Generating permutations from arrays - php

Question

I tried to port a function that generates permutations from this answer to PHP and I came out with the following:
function recurse($s, $arrs, $k) {
if ($k === count($arrs)) {
echo $s.' ';
} else {
foreach ($arrs[$k] as $o) {
recurse($s.$o, $arrs, $k + 1);
}
}
}
which gives me the correct output 137 138 147 148 237 238 247 248
Now I want to have the output as an array and not as a string, but after the edit for some reason I get wrong results:
function generatePermutations($s, $arrs, $k) {
if ($k === count($arrs)) {
print_r($s);
} else {
foreach ($arrs[$k] as $o) {
$s[] = $o;
generatePermutations($s, $arrs, $k + 1);
}
}
}
Output:
Array
(
[0] => 1
[1] => 3
[2] => 7
)
Array
(
[0] => 1
[1] => 3
[2] => 7
[3] => 8
)
Array
(
[0] => 1
[1] => 3
[2] => 4
[3] => 7
)
Array
(
[0] => 1
[1] => 3
[2] => 4
[3] => 7
[4] => 8
)
Array
(
[0] => 1
[1] => 2
[2] => 3
[3] => 7
)
Array
(
[0] => 1
[1] => 2
[2] => 3
[3] => 7
[4] => 8
)
Array
(
[0] => 1
[1] => 2
[2] => 3
[3] => 4
[4] => 7
)
Array
(
[0] => 1
[1] => 2
[2] => 3
[3] => 4
[4] => 7
[5] => 8
)
This is the input for both functions
$in = array( array(1, 2), array(3, 4), array(7, 8) );
recurse("", $in, 0);
generatePermutations(array(), $in, 0);
What did I do wrong?

Answer 1

if you use $s[] = $o; in your recursive function, it will slice your array for every digit.
You can add a "result" parameter, which will hold your resultset as an array.
Notice the & sign. Without it, the parameter is not "writable", only "readable".
$in = array( array(1, 2), array(3, 4), array(7, 8) );
$result = array();
generatePermutations("", $in, 0, $result);
print_r($result);
function generatePermutations($s, $arrs, $k, &$result) {
if ($k === count($arrs)) {
$result[] = $s;
} else {
foreach ($arrs[$k] as $o) {
generatePermutations($s.$o, $arrs, $k + 1, $result);
}
}
}
Output :
Array
(
[0] => 137
[1] => 138
[2] => 147
[3] => 148
[4] => 237
[5] => 238
[6] => 247
[7] => 248
)
Edit : Modification to get Robotex's expected output :
$in = array( array(1, 2), array(3, 4), array(7, 8) );
$result = array();
$s = array();
generatePermutations($s, $in, 0, $result);
print_r($result);
function generatePermutations(&$s, $arrs, $k, &$result) {
if ($k === count($arrs)) {
array_push($result, $s);
} else {
foreach ($arrs[$k] as $o) {
array_push($s, $o);
generatePermutations($s, $arrs, $k + 1, $result);
array_pop($s);
}
}
}
Output :
Array
(
[0] => Array
(
[0] => 1
[1] => 3
[2] => 7
)
[1] => Array
(
[0] => 1
[1] => 3
[2] => 8
)
[2] => Array
(
[0] => 1
[1] => 4
[2] => 7
)
[3] => Array
(
[0] => 1
[1] => 4
[2] => 8
)
[4] => Array
(
[0] => 2
[1] => 3
[2] => 7
)
[5] => Array
(
[0] => 2
[1] => 3
[2] => 8
)
[6] => Array
(
[0] => 2
[1] => 4
[2] => 7
)
[7] => Array
(
[0] => 2
[1] => 4
[2] => 8
)
)