Array difference.

alt-solution.php

// this also works : 

$feed = array(
	4000,
	3500,
	3000,
	2000,
	1000,
	100,
	0,
	3000,
	2000,
	2500, 
	3000,
	2000,
	200,
	50,
	5000,
	7000,
	700, 
	100
);

$compost = 0;
$countFeed = count($feed);

for($i = 1; $i < $countFeed; $i++){
		
	$prevNumber = $feed[$i - 1];		
	$newNumber = $feed[$i];
	$diff = $prevNumber - $newNumber; 

	if($diff > 0) {
		$compost = $diff + $compost;
	}
}

echo $compost;

difference.php

$feed = array(
	4000,
	3500,
	3000,
	2000,
	1000,
	100,
	0,
	3000
);

want to sum the positive difference e.g. difference = -(new - old) 

e.g. -(3500-4000) = 500 so won't sum when number goes up.

Any ideas?

solution.php

$feed = array(
	4000,
	3500,
	3000,
	2000,
	1000,
	100,
	0,
	3000,
	2000,
	2500, 
	3000,
	2000,
	200,
	50,
	5000,
	7000,
	700, 
	100
);

$compost = 0;
$countFeed = count($feed);

for($i = 0; $i < $countFeed; $i++){
		
	if($i > 0) {
		$prevNumber = $feed[$i - 1];		
		$newNumber = $feed[$i];
		$diff = $prevNumber - $newNumber; 

		if($diff > 0) {
			$compost = $diff + $compost;
		}		
	}
}

echo $compost;