adding sums of array values

a tricky one?

[httpwebwitch]

I have two arrays:
('a'=>1,'b'=>3,'c'=>2,'d'=>6)
('a'=>2,'b'=>0,'c'=>4)

I need a very efficient function to add the values together for each element. For instance, the output would be an array:

('a'=>3,'b'=>3,'c'=>6,'d'=>6)

This function could potentially be used in billions of iterations, so it needs to be as efficient as possible.

I will also be needing another similar function, which I'll post in a separate thread. Any advice is welcome.
:-)

[httpwebwitch]

function add_array_values($a,$b){
$new=array();
foreach($a as $k=>$v){
$new[$k]+=$v;
}
foreach($b as $k=>$v){
$new[$k]+=$v;
}
return $new;
}

Is that as good as it gets?

[Netizen]

Or:

function add_array_values($a,$b){

$keys=array_merge(array_keys($a),array_keys($b));

foreach ($keys as $key) {
$c[$key]=$a[$key]+$b[key];
}

return $c;

}

Is there another way of presenting the data in the arrays? Where are the arrays being generated?

[ergophobe]

Some deconstructive criticism....

Assuming that you are running your development platform with error reporting set to all (which is a good idea):

$new[$k]+=$v;

This will throw a notice or warning, since $new[$k] is not defined on the first time through.

$c[$key]=$a[$key]+$b[key];

Similar problem here. You will get a warning or notice for an undefined index.

Of course, if speed is to be valued over stability, these two options are both fine.

[ergophobe]

To be more constructive...

If you decide that you want this to be as solid as possible, you could benchmark the following two alternatives and see which is better:

option 1

$keys=array_merge(array_keys($a),array_keys($b));

$new=array();
foreach($keys as $k=>$v){
$new[$k]=0;
}
foreach($a as $k=>$v){
$new[$k]+=$v;
}
foreach($b as $k=>$v){
$new[$k]+=$v;
}
return $new;
}

**** option 2 (I assume this will be slower).

function add_array_values($a,$b){

$keys=array_merge(array_keys($a),array_keys($b));

foreach ($keys as $key) {
$x = (isset($a[$key]))? $a[$key] : 0;
$y = (isset($b[$key]))? $b[$key] : 0;
$c[$key]=$x + $y;
}

return $c;

}

Incidentally, to combine this with another thread [webmasterworld.com], to get the difference, you just do the same, but instead of

$c[$key]=$x + $y;

Use

$c[$key]= abs($x - $y);