Remove the first N elements from an array while reducing the N variable

Question

I'm trying to remove the first $offset elements from an array and reduce the offset for future adds to the array. The loop must also end if I reach the end of the array.

$max1 = $offset;
$max2 = count($elements);
for ($i=0; ($i<$max1 && $i<$max2); $i++) {
 unset($elements[$i]);
 $offset--;
}

I'm trying to find a better way to do it. At least a way to use the values of $offset and count($elements) (which are changing) without having to create aux variables.

• Sᴀᴍ Onᴇᴌᴀ: What task does this code accomplish? Please tell us, and also make that the title of the question via edit. Maybe you missed the placeholder on the title element: "State the task that your code accomplishes. Make your title distinctive.". Also from How to Ask: "State what your code does in your title, not your main concerns about it.".
• Dave: What @SamOnela said. You were given the exact same advice when you posted this question on StackOverflow.
• CarlosAS: I'm trying to be as much specific as I can. I changed the title but I think the task is clear, I don't really know what could be unclear in the description.

Answer 1

Code style

Variable names

Make sure that your variable names are descriptive of what they contain. In particular $max1 and $max2 use a poor naming style as it is unclear what you expect them to contain.

Whitespace

Make sure to keep whitespace around your operators, even in a for-loop. This increases readability, and thus decreases the chance for bugs.

Unneeded bookkeeping

You currently use $offset--; but do not use the result. Remove code you do not use.

Bugs

Your current code only works once, because you keep the keys. Considering the following code:

$offset = 3;
$elements = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13];
$max1 = $offset;
$max2 = count($elements);
for ($i=0; ($i<$max1 && $i<$max2); $i++) {
 unset($elements[$i]);
 $offset--;
}
$max1 = $offset;
$max2 = count($elements);
for ($i=0; ($i<$max1 && $i<$max2); $i++) {
 unset($elements[$i]);
 $offset--;
}
var_dump($elements);
// Result: array(11) { [3]=> int(3) [4]=> int(4) [5]=> int(5) [6]=> int(6) [7]=> int(7) [8]=> int(8) [9]=> int(9) [10]=> int(10) [11]=> int(11) [12]=> int(12) [13]=> int(13) }

You would expect it to remove the first 3 elements twice, but because the keys are not reset, the second iteration of your code will try to remove key 0, 1 and 2, while the first key is 3.

Algorithm

Your current code modifies an array in-place while keeping the keys. Modifying arrays while looping over them is aweful in any language, as you need to be mindful that conditions in loops may be pre-calculated, and element pointers may skip elements.

Since you use a for-loop, I can assume you use an array with numeric keys, which makes it odd that you would want to keep the current keys.

Alternative creating a new array

In php you can use array_slice to obtain a new array with only a slice of the original array. This will reset the keys of the array.

$elements = array_slice($elements, $offset);

No loop required.

Alternative deleting elements

You can use array_splice to modify an array in-place by deleting a consecutive chunk of array elements. It returns the deleted elements and the original array has a few less elements. This will also reset the keys.

array_splice($elements, 0, $offset);

Again, no loop required.

Alternative with a loop

You can also use a loop and use array_shift to remove the first element of the array repeatedly. For numeric keys the keys are reset, while literal keys are left untouched.

for ($i = 0; $i < $offset; $i++) {
 $result = array_shift($elements);
 if (is_null($result)) {
 // The array is empty, so safe us some time
 break;
 }
}

Filter to keep keys

If we want to keep keys, we can use array_filter. The following code works exactly like your own code, as in: It works the first time you invoke it, and then it breaks.

$elements = array_filter(
 $elements,
 function ($item, $index) use ($offset) {
 return $index >= $offset;
 },
 ARRAY_FILTER_USE_BOTH
);

You can improve that code by using and checking against $offset + $first_key.

reset($elements);
$first_key = key($array);