无循环按键对多维数组的值求和 [英] Sum values of multidimensional array by key without loop

问题描述

I have this:

Array (
    [0] => Array ( [f_count] => 1 [uid] => 105 ) 
    [1] => Array ( [f_count] => 0 [uid] => 106 ) 
    [2] => Array ( [f_count] => 2 [uid] => 107 ) 
    [3] => Array ( [f_count] => 0 [uid] => 108 ) 
    [4] => Array ( [f_count] => 1 [uid] => 109 ) 
    [5] => Array ( [f_count] => 0 [uid] => 110 ) 
    [6] => Array ( [f_count] => 3 [uid] => 111 )
)

What I need is: 7", which is the the sum of the f_count column.

I've been trying to figure this out for a couple hours. I thought array_sum() would work, but not with a multidimensional array. So, I've tried figuring out how to isolate the f_counts by unset() or splicing or anything else, but every solution seems to involve a foreach loop. I've messed with array_map, array_walk, and others to no avail. I haven't found a function that works well with multidimensional arrays.

I'm running PHP 5.4.

Can someone please show me how to sum that column without a foreach loop?

If it helps, the f_count values will never be higher than 100, and the uid values will always be greater than 100.

---

Alternatively, if there's a way to run my query differently such that the array is not multidimensional, that would obviously work as well.

$query = "SELECT f_count, uid FROM users WHERE gid=:gid";
...
$array = $stmt->fetchAll();

I'm using PDO.

解决方案

You need to couple it with array_map() to select the f_count column first:

array_sum(array_map(function($item) { 
    return $item['f_count']; 
}, $arr));

Of course, internally, this performs a double loop; it's just that you don't see it inside the code. You could use array_reduce() to get rid of one loop:

array_reduce($arr, function(&$res, $item) {
    return $res + $item['f_count'];
}, 0);

However, if speed is the only interest, foreach remains the fastest:

$sum = 0;
foreach ($arr as $item) {
    $sum += $item['f_count'];
}

This is thanks to the "locality" of the variables that you're using, i.e. there are no function calls used to calculate the final sum.

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