PHP array key to value mappings question - php

Question

I just saw this on php.net description of how key to value mapping works:
$switching = array( 10, // key = 0
5 => 6,
3 => 7,
'a' => 4,
11, // key = 6 (maximum of integer-indices was 5)
'8' => 2, // key = 8 (integer!)
'02' => 77, // key = '02'
0 => 12 // the value 10 will be overwritten by 12
);
I just cant quite understand how 11 could be assigned key 6. I know 5 is not possible since it is already used on the second element as a key so it makes sense to jump it over.
But should not be 11 intuitively assigned key 4 in the first place since the first element of the array 10 is assigned key 0 and therefore the key value is incremented 0..1..2..3..4 from that point according to the first index unless specified otherwise (e.g 5=>6 could have had key 1, 3=>7 with key 2, and 'a'=> 4 could have had key 3 if not specified)? And also, why does it say that 11 should be assigned a key that represents the maximum integer of indices (in this case 6 since 5 was used already)?
Would appreciate any help/clarification. Please let me know if the question needs to be clarified. Thanks much.

Answer 1

It is implemented in this way just because it is the most performant solution - to just use maximum_specified_key + 1, rather than to find a hole in enumration

Answer 2

I believe it has to do with the way that PHP arrays work. Each has an internal cursor for the each, current, next, pos and similar methods. My guess: as a new key is specified, if it is higher than the current cursor, the cursor is advanced to that position so that anything added after that point will still be at current position + 1.

Answer 3

And also, why does it say that 11
should be assigned a key that
represents the maximum integer of
indices (in this case 6 since 5 was
used already)?
Why not? As zerkms says, it's performant, so there's that.
It's also more or less what you might expect. Given an array with mixed keys like that, what would you expect array_push() to do?
Of course, if you're running into this kind of thing in real life, it's probably time to stop and consider some amount of refactoring. Arrays in PHP are very flexible, and these somewhat arbitrary decisions had to be made. They're documented. The only alternative is to make array usage much more rigid.

Answer 4

The PHP manual gives you the answer in the link you provided:
As mentioned above, if no key is specified, the maximum of the
existing integer indices is taken, and the new key will be that
maximum value plus 1 (but at least 0). If no integer indices exist
yet, the key will be 0 (zero).
The code:
$switching = array( 10, // key = 0
5 => 6,
3 => 7,
'a' => 4,
11, // key = 6 (maximum of integer-indices was 5)
'8' => 2, // key = 8 (integer!)
'02' => 77, // key = '02'
0 => 12 // the value 10 will be overwritten by 12
);
So far the maximum key when you get to 'a' is 5 so the next available key according to the php manual would be 5 + 1 i.e. 6.
Note that the maximum integer key used
for this need not currently exist in the array. It need only have
existed in the array at some time since the last time the array was
re-indexed.
// empty the array;
foreach( $switching as $key => $value ) {
unset( $switching[$key] );
}
// refill it with new elements
for( $i = 0; $i < 10; $i++ ) {
$switching[] = $i + 1;
}
output array:
Array
(
[9] => 1
[10] => 2
[11] => 3
[12] => 4
[13] => 5
[14] => 6
[15] => 7
[16] => 8
[17] => 9
[18] => 10
)