good_evening good_evening - 3 months ago 8
PHP Question

Php recursion to get all possibilities of strings

Here is my code to get all possibilities:

$seq[1] = 'd';
$seq[2] = 'f';
$seq[3] = 'w';
$seq[4] = 's';

for($i = 1; $i < 5; $i++)
{
$s['length_1'][] = $seq[$i];
$c1++;

for($i2 = $i+1; $i2 < 5; $i2++)
{
$s['length_2'][] = $seq[$i].$seq[$i2];
$last = $seq[$i].$seq[$i2];
$c2++;

for($i3 = $i2+1; $i3 < 5; $i3++)
{
$s['length_3'][] = $last.$seq[$i3];
$last = $last.$seq[$i3];
$c3++;

for($i4 = $i3+1; $i4 < 5; $i4++)
{
$s['length_4'][] = $last.$seq[$i4];
$c4++;
}
}
}
}

for($i = 0; $i < $c1; $i++)
echo $s['length_1'][$i].'<br>';

for($i = 0; $i < $c2; $i++)
echo $s['length_2'][$i].'<br>';

for($i = 0; $i < $c3; $i++)
echo $s['length_3'][$i].'<br>';

for($i = 0; $i < $c4; $i++)
echo $s['length_4'][$i].'<br>';


But if I want to add more, then I will have to add one more loop. So, how can I do it with recursion? I try, I try, but I really can't do it.
Please help and post example as simple as possible.

Thank you.

Answer

Here's a simple algo. Iterate from 1 to 2count(array)-1. On each iteration, if j-th bit in a binary representation of the loop counter is equal to 1, include j-th element in a combination.

As PHP needs to be able to calculate 2count(array) as an integer, this may never exceed PHP_INT_MAX. On a 64-bit PHP installation your array cannot have more than 62 elements, as 262 stays below PHP_INT_MAX while 263 exceeds it.

EDIT: This computes all possible combinations, not permutations (ie, 'abc' = 'cba'). It does so by representing the original array in binary and "counting up" from 0 to the binary representation of the full array, effectively building a list of every possible unique combination.

$a = array('a', 'b', 'c', 'd');

$len  = count($a);
$list = array();

for($i = 1; $i < (1 << $len); $i++) {
    $c = '';
    for($j = 0; $j < $len; $j++)
        if($i & (1 << $j))
            $c .= $a[$j];
    $list[] = $c;
}

print_r($list);
Comments