php: check if an array has duplicates

Question

I'm sure this is an extremely obvious question, and that there's a function that does exactly this, but I can't seem to find it. In PHP, I'd like to know if my array has duplicates in it, as efficiently as possible. I don't want to remove them like array_unique does, and I don't particularly want to run array_unique and compare it to the original array to see if they're the same, as this seems very inefficient. As far as performance is concerned, the "expected condition" is that the array has no duplicates.

I'd just like to be able to do something like

if (no_dupes($array))
    // this deals with arrays without duplicates
else
    // this deals with arrays with duplicates

Is there any obvious function I'm not thinking of?
http://stackoverflow.com/questions/1170807/how-to-detect-duplicate-values-in-php-array
is a very similar question, however if you read the question, he's looking for array_count_values.

Answer 1

php's in_array() might do it: http://php.net/manual/en/function.in-array.php

Answer 2

Two ways to do it efficiently that I can think of:

1. inserting all the values into some sort of hashtable and checking whether the value you're inserting is already in it(expected O(n) time and O(n) space)

2. sorting the array and then checking whether adjacent cells are equal( O(nlogn) time and O(1) or O(n) space depending on the sorting algorithm)

stormdrain's solution would probably be O(n^2), as would any solution which involves scanning the array for each element searching for a duplicate

Answer 3

I know you said you aren't after array_unique. However, I don't think you're going to find a magical function. I also don't think building one is going to be faster that making use of the native functions. So here's what I propose:

function array_has_dupes($array) {
   // streamline per @Felix
   return count($array) != count(array_unique($array));
}

Adjust the second parameter of array_unique() to your liking.

Answer 4

You can do:

function has_dupes($array){
 $dupe_array = array();
 foreach($array as $val){
  if(++$dupe_array[$val] > 1){
   return true;
  }
 }
 return false;
}

Answer 5

As you specifically said you didn't want to use array_unique I'm going to ignore the other answers despite the fact they're probably better.

Why don't you use array_count_values() and then check if the resulting array has any value greater than 1?