forcing json_encode to encode (sparse) array as indexed array in php

Question

I am trying to transfer an array between the host and my webpage using JSON. On the host, I have a php script which reads a number of rows from a mysql table and creates an array out of it.

$query="select ID,val,size from db;";
$result=mysql_query($query);
while ($row=mysql_fetch_assoc($result)) {
  $idx= (int) $row['ID'];
  $data[$idx]['val']=$row['val'];
  $data[$idx]['size']=$row['size'];
}

echo json_encode($data);

Normally, the ID's are incremented linearly between, for example, 0 and 99.. In this case, the json_encoded output is an array (and will be interpreted as an array in javascript... If I do an array.length, I get 100 as the answer)....
Sometimes, a few of the rows are missing from the table (maybe there was no row with an idx=40)... In this case, the array is json_encoded as a collection of objects. In javascript, if I try to look at array.length, I get an undefined result. I'd like to force the data array to be interpreted as an indexed array with null/0 values for missing rows. Is there an easy way to do this? It looks like the json_encode function has an option to force encoding as an associative array, but not the other way around.

Thx.

Answer 1

$query="select ID,val,size from db;";
$result=mysql_query($query);
$maxId = -1;
$defaultValue = null;
while ($row=mysql_fetch_assoc($result)) {
   $idx= (int) $row['ID'];
   if($maxId < $idx)
       $maxId = $idx;
   $data[$idx]['val']=$row['val'];
   $data[$idx]['size']=$row['size'];
}
for($i=0;$i<$maxId;$i++)
    if(!isset($data[$i))
        $data[$i] = $defaultValue;
echo json_encode($data);

Answer 2

I would define $data as a full array and then replace elements inside of it.

$data = array_fill($starting_index, 100, null);

$query="select ID,val,size from db;";
$result=mysql_query($query);
while ($row=mysql_fetch_assoc($result)) {
  $idx= (int) $row['ID'];
  $data[$idx]['val']=$row['val'];
  $data[$idx]['size']=$row['size'];
}

echo json_encode($data);

Answer 3

It's annoying, but that's Javascript for you. I usually work around this by adding another level to my data structure:

$data['size'] = count($myarray);
$data['array'] = $myarray;

echo json_encode($data);

That way you get the length included for "free", without having to do any client-side looping/counting.