add to array if it isn't there already

Question

How do I add elements to an array only if they aren\'t in there already? I have the following:

$a=array();
// organize the array
foreach($array as $k=>$v)         


        

Answer 1

Since there are a ton of ways to accomplish the desired results and so many people provided !in_array() as an answer, and the OP already mentions the use of array_unique, I would like to provide a couple alternatives.

Using array_diff (php >= 4.0.1 || 5) you can filter out only the new array values that don't exist. Alternatively you can also compare the keys and values with array_diff_assoc. http://php.net/manual/en/function.array-diff.php

$currentValues = array(1, 2);
$newValues = array(1, 3, 1, 4, 2);
var_dump(array_diff($newValues, $currentValues));

Result:

Array
(
    [1] => 3
    [3] => 4
)

http://ideone.com/SWO3D1

Another method is using array_flip to assign the values as keys and compare them using isset, which will perform much faster than in_array with large datasets. Again this filters out just the new values that do not already exist in the current values.

$currentValues = [1, 2];
$newValues = [1, 3, 1, 4, 2];
$a = array();
$checkValues = array_flip($currentValues);
foreach ($newValues as $v) {
    if (!isset($checkValues[$v])) {
        $a[] = $v;
    }
}

Result:

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

http://ideone.com/cyRyzN

With either method you can then use array_merge to append the unique new values to your current values.

1. http://ideone.com/JCakmR
2. http://ideone.com/bwTz2u

Result:

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