Merging array in recursive function using php? - php

Question

After one week searching and converting many algorithm from other language into php to make an array that contain "combination k from n". I'm stuck.
please help me.
This is my code (using php):
function comb($item,$arr,$out, $start, $n, $k, $maxk) {
if ($k > $maxk) {
foreach($arr as $ar){
echo "$ar";
echo "<br/>";
}
return;
}
for ($i=$start; $i<=$n; $i++) {
$arr[$k] = $item[$i-1];
comb($isi, $arr, $out, $i+1, $n, $k+1, $maxk);
}
}
$team = array("A","B","C","D");
$ar = array();
$o = array();
comb($team,$ar,$o,1,4,1,2);
Recursive algorithm above is really confuse me. The code above was successful to form the combination but I cannot merge them into one array because of its recursive characteristics. I just want to make an array that contain the result of combination of 2 from 4 items. Like this (see below)
Array (
[0] => Array (
[1] => A
[2] => B
)
[1] => Array (
[1] => A
[2] => C
)
[2] => Array (
[1] => A
[2] => D
)
[3] => Array (
[1] => B
[2] => C
)
[4] => Array (
[1] => B
[2] => D
)
[5] => Array (
[1] => C
[2] => D
)
)
I know I still far from the answer. But please guide me, to reach that answer. Perhaps you know the other technique, it doesn't matter. If your code works, I will use it. No matter what the technique you've used. Any ideas would be gratefully appreciated,Thank you..!

Answer 1

An (I think) simpler recursive implementation is:
<?php
/* Given the array $A, returns the array of $k-subsets
of $A in lexicographical order. */
function k_lex_subset($A,$k) {
if (($k <= 0) or (count($A) < $k)) { return array(); }
else if ($k <= 1) { return array_chunk($A,1); }
else {
$v = array_shift($A);
$AwA = k_lex_subset($A,$k-1);
foreach($AwA as &$vp) {
array_unshift($vp,$v);
}
$AwoA = k_lex_subset($A,$k);
$resultArrs = array_merge($AwA, $AwoA);
return($resultArrs);
}
}
$team = array("A","B","C","D");
print_r(k_lex_subset($team,2));
?>
which returns
Array
(
[0] => Array
(
[0] => A
[1] => B
)
[1] => Array
(
[0] => A
[1] => C
)
[2] => Array
(
[0] => A
[1] => D
)
[3] => Array
(
[0] => B
[1] => C
)
[4] => Array
(
[0] => B
[1] => D
)
[5] => Array
(
[0] => C
[1] => D
)
)
and will work for any size array, and any $k.
The term you are looking for is (lexicographical) k-subset enumeration where $k is 2 in this specific case.
Explanation
The idea is very simple. Assume we have (for example) a set {A,B,C,D}. We want to start with all sets with A in, and so we consider subsets of size 1-less coming from {B,C,D} and append A to them yielding
{{A,B}, {A,C}, {A,D}}
and then we consider all subsets of size 2 without A in
{{B,C}, {B,D}, {C,D}}
and then we just merge the two. It is hopefully easy to see how, in general, this yields a nice recursive strategy for constructing the k-subsets of a set (instead of just k=2).
Reference
A fantastic reference on this sort of thing is Vol 4 Fasicle 3 of Knuth's The Art of Computing Programming.

Answer 2

This should do the trick to reach the array you've described above:
<?php
$array = array("A","B","C","D");
function transformArray( $array ) {
$returnArray = array();
for( $i=0; $i < count($array); $i++ ) {
for( $j=$i+1; $j < count($array); $j++ ) {
$returnArray[] = array( $array[$i], $array[$j] );
}
}
return $returnArray;
}
print_r(transformArray($array));
?>

Answer 3

Not exactly sure on the necessity of start, n, and k, but this should get you the expected output. If you provide some more details on why those counters would be necessary, we can get you a more thorough answer.
function comb($itemArray, $start, $n, $k, $maxk) {
//if ($k > $maxk) return;
$outputArray = array();
foreach($itemArray AS $index => $firstChar) {
for($i = $index+1; $i<count($itemArray); $i++) {
$secondChar = $itemArray[$i];
$outputArray[] = array($firstChar, $secondChar);
}
}
return $outputArray;
}
$teamArray = array("A","B","C","D");
$resultArray = comb($teamArray,1,4,1,2);
ppr($resultArray);
function ppr($variable) {
echo '<pre>';
print_r($variable);
echo '</pre>';
}

Answer 4

function comb($a, $len){
if ($len > count($a))return array();
$out = array();
if ($len==1) {
foreach ($a as $v) $out[] = array($v);
return $out;
}
$len--;
while (count($a) > $len) {
$b = array_shift($a);
$c = comb($a, $len);
foreach ($c as $v){
array_unshift($v, $b);
$out[] = $v;
}
}
return $out;
}
$test = array('a','b','c','d');
$a = comb($test,2);
print_r($a);
would give you:
Array(
[0] => Array
(
[0] => a
[1] => b
)
[1] => Array
(
[0] => a
[1] => c
)
[2] => Array
(
[0] => a
[1] => d
)
[3] => Array
(
[0] => b
[1] => c
)
[4] => Array
(
[0] => b
[1] => d
)
[5] => Array
(
[0] => c
[1] => d
)
)

Answer 5

Why are you doing
echo "ar"
instead of
echo $ar
Also are you looking for permutation or combination? Here is a site with very nice algorithms for both. The implementation is in java but it's very clear. so converting to php won't be difficult: http://geekviewpoint.com/Numbers_functions_in_java/

Answer 6

If you want a pure clean recursive way use this:
function comb($item, $n) {
return comb_rec($item, array(), $n);
}
function comb_rec($items, $c, $n) {
if (count($c) == $n) {
return array($c);
}else{
if (count($items) == 0) {
return $items;
}else{
$list = $items;
$head = array_shift($list);
$tail = $list;
$current = $c;
array_push($current,$head);
return array_merge(comb_rec($tail, $current, $n),comb_rec($tail, $c, $n));
}
}
}
$team = array("A","B","C","D");
$all = comb($team,2);
print_r($all);