How do you find the closest value to a number in an associative array?

I have an associative array with 15 different companies and their stock prices, formatted as shown below:

$CloseStockPrice ('Business'=>50.5. 'Business two'=>100.5, .....)

I have found the average stock price:

$Average = (array_sum($CloseStockPrice)/count($CloseStockPrice));

The average ends up being 161.

But I now need to find the closest number (absolute terms) to that average value (161) within the associative array. I need to display the business and the stock value.

My most recent attempt:

function computeClosest(array $CloseStockPrice) 
{
    $closest = 161;
    
    for ($i = 161; $i < count($CloseStockPrice) ; $i++) 
    {
        if ($closest === 161) 
        {
            $closest = $CloseStockPrice[$i];
        } else if ($CloseStockPrice[$i] > 161 && $CloseStockPrice[$i] <= abs($closest)) 
        {
            $closest = $CloseStockPrice[$i];
        } else if ($CloseStockPrice[$i] < 161 && -$CloseStockPrice[$i] < abs($closest)) 
        {
            $closest = $CloseStockPrice[$i];
            return $closest;
        }
    }
}

Any suggestions?

Answer

While you loop through your array of business entries, cache the businesses with prices with the smallest absolute difference involving the average value.

While you might expect a single value in many cases, the fact that multiple qualifying businesses is possible means that you must keep an array of qualifying businesses for most accurate results.

A linear (single loop) process (O(n)) will outperform a sort algorithm (O(n log n)).
https://stackoverflow.com/q/56506410/2943403

Code: (Demo)

$closeStockPrice = [
    'A' => 50,
    'B' => 155,
    'C' => 75,
    'D' => 245,
    'E' => 300,
    'F' => 100,
    'G' => 153,
];

$average = array_sum($closeStockPrice) / count($closeStockPrice);

$bestDistances = [];
foreach ($closeStockPrice as $business => $price) {
    $distance = abs($average - $price);
    $current = current($bestDistances);
    if (!$bestDistances || $current > $distance) {
        $bestDistances = [$business => $distance];  // new best distance
    } elseif ($current === $distance) {
        $bestDistances[$business] = $distance;  // push business with same distance
    }
}
var_export([
    'average' => $average,
    'bestDistances' => $bestDistances,
    'bestBusinessPrices' => array_intersect_key($closeStockPrice, $bestDistances)
]);

Output:

array (
  'average' => 154,
  'bestDistances' => 
  array (
    'B' => 1,
    'G' => 1,
  ),
  'bestBusinessPrices' => 
  array (
    'B' => 155,
    'G' => 153,
  ),
)