php find max in 2 dimension array but return another element('id') of the array

Question

here is my array sample $data[][];

Array( [0] => Array ( [id] => 1349
 [rating1] => 1.9378838029981E-7
 [rating2] => 1.1801796607774 )
 [1] => Array ( [id] => 1350
 [rating1] => 5.5499981876923E-7
 [rating2] => 1.5121329727308 )
 [2] => Array ( [id] => 1377
 [rating1] => 0.00023952225410117
 [rating2] => 2.1947077830236 )
 [3] => Array ( [id] => 1378
 [rating1] => 0.00022982302863634
 [rating2] => 2.2135588326622 )
 [4] => Array ( [id] => 1379
 [rating1] => 0.00026272979843585
 [rating2] => 2.2388295595073 )
 [5] => Array ( [id] => 1380
 [rating1] => 0.0002788640872546
 [rating2] => 2.1815325502993 )
)

I want to find max($data[][rating?]) but return $data[id][max_rating?] i.e. id associated with the max value.
Finding the max was easy for one particular, say rating1, I used array_reduce as follows (this is inspired from this SO ):

$max = array_reduce($data, function($a, $b) {
 return $a > $b['rating1'] ? $a : $b['rating1']; 
});

Now I have two questions :
1. How can I extend above array_reduce to include rating2 ? I have other ratingX as well.
2. I do not want the max value, rather the $data[][id] associated with the max.

I am not so much concerned about Q1, but the second one is important as I don't want to search through the array again to get associated $data[][id].
One line of thought is to use array_map instead of array_reduce, but I couldn't come up with a version which will pass on both [id] and [rating?]. Also, there are complications when I try to max() multiple rating? at one shot, as each rating will have different max, which in turn associates with different [id].
EDIT : Just to be clear, I want all the respective ids associated with respective max of each rating?

• knittl: this looks like a table from a database. if so, do your filtering at the db-level!
• wadkar: no this is NOT, based on the data from table, I calculate rating1,rating2... and add that rating to the array already available from mysql_fetch_array() .
• knittl: you could do the rating calculation in sql too, you know. anyway, have a look at my answer and see if it works for you
• wadkar: hmm, lets just say its not possible and leave it at that. I am going through your suggestions. Thanks in advance :)
• Bob Vale: if [id] is unique is there any reason why you are not using the id as the array key instead of having it as a field in the sub array? so array(1349 => array ('rating1'=>....,'rating2'=>....),1350=> array.....

Answer 1

assuming your array is unsorted you have to loop through it at least once (either manually or using builtin functions). i'd use the following code:

$array = array(
 array( 'id' => 1349, 'sudhi_rating1' => 1.9378838029981E-7, 'sudhi_rating2' => 1.1801796607774 ),
 array( /* ... */ ),
 /* ... */
);
$idx_max = 0;
foreach($array as $idx => $item) {
 if($item['sudhi_rating1'] > $array[$idx_max]['sudhi_rating1'])
 $idx_max = $idx;
}
echo "Rating 1 has max value at id ", htmlspecialchars($array[$idx_max]['id']);

you can extend the code to check multiple ratings at once (make $idx_max an array itself and add more ifs):

$idx_max = array (
 'sudhi_rating1' => 0,
 'sudhi_rating2' => 0,
 /* ... */ );
foreach($array as $idx => $item) {
 foreach($idx_max as $rating => &$max) {
 if($item[$rating] > $array[$max][$rating])
 $max = $idx;
 }
}
foreach($idx_max as $rating => $max)
 echo 'Max value for ', htmlspecialchars($rating), ' has id ', htmlspeciachars($array[$max]['id']);

• wadkar: the foreach(&$idx_max as $rating => $max) is generating error, if you want to pass $idx_max as reference i think $max should be replaced with &$max , i tried and it works now. Fabulous !! excellent peace of code. Thanks a lot, just one tiny problem, it returns 5,4,4 for rating1,2,3 i.e. not the 'id' but I can easily do $data[5]['id'] to get the value of 'id'.
• wadkar: you fixed it, thanks for this excellent answer ! Just to improve the efficiency, what if the $data array is passed by reference too? Its not modified anywhere... I dont like the foreach syntax because of the copy constraints... I never know how man thousand arrays I will have to process.
• knittl: @sudhi: i've already noticed the id issue which was fixed 7 minutes ago. thanks for pointing out the wrong place of the reference operator. as for passing the array itself as reference: try for yourself, it's hard to say beforehand, but i don't think there will be too much of an overhead (copy-on-write?)
• wadkar: hmm... copy-on-write, i always forget that, and my instincts tell me not to create copies of things that you are not going to change. PHP is very different in that regard. Anyways I don't have the time and necessary skills to rationalize the difference between passing by reference and passing by copy, so I will keep it simple and follow what you prescribed. Thanks a lot @knittl

Answer 2

$max = array_reduce($data, function($a, $b) { 
 if (is_null($a)) return $b;
 return max($a['rating1'],$a['rating2'])>max($b['rating1'],$b['rating2']) ? $a : $b;
});

Result: no Entries $max= NULL otherwise $max['id'] is the id with the max rating

Alternatively this generic code

$max = array_reduce($data, function($a, $b) {
 if (is_null($a)) return $b;
 return maxRating($a)>maxRating($b) ? $a : $b;
 });
function maxRating($row){
 return (max(array_intersect_key($row,array_flip(array_filter(array_keys($row),function ($item) { return strstr($item,'rating')!==FALSE;})))));
}

Will find for all ratings of the form rating?

EDIT -- The code was trying to answer Q1 here is the answer for just Q2

$max = array_reduce($data, function($a, $b) { 
 if (is_null($a)) return $b;
 return $a['rating1']>$b['rating1'] ? $a : $b;
});

EDIT2 -- This is a generic solution for any number of rating? columns

$ratingKeys=array_filter(array_keys($data[0]),function ($item) { return strstr($item,'rating')!==FALSE;});
$max = array_reduce($data,function($a,$b) use (&$ratingKeys) {
 if (is_null($a)) {
 $a=array();
 foreach($ratingKeys as $key) {
 $a[$key]=$b[$key];
 $a[$key.'_id'] = $b['id'];
 }
 return $a;
 }
 foreach($ratingKeys as $key) {
 if ($a[$key]<$b[$key]) {
 $a[$key]=$b[$key];
 $a[$key.'_id']=$b['id'];
 }
 }
 return $a;
});

This code results in

array(4) {
 ["rating1"]=> float(0.0002788640872546)
 ["rating1_id"]=> int(1380)
 ["rating2"]=> float(2.2388295595073)
 ["rating2_id"]=> int(1379)
}

EDIT 3 -- If you change the format of the input array to use id as the array key, you can massively simplify

$max=array_reduce(array_keys($data),function ($a,$b) use (&$data) {
 if (is_null($a)) $a=array();
 foreach(array_keys($data[$b]) as $item) {
 if (!isset($a[$item]) {
 $a[$item] = $b;
 } else {
 if ($data[$a[$item]][$item]) < $data[$b][$item]) $a[$item]=$b;
 }
 return $a;
 }
});

This code results in

array(2) {
 ["rating1"]=> int(1380)
 ["rating2"]=> int(1379)
}

• wadkar: I want to compare rating1 of each $data[] and not between each $data[][id] ! So there are three max and three corresponding ids
• Bob Vale: This finds the row that has the maximum value in either rating1 or rating2
• wadkar: ohhk, maybe its still not clear to you, i want to find max in rating1 alone. ignore rating2, now how can i get id as well?
• Bob Vale: I have provided rating1 alone, $max will equal the matching row so you can then just use $max['id'] to get the id
• Bob Vale: I have no provided a completely generic solution for any number of rating columns