array values in multidimensional array - php

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 :-)