replace a foreach loop with a functional programming approach - php

Question

Below is an example of a function that adds a constant number to the keys of an array -
function addConstantToArrayKeys($seed_array, $constant)
{
foreach($seed_array as $key => $value){
$a[$key + $constant] = $value;
}
return $a;
}
$basearray = [1 => 17, 2 => 24, 3 => 12];
$test = addConstantToArrayKeys($basearray, 19);
The result of this example is:
Array
(
[1] => 17
[2] => 24
[3] => 12
)
Array
(
[20] => 17
[21] => 24
[22] => 12
)
It works as desired but can this method be rewritten somehow using a functional programming approach that can achieve the same result?
I have tried this:
function addConstantToArrayKeys($seed_array, $constant)
{
return array_map(function($key, $element) use($constant)
{return $a[$key + $constant] = $element;},
array_keys($seed_array), array_values($seed_array));
}
But it does not handle the keys as desired. Here is the output -
Array
(
[1] => 17
[2] => 24
[3] => 12
)
Array
(
[0] => 17
[1] => 24
[2] => 12
)
Edit -
Thanks to Progrock's answer, this worked -
function addConstantToArrayKeys($seed_array, $constant)
{
return array_combine(array_map(
function($key) use($constant) {
return $key + $constant;
}, array_keys($seed_array)), $seed_array);
}

Answer 1

May be its too late but this should work for you as you wanted the functional approach.
function addConstantToArrayKeys($basearray, $constant){
$arrayReindexed = [];
array_walk($basearray,function($v,$k) use (&$arrayReindexed, $constant) {
$key = $k;
$k = $key + $constant;
$arrayReindexed[$k] = $v;
});
return $arrayReindexed;
}
$basearray = [1 => 17, 2 => 24, 3 => 12];
$test = addConstantToArrayKeys($basearray, 19);
print '<pre>';
print_r($test);
print '</pre>';
Output:
Array (
[20] => 17
[21] => 24
[22] => 12
)

Answer 2

You could use something unreadable like this. It does not mutate existing array keys.
<?php
$nums = [
1 => 17,
2 => 24,
3 => 12
];
$out = array_combine(array_map(
function($int) {
return $int + 10;
}, array_keys($nums)), $nums);
var_export($out);
Output:
array (
11 => 17,
12 => 24,
13 => 12,
)
Your first example is perfectly functional and readable, but for the naming.
<?php
$nums = [
1 => 17,
2 => 24,
3 => 12
];
$add_num_to_keys = function (array $in, $num) {
$out = [];
foreach($in as $k => $v)
$out[$k + $num] = $v;
return $out;
};
$out = $add_num_to_keys($nums, 10);
var_export($out);
Output:
array (
11 => 17,
12 => 24,
13 => 12,
)
You can't do:
foreach($in as &$k => $v)
$k += 10;
As the key element can't be a reference.