How to find the indices of duplicate values in an array - php

Question

I have two arrays, array1 and array2 that I am using to populate a table, so that array1[5] and array2[5] both fill the same row but I want to write a function that removes both array1[i] and array2[i] if array1[i] is a duplicate of array1[j] for some j less than i, where i is an arbitrary positive integer.
To accomplish this I was to work out the indices of duplicate values in array1 and then use this information to delete the entries from both array1 and array2 for these indices, before populating my table.
Any ideas gratefully received.
Thanks.

Answer 1

The array_unique function removes dupes, but preserves keys. Then you can just iterate through the other array and remove the keys that don't exist in the first one.
$array1 = array_unique($array1);
foreach ($array2 as $key => $val) {
if (!array_key_exists($key,$array1)) unset($array2[$key]);
}

Answer 2

$array1_keys = array_keys($array1); // all keys
$unique = array_keys(array_unique($array1)); // keys of unique values
$duplicate = array_diff($array1_keys, $unique_keys); // keys of the duplicate values
foreach($duplicate as $key){
unset($array2[$key]);
}
Sidenote: Be aware that array_diff uses string casting for comparison. If your array contains non scalar values you should have a look at array_udiff.
Edit: Mingos post in my eyes did not fully suit the question, looks like i was wrong :D

Answer 3

If your array keys are meaningful and not just a 0-based index and you want to keep the duplicate entry with the lowest key (as you indicate might be the case from your question) then you need to sort the array first. If you don't, you will get the first entry in the array for each duplicate value and not entry with the lowest key. Compare
$array = array( 5 => 'foo', 1 => 'bar', 2 => 'foo', 3 => 'bar' );
$array = array_unique( $array );
var_dump( $array );
with
$array = array( 5 => 'foo', 1 => 'bar', 2 => 'foo', 3 => 'bar' );
ksort( $array );
$array = array_unique( $array );
var_dump( $array );

Answer 4

In asociative arrays, you can get easily a counter of item to detect repeated and the first ocurrence with something as simple as (in this case using an associative array, where $array_key is the key to count) using array_count_values:
$index = null;
$array_by_columns = array_count_values(array_column($associative_array, $array_key);
foreach($array_by_columns) as $key => $ocurrences){
if($ocurrences > 1){
$index = $key;
}
}
If you have $index there are repeated element and also is the index of in the array.
A more compacted way in a line:
$index = null;
foreach(array_count_values(array_column($associative_array, $array_key)) as $key => $ocurrences)if($ocurrences > 1) $index = $key;
Data example:
[ARRAY] (start)
n0
KEY = 0
VALUE = {"user":"1","points":"10"}
n1
KEY = 1
VALUE = {"user":"4","points":"10"}
n2
KEY = 2
VALUE = {"user":"5","points":"30"}
n3
KEY = 3
VALUE = {"user":"1","points":"40"}
[ARRAY] (end)
More info:
array_count_values
array_column