PHP : Numbers - [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.