Convert list (array) to associative array in PHP

Question

I want to convert PHP list (array), i.e.

array("start", "end", "coords")

into associative array with truthy values (just to be able to test the presence/absence of key quickly), i.e. to something like this:

array(
    "start" => 1,
    "end" => 1,
    "coords" => 1
)

Is there any more elegant way to do it than this?

array_fill_keys($ar, 1)

What can possibly be more elegant than one call to a built-in function? — Felix Kling
– Felix Kling, Commented Mar 14, 2012 at 11:01
@Felix I know it sounds weird, but I find this way of doing it rather clumsy... — Tomas
– Tomas, Commented Mar 14, 2012 at 11:04
You can iterate over the array if it makes you feel better ;) PHP does not have sets so this is really the best you can do. You can create your own function of course: function setify($array) { return array_fill_keys($ar, true);} or something like that. Or what would you expect? — Felix Kling
– Felix Kling, Commented Mar 14, 2012 at 11:05

Tomas · Accepted Answer · 2012-03-20 23:23:50Z

5

There is probably no more elegant solution than array_fill_keys($ar, 1).

answered Mar 20, 2012 at 23:23

Tomas

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skyronic · Accepted Answer · 2012-03-14 11:08:19Z

1

There is a function called array_flip that does this.

http://php.net/array_flip

Doing array_flip on an array and then using isset turned out to be much faster than doing in_array for me.

But note that this is only useful when you're going to be searching the array multiple times.

answered Mar 14, 2012 at 11:08

skyronic

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4 Comments

Felix Kling Over a year ago

in_array is slow anyways (compared to isset), as it has to do a linear search. array_flip is a possibility but note that the first entry will have a value of 0 and therefore does not evaluate to true (e.g. if($arr[firstEntry])). Of course one should use isset anyways, so the actual values don't really matter (as long as they are not null).

Felix Kling Over a year ago

@Tomas: How else are you testing the presence of an element? Just doing if($arr[value]) will give you a Notice: if the key is not in the array.

skyronic Over a year ago

yes, in_array is slow. But if I am using this method to check if a value exists in an array, it has to process each item in the array anyways. And I'm sure the time to hash a string and make it an array key is longer than the time taken whether it's equal to the $needle. Also, this method takes up more memory - but it's great if you are checking if lots of strings.

skyronic Over a year ago

@Thomas, then just say $result[$originalArray[0]] = 1 and now you can be assured that every key points to a value that evaluates to true.

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