Get first key in a (possibly) associative array?

Question

What's the best way to determine the first key in a possibly associative array? My first thought it to just foreach the array and then immediately breaking it, like this:

foreach ($an_array as $key => $val) break;

Thus having $key contain the first key, but this seems inefficient. Does anyone have a better solution?

Answer 1

array_keys returns an array of keys. Take the first entry. Alternatively, you could call reset on the array, and subsequently key. The latter approach is probably slightly faster (Thoug I didn't test it), but it has the side effect of resetting the internal pointer.

Answer 2

key($an_array) will give you the first key

edit per Blixt: you should call reset($array); before key($an_array) to reset the pointer to the beginning of the array.

Answer 3

Interestingly enough, the foreach loop is actually the most efficient way of doing this.

Since the OP specifically asked about efficiency, it should be pointed out that all the current answers are in fact much less efficient than a foreach.

I did a benchmark on this with php 5.4, and the reset/key pointer method (accepted answer) seems to be about 7 times slower than a foreach. Other approaches manipulating the entire array (array_keys, array_flip) are obviously even slower than that and become much worse when working with a large array.

Foreach is not inefficient at all, feel free to use it!

Edit 2015-03-03:

Benchmark scripts have been requested, I don't have the original ones but made some new tests instead. This time I found the foreach only about twice as fast as reset/key. I used a 100-key array and ran each method a million times to get some noticeable difference, here's code of the simple benchmark:

$array = [];
for($i=0; $i < 100; $i++)
    $array["key$i"] = $i;

for($i=0, $start = microtime(true); $i < 1000000; $i++) {
    foreach ($array as $firstKey => $firstValue) {
        break;
    }
}
echo "foreach to get first key and value: " . (microtime(true) - $start) . " seconds <br />";

for($i=0, $start = microtime(true); $i < 1000000; $i++) {
    $firstValue = reset($array);
    $firstKey = key($array);
}
echo "reset+key to get first key and value: " . (microtime(true) - $start) . " seconds <br />";

for($i=0, $start = microtime(true); $i < 1000000; $i++) {
    reset($array);
    $firstKey = key($array);
}
echo "reset+key to get first key: " . (microtime(true) - $start) . " seconds <br />";


for($i=0, $start = microtime(true); $i < 1000000; $i++) {
    $firstKey = array_keys($array)[0];
}
echo "array_keys to get first key: " . (microtime(true) - $start) . " seconds <br />";

On my php 5.5 this outputs:

foreach to get first key and value: 0.15501809120178 seconds 
reset+key to get first key and value: 0.29375791549683 seconds 
reset+key to get first key: 0.26421809196472 seconds 
array_keys to get first key: 10.059751987457 seconds

reset+key http://3v4l.org/b4DrN/perf#tabs
foreach http://3v4l.org/gRoGD/perf#tabs

Answer 4

If efficiency is not that important for you, you can use array_keys($yourArray)[0] in PHP 5.4 (and higher).

Examples:

# 1
$arr = ["my" => "test", "is" => "best"];    
echo array_keys($arr)[0] . "\r\n"; // prints "my"


# 2
$arr = ["test", "best"];
echo array_keys($arr)[0] . "\r\n"; // prints "0"

# 3
$arr = [1 => "test", 2 => "best"];
echo array_keys($arr)[0] . "\r\n"; // prints "1"

The advantage over solution:

list($firstKey) = array_keys($yourArray);

is that you can pass array_keys($arr)[0] as a function parameter (i.e. doSomething(array_keys($arr)[0], $otherParameter)).

HTH

Answer 5

$myArray = array(
    2 => '3th element',
    4 => 'first element',
    1 => 'second element',
    3 => '4th element'
);
echo min(array_keys($myArray)); // return 1

Answer 6

To enhance on the solution of Webmut, I've added the following solution:

$firstKey = array_keys(array_slice($array, 0, 1, TRUE))[0];

The output for me on PHP 7.1 is:

foreach to get first key and value: 0.048566102981567 seconds 
reset+key to get first key and value: 0.11727809906006 seconds 
reset+key to get first key: 0.11707186698914 seconds 
array_keys to get first key: 0.53917098045349 seconds 
array_slice to get first key: 0.2494580745697 seconds 

If I do this for an array of size 10000, then the results become

foreach to get first key and value: 0.048488140106201 seconds 
reset+key to get first key and value: 0.12659382820129 seconds 
reset+key to get first key: 0.12248802185059 seconds 
array_slice to get first key: 0.25442600250244 seconds 

The array_keys method times out at 30 seconds (with only 1000 elements, the timing for the rest was about the same, but the array_keys method had about 7.5 seconds).