Recipe 2.5 Operating on a Series of Integers
2.5.1 Problem
You
want to apply a piece of code over a range of integers.
2.5.2 Solution
Use the range( ) function, which returns an array populated
with integers:
foreach(range($start,$end) as $i) {
plot_point($i);
}
Instead of using range( ), it can be more
efficient to use a for
loop. Also, you can
increment using values other than 1. For example:
for ($i = $start; $i <= $end; $i += $increment) {
plot_point($i);
}
2.5.3 Discussion
Loops like this are
common. For instance, you could be plotting a function and need to
calculate the results for multiple points on the graph. Or, you could
be NASA counting down until the launch of the Space Shuttle Columbia.
In the first example, range( ) returns an array
with values from $start to
$end. Then
foreach
pulls out each element and assigns it to
$i inside of the loop. The advantage of using
range( ) is its brevity, but this technique has a
few disadvantages. For one, a large array can take up unnecessary
memory. Also, you're forced to increment the series
one number at a time, so you can't loop through a
series of even integers, for example.
As of PHP 4.1, it is valid for $start to be larger
than $end. In this case, the numbers returned by
range( ) are in descending order. Also, you can
use iterate over character sequences:
print_r(range('l', 'p'));
Array
(
[0] => l
[1] => m
[2] => n
[3] => o
[4] => p
)
The for loop method just uses a single integer and
avoids the array entirely. While it's longer, you
have greater control over the loop, because you can increment and
decrement $i more freely. Also, you can modify
$i from inside the loop, something you
can't do with range( ), because
PHP reads in the entire array when it enters the loop, and changes to
the array don't effect the sequence of elements.
2.5.4 See Also
Recipe 4.4 for details on initializing an
array to a range of integers; documentation on range(
) at http://www.php.net/range.
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