Find duplicate values in an array which creates a new array with key as duplicate keys and values as the dupes

Question

I am trying to find all duplicates in my array, and create a new array which has keys as the duplicate values key and value as the key of its duplicate

example

[1] => 10
[2] => 11
[3] => 12
[4] => 12
[5] => 12
[6] => 13
[7] => 13

After I apply the duplicate check I just need

[4] => [3] // value of key 4 is dupe of key 3
[5] => [3] // value of key 5 is dupe of key 3
[7] => [6] // value of key 7 is dupe of key 6

this gets me all duplicate keys, but I need duplicate keys with values as the keys which are duplicate

$arr_duplicates = array_keys(array_unique( array_diff_assoc( $array, array_unique( $array ) ) ));

Thanks

@Neal sorry its hard to explain what I am trying to explain Basically I just need to get all dupes as an array but the values of my new array should be the key from which they are dupes of. — user2619159
– user2619159, Commented Jul 29, 2013 at 17:18
Why did you edit the output format? You can't have two elements with the same key (3). — gen_Eric
– gen_Eric, Commented Jul 29, 2013 at 17:21
@RocketHazmat - I have added few comments to explain what I need in more details — user2619159
– user2619159, Commented Jul 29, 2013 at 17:22
@user2619159: You still can't have key 3 equal to both 4 and 5 (unless you make it an array). — gen_Eric
– gen_Eric, Commented Jul 29, 2013 at 17:23

user2609094 · Accepted Answer · 2013-07-29 17:51:43Z

2

Try this for potential speed boost over the other solution. Will however use a lot more memory on large data sets.

<?php

$orig = array(
    1   => 10,
    2   => 11,
    3   => 12,
    4   => 12,
    5   => 12,
    6   => 13,
    7   => 13
);

$seen  = array();
$dupes = array();

foreach ($orig as $k => $v) {
    if (isset($seen[$v])) {
        $dupes[$k] = $seen[$v];
    } else {
        $seen[$v] = $k;
    }
}
unset($seen);

var_dump($dupes);

answered Jul 29, 2013 at 17:51

user2609094

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1 Comment

user2619159 Over a year ago

Nice one, thats very fast

gen_Eric · Accepted Answer · 2013-07-29 17:34:29Z

1

This should do what you want. Loop over the array, and see if the value was already in there. If so, add it to the result.

$arr_duplicates = array();
foreach($array as $k=>$v){
    // array_search returns the 1st location of the element
    $first_index = array_search($v, $array);
    // if our current index is past the "original" index, then it's a dupe
    if($k != $first_index){
        $arr_duplicates[$k] = $first_index;
    }
}

DEMO: http://ideone.com/Kj0dUV

edited Jul 29, 2013 at 17:34

answered Jul 29, 2013 at 17:27

gen_Eric

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6 Comments

user2609094 Over a year ago

Change $k > $first_index to $k != $first_index to make it work with indexes in any order, such as $array = [7 => 13, 6 => 13].

user2619159 Over a year ago

@RocketHazmat this is a great function, I had tried foreach() before too, the only issue ive had is, am working with very large arrays i.e 10000++ records, and speed is very important

gen_Eric Over a year ago

@user2619159: Anything else you try is probably gonna be slower. Do you notice a speed issue with this code? If not, then it's probably not an issue :-P

user2609094 Over a year ago

Speed can be improved by keeping track of the first time each value was seen. You'd be trading a possible speed boost for an enormous amount of memory usage.

gen_Eric Over a year ago

@Nate: I guess you could cache the array_search lookups, that may be faster, but I'm not sure how much.

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