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 $key contain the first key, but this seems inefficient. Does anyone have a better solution?
转载于:https://stackoverflow.com/questions/1028668/get-first-key-in-a-possibly-associative-array
reset($array);
$first_key = key($array);
It's essentially the same as your initial code, but with a little less overhead, and it's more obvious what is happening.
Just remember to call reset, or you may get any of the keys in the array. You can also use end instead of reset to get the last key.
If you wanted the key to get the first value, reset actually returns it:
$first_value = reset($array);
There is one special case to watch out for though (so check the length of the array first):
$arr1 = array(false);
$arr2 = array();
var_dump(reset($arr1) === reset($arr2)); // bool(true)
array_keys returns an array of keys. Take the first entry. Alternatively, you could call reset on the array, and subsequently key. The latter approach is probably slightly faster (Thoug I didn't test it), but it has the side effect of resetting the internal pointer.
key($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.
Please find the following:
$yourArray = array('first_key'=> 'First', 2, 3, 4, 5);
$keys = array_keys($yourArray);
echo "Key = ".$keys[0];
I think this will work.
list($firstKey) = array_keys($yourArray);
$myArray = array(
2 => '3th element',
4 => 'first element',
1 => 'second element',
3 => '4th element'
);
echo min(array_keys($myArray)); // return 1
This could also be a solution.
$first_key = current(array_flip($array));
I have tested it and it works.
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
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
$arr = array('key1'=>'value1','key2'=>'value2','key3'=>'key3');
list($first_key) = each($arr);
print $first_key;
// key1
You could try
array_keys($data)[0]
A one-liner:
$array = array('key1'=>'value1','key2'=>'value2','key3'=>'key3');
echo key( array_slice( $array, 0, 1, true ) );
# echos 'key1'
The best way that worked for me was
array_shift(array_keys($array))
array_keys gets array of keys from initial array and then array_shift cuts from it first element value. You will need PHP 5.4+ for this.
You can play with your array
$daysArray = array('Monday', 'Tuesday', 'Sunday');
$day = current($transport); // $day = 'Monday';
$day = next($transport); // $day = 'Tuesday';
$day = current($transport); // $day = 'Tuesday';
$day = prev($transport); // $day = 'Monday';
$day = end($transport); // $day = 'Sunday';
$day = current($transport); // $day = 'Sunday';
To get the first element of array you can use current and for last element you can use end
Edit
Just for the sake for not getting any more down votes for the answer you can convert you key to value using array_keys and use as shown above.
This is the easier way I had ever found. Fast and only two lines of code :-D
$keys = array_keys($array);
echo $array[$keys[0]];
Today I had to search for the first key of my array returned by a POST request. (And note the number for a form id etc)
Well, I've found this: Return first key of associative array in PHP
I've done this, and it work.
$data = $request->request->all();
dump($data);
while ($test = current($data)) {
dump($test);
echo key($data).'<br />';die();
break;
}
Maybe it will eco 15min of an other guy. CYA.
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).
A more universal and safe version with fallback for an empty iterable / array:
Note: Assuming the $array is an
arrayoriterable(PHP >=7.1)
$array = [/* ... */]; // my array or iterable
// these lines are a fallback for the case nothing is found
$firstKey = null; // null or default value of your choice
$firstValue = null; // null or default value of your choice
// iterating the array
foreach ($array as $firstKey => $firstValue) {
// stop after first iteration
break;
}
TLDR: This way you don't need to double-check the iterable. (array in this case).
Foreach is the fastest way to iterate over everything.
Recommendations and explanations: I recommend setting default values for $firstKey and $firstValue before iterating, just so you know if the array was empty or not.
Note: If you rely that your array will always be populated there is no need for default values. (but be 100% sure about that!)
Explanation for more advanced stuff:
If your iterable has a custom Iterator you might need to reset it manually after the foreach statement.
More about PHP: iterables