How to determine if data is increasing or decreasing in PHP - php

Question

Let's say we have the following data in an array:
$data1 = [3,5,7,6,8,9,13,14,17,15,16,16,16,18,22,20,21,20];
$data2 = [23,18,17,17,16,15,16,14,15,10,11,7,4,5];
As with $data1 we can say the data is increasing while in $data2 it is decreasing.
Using PHP, how do you know the data is increasing or decreasing, and is there a way on how to measure
know the rate of increasing as well as decreasing i.e in terms of percentage.
Edit
From the comments I received I got an idea and here is what I have tried.
What I want to achieve;
I want to know if the trend of the data coming in is upwards or downwards.
Want also to know the rate at which the data is rising or droping. For example $data1 = [1,3,5]; is not the same as $data2 = [1, 20, 55];. You can see $data1 rate of increase is not the same as $data2.
function increaseOrDecrease($streams = []) : array
{
$streams = [3,5,7,6,8,9,13,14,17,15,16,16,16,18,22,20,21,20]; // For the increasing
//$streams = [23,18,17,17,16,15,16,14,15,10,11,7,4,5]; // For the decreasing
$first = 0;
$diff = [];
foreach ($streams as $key => $number) {
if ($key != 0) {
$diff[] = $number - $first;
}
$first = $number;
}
$avgdifference = array_sum($diff)/count($diff); //Get the average
$side = $avgdifference > 0 ? 'UP' : 'DOWN';
$avgsum = array_sum($streams)/count($streams);
$percentage = abs($avgdifference)/$avgsum * 100;
if ($side == 'UP') {
$data = [
'up' => true,
'percent' => $percentage,
];
}else {
$data = [
'up' => false,
'percent' => $percentage,
];
}
return $data;
}
I would like some help to refactor this code or the best approach to solve the issue.

Answer 1

There are several ways to analyze data and extract a trend. The most classical method is called
least squares. It's a way of fitting a line
through the data. The method computes the slope and the intercept of the line. The trend is just the slope.
The formulas are given here.
A PHP implementation is the following:
function linearRegression($x, $y)
{
$x_sum = array_sum($x);
$y_sum = array_sum($y);
$xy_sum = 0;
$x2_sum = 0;
$n = count($x);
for($i=0;$i<$n;$i++)
{
$xy_sum += $x[$i] * $y[$i];
$x2_sum += $x[$i] * $x[$i];
}
$beta = ($n * $xy_sum - $x_sum * $y_sum) / ($n * $x2_sum - $x_sum * $x_sum);
$alpha = $y_sum / $n - $beta * $x_sum / $n;
return ['alpha' => $alpha, 'beta' => $beta];
}
function getTrend($data)
{
$x = range(1, count($data)); // [1, 2, 3, ...]
$fit = linearRegression($x, $data);
return $fit['beta']; // slope of fitted line
}
Examples:
echo getTrend([1, 2, 3]); // 1
echo getTrend([1, 0, -1]); // -1
echo getTrend([3,5,7,6,8,9,13,14,17,15,16,16,16,18,22,20,21,20]); // 1.065
echo getTrend([23,18,17,17,16,15,16,14,15,10,11,7,4,5]); // -1.213

Answer 2

You are asking for a type of data structure that can represent ascending as well as descending data. PHP got SplMinHeap and SplMaxHeap for this purpose. These built in classes make life easer when dealing with ascending or descending datasets.
A quick example ...
<?php
declare(strict_types=1);
namespace Marcel;
use SplMinHeap;
$numbers = [128, 32, 64, 8, 256];
$heap = new SplMinHeap();
foreach ($numbers as $number) {
$heap->insert($number);
}
$heap->rewind();
while($heap->valid()) {
// 8, 32, 64, 128, 256
echo $heap->current() . PHP_EOL;
$heap->next();
}
The SplMinHeap class keeps the minimum automatically on the top. So just use heaps instead of arrays that have no structure. Same goes for SplMaxHeap that keeps the highest value on the top.
Finding the differences
If you want to iterate all data and finding the differences between one to the next, you just have to iterate the heap. It 's ordered anyway.
$heap->rewind();
$smallest = $heap->current();
while($heap->valid()) {
// 8, 32, 64, 128, 256
$current = $heap->current();
echo $current . PHP_EOL;
// 0 (8 - 8), 24 (32 - 8), 32 (64 - 32), 64 (128 - 64), 128 (256 - 128)
echo "difference to the value before: " . ($current - $smallest) . PHP_EOL;
$smallest = $current;
$heap->next();
}

Answer 3

I would do simple things like this
$data1 = [3,5,7,6,8,9,13,14,17,15,16,16,16,18,22,20,21,20];
$data2 = [23,18,17,17,16,15,16,14,15,10,11,7,4,5];
getTrend($data1) //Returns up
getTrend($data2) // Returns down
function getTrend($arr)
{
$up = 0;
$down = 0;
$prev = "";
foreach($arr as $val)
{
if($prev != "" && $val > $prev)
{
$up = $val-$prev;
}
else if($prev != "" && $val < $prev)
{
$down = $prev-$val ;
}
$prev = $val);
}
if($up > $down)
{
return "up";
}
else if($down > $up)
{
return "down";
}
else {
return "flat";
}
}