从数组中返回的“n”元素均匀地生成stderr

Question

I have a solution which takes m-element array returns an n-element array distributed evenly. But for some value of m it generates stderr.

For example, if array will have 15 elements it stderr will be:

PHP Notice: Undefined offset: -3 PHP Notice: Undefined offset: -1

class SpredArrayClass
{

var $array = [1,2,3,4,5,6,7,8,9,9,10,11,12,13,14,15];

function spreadOutArray(array $array = [], $targetOutputLength = 10) {

    $array = $this->array;

    $originalArrayLength = count($array);

    if ($originalArrayLength == 0) {
        return false;
    }

    if ($originalArrayLength <= $targetOutputLength) {
        return $array;
    }

    $output = [];
    $interval = round($originalArrayLength / $targetOutputLength);
    for ($index = $originalArrayLength - 1; count($output) < $targetOutputLength; $index -= $interval) {
        $output[] = $array[$index];
    }

    return array_reverse($output);
    }
}

Problem seem to be in this part:

$output = [];
$interval = round($originalArrayLength / $targetOutputLength);
for ($index = $originalArrayLength - 1; count($output) < $targetOutputLength; $index -= $interval) {
    $output[] = $array[$index];
}

Undefinded offset stderr appears at this line: $output[] = $array[$index];

Demo of code execution here: http://ideone.com/UrF9UK

Answer 1

You round too early. The steps ($interval) should be exact, because if the rounding is upward, the steps will bring you too fast to the start of the array and lead to negative indexes.

So only round when you use the resulting index:

$interval = $originalArrayLength / $targetOutputLength;
for ($index = $originalArrayLength - 1; count($output) < $targetOutputLength; $index -= $interval) {
    $output[] = $array[round($index)];
}