I'm working out an algorithm to get permutations like
123
132
213
231
312
321
I'm doing it using nested foreach loops.
for (..) {
for(..) {
for(..) {
echo $i . $j . $k . "<br />";
}
}
}
Problem is those # of nested loops are optimized for 3-spot permutations. How can I could I d开发者_StackOverflow中文版ynamically set the number of nested for loops to generate 4-letter or 5-letter permutations?
Yes, just do it recursively.
function permuteThis($items, $permutations = array()) {
if(!is_array($items))
$items = str_split($items);
$numItems = sizeof($items);
if($numItems > 0) {
$cnt = $numItems - 1;
for($i = $cnt; $i >= 0; --$i) {
$newItems = $items;
$newPerms = $permutations;
list($tmp) = array_splice($newItems, $i, 1);
array_unshift($newPerms, $tmp);
permuteThis($newItems, $newPerms);
}
} else {
echo join('', $permutations) . "\n";
}
}
$number = 123;
permuteThis($number);
Recursive method is the way to go. But you can also use eval
function like:
$loop = iteration('i',10) . iteration('j',10). iteration('k',10). iteration('l',10);
$loop .= "print \"\$i \$j \$k \$l\\n\";";
// $loop now has: for($i=0;$i<10;$i++)for($j=0;$j<10;$j++)for($k=0;$k<10;$k++)for($l=0;$l<10;$l++) print "$i $j $k $l\n";
eval($loop);
function iteration($var,$limit) {
return "for(\${$var}=0;\${$var}<$limit;\${$var}++)";
}
No, no, please do NOT use recursion for generating permutations. Use the algorithm outlined here, see also php implementation
The answer is: use a recursive function.
You could use a recursive function. Take a look at this post.
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