Error unset value from array in php - php

Question

I want unset all value from this array, but result only remove half
Ex 1:
$filter = array("English", "Malay", "Student Pass", "NRIC");
for($i=0; $i<count($filter); $i++){
unset($filter[$i]);
}
print_r($filter);
=> Result Array ( [2] => Student Pass [3] => NRIC )
Ex 2:
$filter = array("English", "Malay");
for($i=0; $i<count($filter); $i++){
unset($filter[$i]);
}
print_r($filter);
=> Result Array ( [1] => Malay )

Answer 1

Your problem is that every time your for loop runs, it recalculates the array's length with count($filter). Thus, the for loop runs less often than there are elements within the array. Instead you should determine the array's length ahead of the loop and only use this variable within the loop:
e.g.:
<?
$filter = array("English", "Malay", "Student Pass", "NRIC");
$arrayLength = count($filter); // contains the initial length of the array
for($i=0; $i<$arrayLength;$i++) {
unset($filter[$i]);
}
print_r($filter);
?>
However, regarding overall performance it might be better to overwrite the array or even unset it.
e.g.:
// Overwrite it with an empty array
$filter = array();
// Unset it
unset($filter);

Answer 2

Its because when you unset the item on the array it removes it. So on the next iteration the count($filter) has now gone down. You could do as #Daan suggested and just set $filter = array(). However if you insisted on calling unset you should loop over the array in reverse order.
for($i = count($filter) - 1; $i >= 0; $i--) {
unset($filter[$i]);
}

Answer 3

Each time you unset an element, count($filter) decreases. Let me give you an example:
You have six apples. You are eating the n'th apple while n is smaller than the number of apples. So, you eat the 0'th apple, because 0 < 6. You have 5 apples. You eat the 1st apple, because 1 < 5. You have 4 apples. You eat the 2nd apple, because 2 < 4. You have 3 apples. You don't eat the 3rd apple, since 3 < 3 is not true. As a result, three apples will remain. Your mistake was assuming that the ever incremented index is slower than the ever decremented count if and only if the array is not empty. This is wrong and you need a different approach, like this:
while (count($filter) > 0) {
unset($filter[count($filter) - 1]);
}
This is sub-optimal though, just unset the array instead, like this:
unset($filter);