop to calculate tables for that number
for ($x=1; $x<=15;
$x++)
{
echo "$number X $x = " . ($number*$x) . "<br>";
}
?>
</body>
</html>
Let's dissect this a little bit:
The first thing we've done here is define the number to be used for the multiplication
table; we've used 7, since we're particularly poor at math - you might prefer
to use another number.
Next we've constructed a "for" loop with $x as the counter variable - we've initialized
it to 1 and specified that the loop should run no more than 15 times.
The $x++ you see in the "for" statement is an interesting little operator known
as the auto-increment operator - more on this in the "Miscellaneous Notes" section
below. For the moment, all you need to know is that it automatically increments
the counter by 1 every time the loop is executed.
Finally, the line that does all the work - the "echo" statement, which takes
the number specified, multiplies it by the current value of the counter, and displays
the result on the page.
As you can see, a "for" loop is a very interesting - and useful - programming
construct. Our next example illustrates its usefulness in a manner that should
endear it to any HTML programmer.
<html>
<head>
<basefont face="Arial">
</head>
<body>
<center>Turning
The Tables, Part II</center>
<br>
<table border=1>
<?
// the purpose
of this exercise is
// to generate a 4x4 grid via an html table
// this is accomplished
via two nested "for" loops
// the first one is for the table rows or <tr>s
//
we need four of them
for ($alpha=1; $alpha<=4; $alpha++)
{
?>
<tr>
<?
//
the second one is for the cells in each row or <td>s
// we need four of them
too
for ($beta=1; $beta<=4; $beta++)
{
?>
<td>
<?
// print
the coordinate of each cell
echo("row $alpha, column $beta");
?>
</td>
<?
}
?>
</tr>
<?
}
?>
</table>
</body>
</html>
And here's the output:
<html>
<head>
<basefont face="Arial">
</head>
<body>
<center>Turning
The Tables, Part II</center>
<br>
<table border=1>
<tr>
<td>
row 1, column 1
</td>
<td>
row 1, column 2
</td>
<td>
row 1, column 3
</td>
<td>
row 1, column 4
</td>
</tr>
<tr>
<td>
row 2, column 1
</td>
<td>
row 2, column 2
</td>
<td>
row 2, column 3
</td>
<td>
row 2, column 4
</td>
</tr>
<tr>
<td>
row 3, column 1
</td>
<td>
row 3, column 2
</td>
<td>
row 3, column 3
</td>
<td>
row 3, column 4
</td>
</tr>
<tr>
<td>
row 4, column 1
</td>
<td>s
row 4, column 2
</td>
<td>
row 4, column 3
</td>
<td>
row 4, column 4
</td>
</tr>
</table>
</body>
</html>
As you'll see if you try coding the same thing by hand, PHP's "for" loop just
saved you a whole lot of work!
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