Bug->Doc #80346 [Opn]: Function `array_intersect_ukey`requires consistent comparison function

From: Date: Tue, 10 Nov 2020 01:57:36 +0000
Subject: Bug->Doc #80346 [Opn]: Function `array_intersect_ukey`requires consistent comparison function
References: 1  Groups: php.doc.bugs 
Request: Send a blank email to doc-bugs+get-18106@lists.php.net to get a copy of this message 
Edit report at https://bugs.php.net/bug.php?id=80346&edit=1

 ID:                 80346
 Updated by:         requinix@php.net
 Reported by:        gilperon at gmail dot com
-Summary:            Function array_intersect_ukey not working
                     (really!)
+Summary:            Function array_intersect_ukeyrequires consistent
                     comparison function
 Status:             Open
-Type:               Bug
+Type:               Documentation Problem
-Package:            *General Issues
+Package:            Arrays related
 Operating System:   ALL
 PHP Version:        7.4.12
 Block user comment: N
 Private report:     N

 New Comment:

Indeed, this is user error: as with every other array sorting function (and the intersect/diff
functions *do* perform sorting - for efficiency), the callback needs to be consistent in its
comparisons. It must return less than/equal to/greater than, and doing something else will likely
result in "garbage in, garbage out" behavior.

This matter is going to be common to all sorting, intersection, diff, etc. functions. If the docs
are amended then I suggest some &copypasta; for each affected page.


PS: Returning $key2 - $key1 is technically backwards, but the final sort order doesn't really
matter since this function only cares about equality between elements.


Previous Comments:
------------------------------------------------------------------------
[2020-11-10 01:32:41] danack@php.net

Changing the code:
  
   echo "DIFFERENT\n";
   return 1;

to:

    echo "DIFFERENT\n";
    return $key2 - $key1;

Makes your code work.

It looks like this code may be sorting the arrays first, and sorting algorithms need to be stable:
https://www.geeksforgeeks.org/stability-in-sorting-algorithms/

That's an educated guess. If it is that, it's a docs problem as stability probably should
be mentioned.

------------------------------------------------------------------------
[2020-11-10 01:21:38] gilperon at gmail dot com

Description:
------------
Run the test script. It has 2 arrays in such a way that only the key 4 appears on both.
Using array_intersect_ukey does not return 4, instead it returns
array(0) {}. Using array_intersect_key works perfectly. I also realized
there is a bug in the implementation of this function, because if you run the test script it will
show you this:

|4|6|DIFFERENT
|8|6|DIFFERENT
|4|8|DIFFERENT
|3|4|DIFFERENT
|5|4|DIFFERENT
|3|5|DIFFERENT
|6|4|DIFFERENT
|6|5|DIFFERENT
|6|3|DIFFERENT

It shouldnt be comparing 8 with 6 because these keys only appear on the 1st array and these keys
shouldnt be compared at all. You can clearly see that this function, instead of comparing each key
of the first array with each key of the second array, it's comparing the keys from the first
array WITH ITSELF and then comparing the keys of the second array ITSELF (line |3|5|) and only then
comparing the keys of both arrays together (but for some reason it misses the |4|4|).

I know that in my simple use case I should be using array_intersect_key but I really
need array_intersect_ukey to work properly because I have a much more complex use case
for it.

 

Test script:
---------------
<?php

$array1 = array(4 => true, 6 => true, 8 => true);
$array2 = array(3 => true, 4 => true, 5 => true);

var_dump(array_intersect_ukey(

	$array1,
	$array2,
	function ($key1, $key2) {
		
		echo "|" . $key1 . "|" . $key2 . "|";
		
		if ($key1 === $key2) {

			echo "EQUAL\n";
			return 0;
			
		}
		else{

			echo "DIFFERENT\n";
			return 1;
		
		}
		
	}

));

?>

Expected result:
----------------
It should only show the element 4, which is present in both arrays.

Actual result:
--------------
Nothing is returned from the intersection.


------------------------------------------------------------------------



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