array values in multidimensional array

Question

I have two arrays

they look like

$a1 = array(
  array('num' => 1, 'name' => 'one'),
  array('num' => 2, 'name' => 'two'),
  array('num' => 3, 'name' => 'three'),
  array('num' => 4, 'name' => 'four'),
  array('num' => 5, 'name' => 'five')
)

$a2 = array(3,4,5,6,7,8);

I want to end up with an array that looks like

$a3 = array(3,4,5);

so basically where $a1[$i]['num'] is in $a2

I know I could do

$a3 = array();

foreach($a1 as $num)
 if(array_search($num['num'], $a2))
   $a3[] = $num['num'];

But that seems like a lot of un-needed iterations.

Is there a better way?

---

Ah Snap...
I just realized I asked this question the wrong way around, I want to end up with an array that looks like

$a3 array(
  array('num' => 3, 'name' => 'three'),
  array('num' => 4, 'name' => 'four'),
  array('num' => 5, 'name' => 'five')
)

Answer 1

You could extract the relevant informations (the 'num' items) from $a1 :

$a1_bis = array();
foreach ($a1 as $a) {
    $a1_bis[] = $a['num'];
}

And, then, use array_intersect() to find what is both in $a1_bis and $a2 :

$result = array_intersect($a1_bis, $a2);
var_dump($result);

Which would get you :

array
  2 => int 3
  3 => int 4
  4 => int 5

With this solution :

• you are going through $a1 only once
• you trust PHP on using a good algorithm to find the intersection between the two arrays (and/or consider that a function developed in C will probably be faster than any equivalent you could code in pure-PHP)

---

EDIT after the comment : well, considering the result you want, now, I would go with another approach.

First, I would use array_flip() to flip the $a2 array, to allow faster access to its elements (accessing by key is way faster than finding a value) :

$a2_hash = array_flip($a2); // To speed things up : 
                            // accessing by key is way faster than finding 
                            // an item in an array by value

Then, I would use array_filter() to apply a filter to $a1, keeping the items for which num is in the $a2_hash flipped-array :

$result = array_filter($a1, function ($item) use ($a2_hash) {
    if (isset($a2_hash[ $item['num'] ])) {
        return true;
    }
    return false;
});
var_dump($result);

Note : I used an anonymous function, which only exist with PHP >= 5.3 ; if you are using PHP < 5.3, this code can be re-worked to suppress the closure.

With that, I get the array you want :

array
  2 => 
    array
      'num' => int 3
      'name' => string 'three' (length=5)
  3 => 
    array
      'num' => int 4
      'name' => string 'four' (length=4)
  4 => 
    array
      'num' => int 5
      'name' => string 'five' (length=4)

Note the keys are not corresponding to anything useful -- if you want them removed, just use the array_values() function on that $result :

$final_result = array_values($result);

But that's probably not necessary :-)