How to filter an array by a condition

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长发绾君心
长发绾君心 2020-11-22 06:12

I have an array like this:

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

Now I want to fi

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  • 2020-11-22 06:43

    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
    
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  • 2020-11-22 06:52

    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;
        }
    }
    
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  • 2020-11-22 06:52

    This should work, but I'm not sure how efficient it is as you probably end up copying a lot of data.

    $newArray = array_intersect_key(
                      $fullarray, 
                      array_flip(array_keys($fullarray, 2))
                );
    
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  • 2020-11-22 06:58

    I think the snappiest, readable built-in function is: array_intersect()

    Code: (Demo)

    $array = array("a" => 2, "b" => 4, "c" => 2, "d" => 5, "e" => 6, "f" => 2);
    var_export(array_intersect($array, [2]));
    

    Output:

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

    Just make sure you declare the 2nd parameter as an array because that is the value type expected.

    Now there is nothing wrong with writing out a foreach loop, or using array_filter(), they just have a more verbose syntax.

    array_intersect() is also very easy to extend (include additional "qualifying" values) by adding more values to the 2nd parameter array.

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  • 2020-11-22 07:02

    I might do something like:

    $newarray = array();
    foreach ($jsonarray as $testelement){
        if ($testelement == 2){$newarray[]=$testelement}
    }
    $result = count($newarray);
    
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  • 2020-11-22 07:08
    $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";
    }
    
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