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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 )
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4 Answers 4

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37

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.

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9 Comments

There's also array_keys(array_count_values($array)) - see CodePad
@Leith, array_keys(array_count_values( can never be faster than array_keys(array_flip( but the result will always be the same.
@Alasdair, why do you say that? The modified CodePad benchmark I linked in my previous comment indicates a significant improvement.
@Leith, yes you are right. Your benchmark does indeed say its faster. That's quite interesting.
Is array_flip O(n) or something better?
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5

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', )
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1 Comment

this isn't an answer really, but saved me some time so thanks!
3

Nothing is better than running your own benchmark.

➜ 8321620 cat first.php <?php $arr = array(-3, 1, 2, 3, 3); for($i = 0; $i <= 1000000; $i++) { array_unique($arr); } ➜ 8321620 time php first.php php first.php 3.24s user 0.01s system 99% cpu 3.251 total ➜ 8321620 cat second.php <?php $arr = array(-3, 1, 2, 3, 3); for($i = 0; $i <= 1000000; $i++) { array_flip(array_flip($arr)); } ➜ 8321620 time php second.php php second.php 1.50s user 0.01s system 99% cpu 1.514 total

Update: Array with 1000 elements.

➜ 8321620 cat first.php <?php $arr = array(); for($i = 0; $i <= 1000; $i++) { $arr[] = rand(0, 1000); } for($i = 0; $i <= 10000; $i++) { array_unique($arr); } ➜ 8321620 time php first.php php first.php 27.50s user 0.03s system 99% cpu 27.534 total ➜ 8321620 cat second.php <?php $arr = array(); for($i = 0; $i <= 1000; $i++) { $arr[] = rand(0, 1000); } for($i = 0; $i <= 10000; $i++) { array_flip(array_flip($arr)); } ➜ 8321620 time php second.php php second.php 1.59s user 0.01s system 99% cpu 1.604 total

So yes, your assumption was correct.

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Comments

-3

you would have to use

array_keys( array_flip( $array ) );

which would take more time

I would go for array_unique. It has the added benefit of explaining whats happening.

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7 Comments

Isn't array_keys also O(n)?
My point is you have to use 2 functions, not 1. It will take more time and obfuscate the code as well.
I have given an example of my array_flip code, array_keys isn't required
Well, now you're using array_flip twice
@Galen, Twice O(n) is much faster than once O(n log n).
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