Trying to understand array_udiff behavior

Question

Let's continue. Why array_udiff compares values of first array after sorting?

$compare = function($a, $b) use(&$iteration_count)
    {
    echo("$a : $b\n");
    $iteration_count++;
    return strcmp($a, $b);
    };

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


$iteration_count = 0;
echo "array_udiff:" . json_encode(array_udiff($a, $b, $compare)) . "\n";
echo "iterations: $iteration_count\n\n";

Output

b : a  // sorting $a started
c : b   
y : x  // sorting $b started
z : y
a : x  // comparison started
a : b  //                    -- what for?
b : x
b : c  //                    -- what for?
c : x
array_udiff:["a","b","c"]
iterations: 9

http://3v4l.org/3Me8o#v500

... to find the actual matches? Just like with your uassoc question? — bwoebi
– bwoebi, Commented Mar 4, 2015 at 15:13
@bwoebi , it's already known that 'b' > 'a' because $a already sorted. I see no pros from this repeated comparison. — sectus
– sectus, Commented Mar 4, 2015 at 15:17

Ja͢ck · Accepted Answer · 2015-03-05 03:55:00Z

2

After the comparison of A[0] and B[0] it will skip all values in A that equal A[0], because B doesn't have that value; see here.

To do this, it must compare at least A[1] against A[0]; you can observe this behaviour by making a small change in the first array:

$a = array('a', 'a', 'b', 'c');

Output:

...
a : x
a : a <-- *
a : b
b : x
b : c
c : x

edited Mar 5, 2015 at 3:55

answered Mar 5, 2015 at 1:42

Ja͢ck

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