Concatenate every n-th row of CSV file

Question

Consider data.csv:

"1","2","3","4"
"5","6","7","8"
"9","10","11","12"
"13","14","15","16"
"17","18","19","20"
"21","22","23","24"
"25","26","27","28"
"29","30","31","32"
"33","34","35","36"

I need to read this CSV and concatenate every nth, say every 3rd row to each other. So the desired output is:

array (
  [0] => 1,2,3,4,13,14,15,16,25,26,27,28    // row 1, 4 and 7
  [1] => 5,6,7,8,17,18,19,20,29,30,31,32    // row 2, 5 and 8
  [2] => 9,10,11,12,21,22,23,24,33,34,35,36 // row 3, 6 and 9
)

I need n to be variable, so it should be easy to, for instance, concatenate every 4th row as well:

array (
  [0] => 1,2,3,4,17,18,19,20,33,34,35,36    // row 1, 5 and 9
  [1] => 5,6,7,8,21,22,23,24                // row 2, 6
  [2] => 9,10,11,12,25,26,27,28             // row 3, 7
)

I now have this:

$path = "data.csv";
$row = 1;
if (($handle = fopen($path, "r")) !== FALSE) {
  while (($data = fgetcsv($handle, 1000, ",")) !== FALSE) {
    for ($i = 1; $i <= 3; $i++) { // concatenate every 3rd row
      if ($row % $i == 0) $newrows[$i] .= implode(",", $data);
    }
    $row++;
  }
}

print_r($newrows);

But it doesn't work as expected since it outputs

Array (
  [1] => 1,2,3,45,6,7,89,10,11,1213,14,15,1617,18,19,2021,22,23,2425,26,27,2829,30,31,3233,34,35,36
  [2] => 5,6,7,813,14,15,1621,22,23,2429,30,31,32
  [3] => 9,10,11,1221,22,23,2433,34,35,36
)

Do you have a better idea? This mathematical logic always has me puzzled! :-)

Answer 1

I don't have access to my server, right now, so I can't test it, but I think this simplified version of what you have should work.

$path = "data.csv";
$row = 1;

$groupings = 4; // group every n rows

if(($handle = fopen($path, "r")) !== FALSE)
{
    while(($data = fgetcsv($handle, 1000, ",")) !== FALSE)
    {
        $where = $row % $groupings; // which grouping is this?
        $newrows[$where] .= implode(",", $data).",";

        $row++;
    }
}

for($i = 0; $i < count($newrows); $i++)
{
    $newrows[$i] = substr($newrows[$i], 0, -1);
}

You need to run a second loop AFTER the initial while-loop since there's no way (that I can think of) to detect if the data being added is the last data that's going to be added to that grouping.