PHP - Using the += operator on an undefined array index - php

Question

I'm using data pulled from an SQL query to build two charts later in my code. And example query would be something like:
SELECT purchase_location, purchase_item, SUM(purchase_amount) as totalPurchase
FROM purchases
GROUP BY purchase_item_id, purchase_location
Not an exact example, but the idea is there. I then iterate through my results to build the two data sets.
$locationData = [];
$itemData = [];
foreach($queryResults as $result) {
$locationData[$result['purchase_location']] += $result['totalPurchase'];
$itemData[$result['purchase_item']] += $result['totalPurchase'];
}
Since I want the data from two different points of view, I have to use += to get the correct totals. My question is this: doing the += operator on an unset index of an array is incredibly slow. I've found that if I do the following:
if (isset($locationData['purchase_location'])) {
$locationData['purchase_location'] += $result['totalPurchase'];
} else {
$locationData['purchase_location'] = $result['totalPurchase'];
}
Using an = for the first time the index is seen. This speeds up the code significantly (As an example, my code went from 8-10 second run time down to less than half a second). My question is, is that the correct/cleanest way to handle this?
And before anyone mentions, I know I could write the query to handle all of this in this simple case, this was just a really easy example to show the issue, that of using += on an, as of yet, undefined array index.

Answer 1

I would suggest initializing the array with that index already, avoiding the need for isset checks and making the code cleaner:
$locationData = ['purchase_location' => 0];
$itemData = ['purchase_item' => 0];
foreach($queryResults as $result) {
$locationData['purchase_location'] += $result['totalPurchase'];
$itemData['purchase_item'] = $result['totalPurchase'];
}
For my projects I usually try to limit the usage of isset to the validation of received data from outside sources (ex: GET, POST) that I can't fully control.
Since you've updated you're answer, now it makes sense to use isset in this case to avoid another loop to construct the array.
$locationData = [];
$itemData = [];
foreach($queryResults as $result) {
if (!isset($locationData[$result['purchase_location']])) {
$locationData[$result['purchase_location']] = 0;
}
if (!isset($itemData[$result['purchase_item']])) {
$itemData[$result['purchase_item']] = 0;
}
$locationData[$result['purchase_location']] += $result['totalPurchase'];
$itemData[$result['purchase_item']] += $result['totalPurchase'];
}
If you're using PHP 7+ you can use the null coalesce ?? to simplify your code like this:
$locationData = [];
$itemData = [];
foreach($queryResults as $result) {
$locationData[$result['purchase_location']] = $result['totalPurchase'] + ($locationData[$result['purchase_location']] ?? 0);
$itemData[$result['purchase_item']] = $result['totalPurchase'] + ($itemData[$result['purchase_item']] ?? 0);
}

Answer 2

You could try the following:
$locationData = [];
$itemData = [];
foreach($queryResults as $result) {
$locationData[$result['purchase_location']] = ($locationData[$result['purchase_location']]??0) + $result['totalPurchase'];
$itemData[$result['purchase_item']] = ($itemData[$result['purchase_item']]??0) + $result['totalPurchase'];
}
But I think it's cleaner and more obvious just to initialise everything to zero separately first:
foreach($queryResults as $result) {
$locationData[$result['purchase_location']] = 0;
$itemData[$result['purchase_item']] = 0;
}
and then do the work of addition in another loop.

Answer 3

for your question, yes you need to create this index before adding += to this index.
instead of check it every iteration, so just configure it before the foreach loop, and then this index will exits and you will be able to sum += and thats all
$locationData = [
'purchase_location' => 0
];
$itemData = [];
foreach($queryResults as $result) {
$locationData['purchase_location'] += $result['totalPurchase'];
$itemData['purchase_item'] = $result['totalPurchase'];
}

Answer 4

My question is, is that the correct/cleanest way to handle this?
Without example data and expected results it's hard to know that results you are after.
A educated guess would be using MySQL WITH ROLLUP GROUP BY Modifier to get a added total record.
SELECT purchase_location, purchase_item, SUM(purchase_amount) as totalPurchase
FROM purchases
GROUP BY purchase_item_id, purchase_location WITH ROLLUP