PHP array copy certain keys, built-in functions? Nested loop performance?

Question

I have a PHP array that I\'d like to duplicate but only copy elements from the array whose keys appear in another array.

Here are my arrays:

$data[123] =         


        

Answer 1

Why not load the entire result set into an array, then begin processing with nested loops?

$query_result = mysql_query($my_query) or die(mysql_error());
$query_rows = mysql_num_rows($query_result);
for ($i = 0; $i < $query_rows; $i++)
{
    $row = mysql_fetch_assoc($query_result);
    // 'key' is the name of the column containing the data key (123)
    // 'value' is the name of the column containing the value (aaa)
    $data[$row['key']] = $row['value']; 
}
foreach ($data as $key => $value)
{
    if ( in_array($key, $keys_to_copy)) 
    {
        $copied_data[$key] = $value;
    }
}

Answer 2

I worked it out - I almost had it above.I thought I'd post the answer anyway for completeness. Hope this helps someone out!

array_intersect_key($data, array_flip($keys_to_copy))

Use array_flip() to switch $keys_to_copy so it can be used within array_intersect_keys()

I'll run some tests to compare performance between the manual loop above, to this answer. I would expect the built-in functions to be faster but they might be pretty equal. I know arrays are heavily optimised so I'm sure it will be close.

EDIT:
I have run some benchmarks using PHP CLI to compare the foreach() code in my question with the code in my answer above. The results are quite astounding.
Here's the code I used to benchmark, which I think is valid:

<?php
ini_set('max_execution_time', 0);//NOT NEEDED FOR CLI

// BUILD RANDOM DATA ARRAY
$data = array();
while ( count($data) <= 200000) {
    $data[rand(0, 500000)] = rand(0, 500000);
}
$keys_to_copy = array_rand($data, 100000);

// FOREACH
$timer_start = microtime(TRUE);
foreach ($data as $key => $value) {
    if ( in_array($key, $keys_to_copy)) {
        $copied_data[$key] = $value;
    }
}
echo 'foreach: '.(microtime(TRUE) - $timer_start)."s\r\n";

// BUILT-IN ARRAY FUNCTIONS
$timer_start = microtime(TRUE);
$copied_data = array_intersect_key($data, array_flip($keys_to_copy));
echo 'built-in: '.(microtime(TRUE) - $timer_start)."s\r\n";
?>

And the results...
foreach: 662.217s
array_intersect_key: 0.099s

So it's much faster over loads of array elements to use the PHP array functions rather than foreach. I thought it would be faster but not by that much!