How to compress an array? - php

Question

Lets say, I have an array with 1000 values (integers). And I need from this array to have an array with f.e. 400 values (the number can be changed, f.e. 150, etc.).
So I need to return each 2.5th array value, i.e. 1st, 3rd, 6th, 8th, 11th, etc.
Is this somehow possible?
I dont need a code, I just need some way, how to do it.
EDITED:
My array is array of elevations (from another array with GPS coordinates). And I want to draw a 2D model of elevation. Lets say, my map will always have 400px width. 1px = 1 point of elevation. Thats why I need each 2.5th elevation value...
Like this:

Answer 1

You don't want the 2.5th one. You effectively want to divide the set into blocks of five and get the first and third from each set.
0 1 2 3 4
5 6 7 8 9
10 11 12 13 14
You want the first and third columns.
This is PHP, so we've got 0-based arrays.
We can divide it into groups of 5 using the modulo operator %. We can then see if the return value is 0 (i.e. it's in the first column) or 2 (i.e. it's in the third column).
I'm going to presume your array has numeric keys starting from 0.
// PHP 5.6
$filtered = array_filter($array, function($value, $key) {
$mod = $key % 5;
return ($mod === 0) || ($mod === 2);
}, ARRAY_FILTER_USE_BOTH);
// pre-PHP 5.6
$filtered = array();
foreach ($array as $key => $value) {
$mod = $key % 5;
if (($mod === 0) || ($mod === 2)) {
$filtered[$key] = $value;
}
}
var_dump($filtered);

Answer 2

How about using the modulo % operator?
Say you want to make 1000 into 200 values, loop through all the items in the array and keep a counter, if the counter % 5 == 0 then put that value into a new array, or if != 0 then remove from array. We use modulo 5 because 1000 / 200 = 5.

Answer 3

Below is the way to start with. It does not ensure that first and last elements are included in the output and, probably, has some other glitches. But since you requested the idea, here you go—array_reduce:
$a=[1,2,3,4,5,6,7,8,9,10,11,12]
$step = 2.5;
$i = 0;
$r = array_reduce($a, function($memo, $curr) use(&$i, $step) {
if($i === round($step * count($memo))) {
$memo[] = $curr;
}
$i++;
return $memo;
}, []);
print_r($r);
/*
Array
(
[0] => 1
[1] => 4
[2] => 6
[3] => 9
[4] => 11
)
*/
Hope it helps.

Answer 4

This should be pretty close to what you seem to be looking for. It will collect whichever values are closest (rounding down) to the float values.
$list = range(1,1000);
$targetSize = 300;
$new = array();
$step = count($list) / $targetSize;
$curStep = 0;
for( $i = 0; $i < count($list); $i++ ) {
$curStep++;
if( $curStep > $step ) {
$new[] = $list[ floor($i) ];
$curStep -= $step;
}
}

Answer 5

So this is it:
$arr = range(1, 1387); // f.e.
$cnt = 296; // f.e.
$new = array();
$max = count($arr);
$step = $max / $cnt;
for ($i = 0; $i < $max; $i += $step) {
$new[] = round($arr[(int)$i]);
}