Find peaks in a series? - php

Question

I have got a series like this:
14, 13, 12, 14, 15, 18, 20, 17, 15, 19, 22, 24, 22, 18, 15, 14, 17, ...
If I plot these points on a chart on X-Y axis using these values as Y coordinates, then you'll see that there are peaks at 20 & 24.
I want to find all these peaks in the series
I tried:
$a=array(14, 13, 12, 14, 15, 18, 20, 17, 15, 19, 22, 24, 22, 18, 15, 14, 17 );
rsort($a);
echo $a[0];
echo $a[1];
But that doesn't give me the two peaks I see on the graph. The result of the code above is 24, and 22. But the peak on the graph were made by 20 and 24...
Is there a way I can detect the array to determine the peaks in the entire series?
I don't need a working code. Just some ideas I can work upon.

Answer 1

As Mark Baker suggested, you need to check that each value in the array is greater than the previous and the next value. That's what defines a peak.
Just be sure to start from the 2nd index and finish on the nth-1 item or else you'll get an Undefined offset error.
$a = array(14, 13, 12, 14, 15, 18, 20, 17, 15, 19, 22, 24, 22, 18, 15, 14, 17);
$arr = [];
for($i=1; $i<count($a)-1; $i++){
if($a[$i] > $a[$i+1] && $a[$i] > $a[$i-1]) {
array_push($arr, $a[$i]);
}
}
var_dump($arr); //returns: array(2) { [0]=> int(20) [1]=> int(24) }
Depending on your application, you might want to add some logic in case the case that 2 or more items "together" are a peak. For example, 22, 24, 24, 22, 18. If this is something you want, just change the logic to check for >= on the next item:
if($a[$i] > $a[$i+1] && $a[$i] >= $a[$i-1]) {
This will yield the same result as above.

Answer 2

<?php
$a=array(14, 13, 12, 14, 15, 18, 20, 17, 15, 19, 22, 24, 22, 18, 15, 14, 17 );
$last=0;
$peaks=array();
$upwards=false;
foreach( $a as $value )
{
if( $value > $last )
{
$upwards = true;
}
if( $value < $last )
{
if( $upwards )
{
$peaks[] = $last;
}
$upwards = false;
}
$last = $value;
}
var_dump($peaks);

Answer 3

Old question, guess it popped up because someone was editing, but here's my two cents' worth. This is better done without a loop:
<?php
$a = [14, 13, 12, 14, 15, 18, 20, 17, 15, 19, 22, 24, 22, 18, 15, 14, 17];
$b = array_filter(
$a,
function($v, $k) use($a) {return $k > 0 && $v > $a[$k-1] && $k + 1 < count($a) && $v > $a[$k+1];},
ARRAY_FILTER_USE_BOTH
);
print_r($b);
Result:
Array
(
[6] => 20
[11] => 24
)