note 88330 added to function.array-key-exists

In some functions cacheing can be extremely useful, and this is especially true in recursive functions, and all the more so in doubly recursive functions. One way to effect cacheing is to have a static array in the function as shown below. This will typically be useful when a function has one or two arguments and is heavily called. Example: <?php function nk($n, $k) { // n choose k, ensuring integer math if ($k > $n-$k) $k = $n - $k; // (n,k) = (n,n-k) if ($k<=1) return ($k==1) ? $n : !$k; // (n,1) = n; (n,0)=1; (n,-#) = 0 static $aNK = array(); // caching section if ($aNK[$n][$k]) return $aNK[$n][$k];// if answer already computed => done // else compute answer, cache it, and return return ($aNK[$n][$k] = nk($n-1, $k) + nk($n-1, $k-1)); } $n = 20; $k = 9; $nk = nk($n,$k); print "nk($n, $k) = $nk"; ?> For functions of two arguments, using a two dimensional array seems to be faster than combining arguments a la "$n $k", and the form of testing shown above is also faster than array_key_exists. However, to avoid the use of array_key_exists, special consideration must be given to empty values (several prior notes cover this) - the example above works because no cached value can be 0. Csaba Gabor from Vienna ---- Server IP: 192.35.244.50 Probable Submitter: 80.108.23.194 ---- Manual Page -- http://www.php.net/manual/en/function.array-key-exists.php Edit -- https://master.php.net/note/edit/88330 Del: integrated -- https://master.php.net/note/delete/88330/integrated Del: useless -- https://master.php.net/note/delete/88330/useless Del: bad code -- https://master.php.net/note/delete/88330/bad+code Del: spam -- https://master.php.net/note/delete/88330/spam Del: non-english -- https://master.php.net/note/delete/88330/non-english Del: in docs -- https://master.php.net/note/delete/88330/in+docs Del: other reasons-- https://master.php.net/note/delete/88330 Reject -- https://master.php.net/note/reject/88330 Search -- https://master.php.net/manage/user-notes.php