2

This should be an easy one liner, but I can't quite get it:

I have an array like this:

$a = ["z", "x", "y"];

and an array like this:

$b = ["x"=>"a", "y"=>"b", "z"=>"c"];

what is a php oneliner to get:

$c = ["c", "a", "b"];

I want to use each element of a to index into b and return an array of the results I've been looking at array_map but couldn't figure out how to bind b to the callback function.

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  • 1
    If there is any, it is on this list. Commented Dec 17, 2013 at 15:24
  • @Mołot, thanks this is the list I was looking at. It seems like array_map is the "correct" choice here, but I don't know how to bind $b as a parameter to the callback? Does anyone know how to do this? Commented Dec 17, 2013 at 15:38
  • 1
    You're missing return in your function. And use($b) does the binding Commented Dec 17, 2013 at 15:43
  • @onetrickpony, thanks I found that in the manual. This is the part I was missing. If you update your answer, or post another, with the array_map version, I will accept it. I am testing this now. Commented Dec 17, 2013 at 15:56
  • 1
    Just use the one you find easier to understand Commented Dec 17, 2013 at 16:07

4 Answers 4

Reset to default
2 
$c = array_merge(array_intersect_key(array_flip($a), $b), $b);
$c = array_values($c);

Not really a one-liner, but close :P

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2 Comments

It can be written as a one-liner though.
array_merge(..., $b) adds everything from $b, so this solution is probably incorrect. array_values(array_intersect_key($b, array_flip($a))) is better but order of resulting elements is different.
2 
$c = array();
foreach ($a as $key => $value)
{
    if (isset($b[$value]))
    {
       $c[] = $b[$value];
    }
}
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3 Comments

@TFinnis, I was going down this road but figured there was a more "functional" way to do this. I will probably choose onetrickpony as the answer, but I gave you a +1 for the working solution.
This code is wrong. I suggested this as an edit, but it seems to have been rejected. $a is not an associative array, so $key will be set to the positional indices: [0, 1, 2]. To correct the code, lose ` => $value` in the foreach line.
@joeshmo, after looking again, you are correct. My intent is that $a is not used as an associative array. I think your fix is right for my situation.
1

Here is the solution I ended up with with help from @onetrickpony:

$a = array("z", "x", "y");
$b = array("x"=>"a", "y"=>"b", "z"=>"c");

$c = array_map(function($key) use ($b){ return $b[$key]; }, $a);

The key to this is the 'use' keyword to bind to the associative array to the closure. Pretty simple once you know what you are looking for :-)

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Comments

1

With modern PHP, you can use arrow function syntax to access the second array without use(). As you iterate all of the values in the first array, check the mapping/second array for a matching key. If a match is found, use that; otherwise use the null coalescing operator to fallback to the original value.

Code: (Demo)

var_export(
    array_map(
        fn($v) => $b[$v] ?? $v,
        $a
    )
);

With the asker's original arrays, all elements are mapped, so the null coalescing is not necessary -- this is just a way to cover similar questions with missing keys in the lookup array.

If the lookup array might contain null values, then you'll need to use key_exists() like this.

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