simple statistics with PHP - php

Question

I have a bunch of JSON objects stored in my database with user input,
i.e.
{ "items": [
{question_id: 5, answer_id: 4, is_correct: 1},
{question_id, 45, answer_id: 233, is_correct: 0},
...]
}
and I'd like to build statistics about which answer_id for which question_id is answered most, etc.
I have tried PHP associative arrays like so:
<?php
$stats = array();
foreach($jsonObject->items as $item)
{
$stats[$item->question_id][$item->answer_id]++;
}
?>
I'm using PHP much like I'd use AWK, and it works, but the problem is that I get a lot of warnings like this one
Undefined offset: 23026
Where 23026 is the id of the question or the answer.
How do I debug / ensure the accuracy of the statistics? Is there a better way of doing this?

Answer 1

The notices occurs, because you try to increase a not existing value for the first occurrence of the question or answer id. If you want to run your script without warning you simply have to check if it was already set or not.
Some sample code which should work:
<?php
$stats = array();
foreach ($jsonObject->items as $item) {
if (isset($stats[$item->question_id][$item->answer_id])) {
$stats[$item->question_id][$item->answer_id]++;
} else {
// Set to 1 if not exist
$stats[$item->question_id][$item->answer_id] = 1;
}
}
?>