Calculating similarity -> python code to php code - Whats wrong? - php

Question

I am trying to convert the following python code to a PHP code. Can you please explain me what is wrong in my PHP code, because I do not get the same results. If you need example data please let me know.
# Returns a distance-based similarity score for person1 and person2
def sim_distance(prefs,person1,person2):
# Get the list of shared_items
si={}
for item in prefs[person1]:
if item in prefs[person2]: si[item]=1
# if they have no ratings in common, return 0
if len(si)==0: return 0
# Add up the squares of all the differences
sum_of_squares=sum([pow(prefs[person1][item]-prefs[person2][item],2)
for item in prefs[person1] if item in prefs[person2]])
return 1/(1+sum_of_squares)
My PHP code:
$sum = 0.0;
foreach($arr[$person1] as $item => $val)
{
if(array_key_exists($item, $arr[$person2]))
{
$p = sqrt(pow($arr[$person1][$item] - $arr[$person2][$item], 2));
$sum = $sum + $p;
}
}
$sum = 1 / (1 + $sum);
echo $sum;
Thanks for helping!

Answer 1

The main difference is that you've added sqrt to the PHP code. The PHP also doesn't handle the special case of no prefs in common, which gives 0 in the python version and 1 in the PHP version.
I tested both versions and those are the only differences I found.

Answer 2

this is close as i could make a direct translation... (untested)
function sim_distance($prefs, $person1, $person2) {
$si = array();
foreach($prefs[$person1] as $item) {
if($item in $prefs[$person2]) $si[$item]=1;
}
if(count($si)==0) return 0;
$squares = array();
foreach($prefs[$person1] as $item) {
if(array_key_exists($item,$prefs[$person2])) {
$squares[] = pow($prefs[$person1][$item]-$prefs[$person2][$item],2);
}
}
$sum_of_squares = array_sum($squares);
return 1/(1+$sum_of_squares);
}
I don't really know what you're trying to do, or if I've interpreted the indentation correctly...but maybe this'll help. I'm assuming your data structures have the same layout as in the python script.
oh...and i'm interpreting the python as this:
def sim_distance(prefs,person1,person2):
# Get the list of shared_items
si={}
for item in prefs[person1]:
if item in prefs[person2]: si[item]=1
# if they have no ratings in common, return 0
if len(si)==0: return 0
# Add up the squares of all the differences
sum_of_squares=sum([pow(prefs[person1][item]-prefs[person2][item],2) for item in prefs[person1] if item in prefs[person2]])
return 1/(1+sum_of_squares)