Reduce size of numeric array by averaging values in PHP: 20 values => X values

Question

In PHP I have an array containing 20 elements or more. The keys have been assigned automatically. The values are random numbers from 1 to 50.

<?php
$randomList = array();
for ($i = 0; $i < 20; $i++) {
  $randomList[] = mt_rand(1, 50);
}
?>

Now I want to plot this array into a line chart. Unfortunately, I can only use 5 points for the graph. So I must reduce the number of elements in the array. But I don't want the look of the chart to be changed. So I need a function like this:

To make it clearer: When I want to reduce the size of an array from 6 elements to 3 elements, I can just sum up pairs of two elements each and take the average:

array(1, 8, 3, 6, 9, 5) => array(4.5, 6, 7)

My function should do this with variable sizes (for input and output).

I hope you can help me. Thanks in advance!

Hi. Can you be more specific about the kind of plot you want to make? Some kind of linear regression? — user50685
– user50685, Commented Jul 29, 2009 at 22:42
See this example of Google: bit.ly/UFMX5 Assuming I have the "150 data points", I want to have the graph to look like "40 data points". But I don't want to just leave out 73% of the data points. — caw
– caw, Commented Jul 29, 2009 at 22:45
Shouldn't your example result in array(4.5, 4.5, 7) instead of array(4.6, 6, 7)? — Paige Ruten
– Paige Ruten, Commented Jul 29, 2009 at 22:47
Oh, yes of course. I thought 3+6 would be 12! :D Really embarrassing. But it's late here in Germany. :) — caw
– caw, Commented Jul 29, 2009 at 23:25

Paige Ruten · Accepted Answer · 2009-07-29 22:45:41Z

4

To "shorten" an array $randomList to $X elements in the way that you've described, you could use array_chunk() and array_map() together like this:

$randomList = array_chunk($randomList, count($randomList) / $X);
$randomList = array_map('array_average', $randomList);

And define array_average() as:

function array_average($array) {
    return array_sum($array) / count($array);
}

answered Jul 29, 2009 at 22:45

Paige Ruten

177k37 gold badges183 silver badges199 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

Evert · Accepted Answer · 2009-07-29 22:46:46Z

-1

$randomList = array();
for ($i = 0; $i < 20; $i++) {
  $randomList[] = mt_rand(1, 50);
}  

$avgList=array();
for($i=0;$i<count($randomList)/2;$i++) {
   $avgList[] = ($randomList[$i*2] + $randomList[$i*2+1]) / 2
}

answered Jul 29, 2009 at 22:46

Evert

101k18 gold badges136 silver badges221 bronze badges

1 Comment

caw Over a year ago

Thanks! But I don't want to halve the array sizes. The function should also be able to reduce an array from 12 elements to 4. Or - even more difficult - 17 elements to 5.

Collectives™ on Stack Overflow

Reduce size of numeric array by averaging values in PHP: 20 values => X values

2 Answers 2

Comments

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related