PHP Foreach Loop Efficiency

Question

Hi All,

I am using a foreach loop within PHP similar to this:

foreach ($class->getAttributes() as $attribute) {
// Work
}

Concerning efficiency, is it better to have a $attributes = $class->getAttributes(); statement outside the foreach loop and iterate over the $attributes variable? Or is the $class->getAttributes() statement only getting called once inside the foreach declaration at the beginning?

(I realize this might not be a big efficiency concern in this case, but I would like to know the principle for this and other larger cases)

Thanks,

Steve

Answer 1

Using $class->getAttributes() outside of the foreach loop and using a temporary variable, or keeping it like you wrote should not change anything about performances : it will still be evaluated only once.

And here is an example that proves it :

function get_array() {
    echo 'function called !<br />';
    return array(
        'first' => 123,
        'second' => 456,
        'last' => 789, 
    );
}
foreach (get_array() as $key => $value) {
    echo "$key : $value<br />";
}

I am using a function and not a method of a class, to get a shorter example, but the principle would be the same with a class+method.

And calling this portion of code gives the following output :

function called !
first : 123
second : 456
last : 789

i.e. the get_array() function is only called once, at the beginning of the foreach loop.

Answer 2

I haven't tested, but I'd think that doing something like this:

$attributes = $class->getAttributes();
foreach($attributes as $attribute)
{

}

Is more readable, and you can get to the attributes after the foreach has conclude if you need to.

However, in a more direct response to your question, getAttributes() will only be called once in either case.

Answer 3

foreach operates on a copy internally:

Note: Unless the array is referenced, foreach operates on a copy of the specified array and not the array itself. […]

So it doesn’t make any difference in your case. $class->getAttributes() is only called once to retrieve the array.

Answer 4

As others have said, the function in your example is only called once.

The case in which pre-evaluating can make a difference is for loops.

$str = 'abcdefghijklmnop';

//strlen will be called on every iteration
for($i = 0; $i < strlen($str); $i++);

//strlen will only be called once
$len = strlen($str);
for($i = 0; $i < $len; $i++);