array_unique vs array_flip

Question

If I had an array of signed integers e.g:

Array (     [0] => -3     [1] => 1     [2] => 2     [3] => 3     [4] => 3 ) 

To get unique values I would instinctively use array_unique but after consideration I could perform array_flip twice which would have the same effect, and I think it would be quicker?

array_unique O(n log n) because of the sort operation it uses

array_flip O(n)

Am I correct in my assumptions?

UPDATE / EXAMPLE:

$intArray1 = array(-4,1,2,3); print_r($intArray1); $intArray1 = array_flip($intArray1); print_r($intArray1); $intArray1 = array_flip($intArray1); print_r($intArray1);  Array (     [0] => -3     [1] => 1     [2] => 2     [3] => 3     [4] => 3 ) Array (     [-3] => 0     [1] => 1     [2] => 2     [3] => 4 ) Array (     [0] => -3     [1] => 1     [2] => 2     [4] => 3 ) 

Answer 1

I benchmarked it for you: CodePad

Your intuition on this was correct!

$test=array(); for($run=0; $run<1000; $run++) $test[]=rand(0,100);  $time=microtime(true);  for($run=0; $run<100; $run++) $out=array_unique($test);  $time=microtime(true)-$time; echo 'Array Unique: '.$time."\n";  $time=microtime(true);  for($run=0; $run<100; $run++) $out=array_keys(array_flip($test));  $time=microtime(true)-$time; echo 'Keys Flip: '.$time."\n";  $time=microtime(true);  for($run=0; $run<100; $run++) $out=array_flip(array_flip($test));  $time=microtime(true)-$time; echo 'Flip Flip: '.$time."\n"; 

Output:

Array Unique: 1.1829199790955 Keys Flip: 0.0084578990936279 Flip Flip: 0.0083951950073242 

Note that array_keys(array_flip($array)) will give a new key values in order, which in many cases may be what you want (identical except much faster to array_values(array_unique($array))), whereas array_flip(array_flip($array)) is identical (except much faster) to array_unique($array) where the keys remain the same.

Answer 2

Caution: this technique is NOT a drop-in replacement for array_unique(). It only works for arrays with values that are are valid keys. (eg: string, integer, things can can be cast to int). And certainly does not work for arrays of objects.

$input = [true, false, 1, 0, 1.2, "1", "two", "0"]; var_export(array_unique($input)); array (   0 => true,   1 => false,   3 => 0,   4 => 1.2,   6 => 'two', ) 

vs:

var_export(array_keys(array_flip($input)));  PHP Warning:  array_flip(): Can only flip STRING and INTEGER values!  in php shell code on line 1  array (   0 => 1,   1 => 0,   2 => 'two', )