How to filter an array by a condition

I have an array like this:

array("a" => 2, "b" => 4, "c" => 2, "d" => 5, "e" => 6, "f" => 2)

Now I want to filter that array by some condition and only keep the elements where the value is equal to 2 and delete all elements where the value is NOT 2.

So my expected result array would be:

array("a" => 2, "c" => 2, "f" => 2)

_{Note: I want to keep the keys from the original array.}

How can I do that with PHP? Any built-in functions?

$fullArray = array('a'=>2,'b'=>4,'c'=>2,'d'=>5,'e'=>6,'f'=>2);


function filterArray($value){
    return ($value == 2);
}

$filteredArray = array_filter($fullArray, 'filterArray');

foreach($filteredArray as $k => $v){
    echo "$k = $v";
}

You somehow have to loop through your array and filter each element by your condition. This can be done with various methods.

Loops while / for / foreach method

Loop through your array with any loop you want, may it be while, for or foreach. Then simply check for your condition and either unset() the elements if they don't meet your condition or write the elements, which meet the condition, into a new array.

Looping

//while loop
while(list($key, $value) = each($array)){
    //condition
}

//for loop
$keys = array_keys($array);
for($counter = 0, $length = count($array); $counter < $length; $counter++){
    $key = $keys[$counter];
    $value = $array[$key];
    //condition 
}

//foreach loop
foreach($array as $key => $value){
    //condition
}

Condition

Just place your condition into the loop where the comment //condition is. The condition can just check for whatever you want and then you can either unset() the elements which don't meet your condition, and reindex the array with array_values() if you want, or write the elements in a new array which meet the condition.

//Pseudo code
//Use one of the two ways
if(condition){  //1. Condition fulfilled
    $newArray[ ] = $value;
            //↑ Put '$key' there, if you want to keep the original keys
            //Result array is: $newArray

} else {        //2. Condition NOT fulfilled
    unset($array[$key]);
    //Use array_values() after the loop if you want to reindex the array
    //Result array is: $array
}

array_filter() method

Another method is to use the array_filter() built-in function. It generally works pretty much the same as the method with a simple loop.

You just need to return TRUE if you want to keep the element in the array and FALSE if you want to drop the element out of the result array.

//Anonymous function
$newArray = array_filter($array, function($value, $key){
    //condition
}, ARRAY_FILTER_USE_BOTH);

//Function name passed as string
function filter($value, $key){
    //condition
}
$newArray = array_filter($array, "filter", ARRAY_FILTER_USE_BOTH);

//'create_function()', NOT recommended
$newArray = array_filter($array, create_function('$value, $key', '/* condition */'), ARRAY_FILTER_USE_BOTH);

preg_grep() method

preg_grep() is similar to array_filter() just that it only uses regular expression to filter the array. So you might not be able to do everything with it, since you can only use a regular expression as filter and you can only filter by values or with some more code by keys.

Also note that you can pass the flag PREG_GREP_INVERT as third parameter to invert the results.

//Filter by values
$newArray = preg_grep("/regex/", $array);

Common conditions

There are many common conditions used to filter an array of which all can be applied to the value and or key of the array. I will just list a few of them here:

//Odd values
return $value & 1;

//Even values
return !($value & 1);

//NOT null values
return !is_null($value);

//NOT 0 values
return $value !== 0;

//Contain certain value values
return strpos($value, $needle) !== FALSE;  //Use 'use($needle)' to get the var into scope

//Contain certain substring at position values
return substr($value, $position, $length) === $subString;

//NOT 'empty'(link) values
array_filter($array);  //Leave out the callback parameter

You can iterate on the copies of the keys to be able to use unset() in the loop:

foreach (array_keys($array) as $key) {
    if ($array[$key] != 2)  {
        unset($array[$key]);
    }
}

The advantage of this method is memory efficiency if your array contains big values - they are not duplicated.

EDIT I just noticed, that you actually only need the keys that have a value of 2 (you already know the value):

$keys = array();
foreach ($array as $key => $value) {
    if ($value == 2)  {
        $keys[] = $key;
    }
}