How is an array in a PHP foreach loop read?

Question

We have all heard of how in a for loop, we should do this:

for ($i = 0, $count = count($array); $i < $c; ++$i)
{
    // Do stuff while traversing array
}

instead of this:

for ($i = 0; $i < count($array); ++$i)
{
    // Do stuff while traversing array
}

for performance reasons (i.e. initializing $count would've called count() only once, instead of calling count() with every conditional check).

Does it then also make a difference if, in a foreach loop, I do this:

$array = do_something_that_returns_an_array();

foreach ($array as $key => $val)
{
    // Do stuff while traversing array
}

instead of this:

foreach (do_something_that_returns_an_array() as $key => $val)
{
    // Do stuff while traversing array
}

assuming circumstances allow me to use either? That is, does PHP call the function only once in both cases, or is it like for where the second case would call the function again and again?

Run a test! Make the function do_something_that_returns_an_array() echo something internally. — Artelius
– Artelius, Commented Nov 6, 2009 at 6:01

Amber · Accepted Answer · 2009-11-06 06:04:36Z

38

foreach() is implemented using iterators - thus it only calls the function that returns an array once, and then uses the iterator which points to the existing result set to continue with each item.

answered Nov 6, 2009 at 6:04

Amber

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Comments

BoltClock · Accepted Answer · 2012-05-25 12:41:11Z

I think testing it will surely answer that.

Here is my code:

function GetArray() {
    echo "I am called.\n";
    return array("One"=>1, "Two"=>2, "Three"=>3, "Four"=>4, "Five"=>5);
}

echo "Case #1\n";
$array = GetArray();
foreach ($array as $key => $val){
    // Do stuff while traversing array
    echo "    Inside the loop: $key => $val\n";
}
echo "\n";

echo "Case #2\n";
foreach (GetArray() as $key => $val) {
    // Do stuff while traversing array
    echo "    Inside the loop: $key => $val\n";
}
echo "\n";

and here is the result:

Case #1
I am called.
    Inside the loop: One => 1
    Inside the loop: Two => 2
    Inside the loop: Three => 3
    Inside the loop: Four => 4
    Inside the loop: Five => 5

Case #2
I am called.
    Inside the loop: One => 1
    Inside the loop: Two => 2
    Inside the loop: Three => 3
    Inside the loop: Four => 4
    Inside the loop: Five => 5

They both read call function once. So no different.

deceze · Accepted Answer · 2009-11-06 06:01:07Z

8

foreach makes a copy of the input array and works on that in each iteration. So you can use foreach (do_something_that_returns_an_array() as $key => $val) and it'll call do_something_that_returns_an_array() only once.

answered Nov 6, 2009 at 6:01

deceze♦

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1 Comment

cletus Over a year ago

That wording is not quite correct. Technically PHP arrays are copy-on-write so when you say the array is copied, the wording is at best ambiguous and at worst misleading. The return value is simply used "as is".

alex · Accepted Answer · 2011-02-08 23:28:57Z

4

I would set up an example to see what happens. Something like...

function do_something_that_returns_an_array() {
    echo 'I have been called!'; 
    return array('i am an', 'array');
}

As you can see on CodePad, it is only being called once.

edited Feb 8, 2011 at 23:28

answered Nov 6, 2009 at 6:00

alex

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Comments

Steven · Accepted Answer · 2009-11-06 06:05:34Z

2

I've always assumed that do_something_that_returns_an_array() only runs once because, unlike in the for loop, there's no reason for it to run multiple times. The reason the for loop truth checker runs at the end of every iteration is to allow for very complicated checkers.

As a test, I did the following:

function get_array() {echo 5; return array(1,2,3,4,5);}
foreach(get_array() as $key => $value) {}

The script printed 5 once. Therefore, the function get_array() only evaluates once.

answered Nov 6, 2009 at 6:05

Steven

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Comments

Valentin Golev · Accepted Answer · 2009-11-06 06:05:01Z

0

foreach() works with Iterator or IteratorAggregate objects (e.g., arrays and objects implementing the interface). It can't work with immutable objects. Functions return immutable objects, so the returning array have to be copied into Iterator object before foreaching (it's made by php itself)

answered Nov 6, 2009 at 6:05

Valentin Golev

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2 Comments

Valentin Golev Over a year ago

immutable == very constant :) you cant change the object which is returned by function in php, it can't be used by L-Value (f() = 1 fails, either, i think, will array_push(f(), 123) )

ircmaxell Over a year ago

Functions don't return immutable objects. Due to the particularities of object references, you can dereference the object and do getObject()->foo = 'bar' quite fine (and if there's a reference to the object, it gets updated)... You can't dereference arrays (getArray()['foo'] doesn't work). But there is no such thing as an immutable object in PHP. You can always dereference or modify an object (access restrictions not withstanding).

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