Re: power set (Re: Modified keypad keys)

[Juri Linkov]

> As an alternative to the recursive definition, I've also been using
> this iterative bitwise version (which I translated into Elisp from the
> C program at http://rosettacode.org/wiki/Power_set#C):

Often a C version doesn't translate nicely to Lisp when translated
literally.  If you prefer an iterative version instead of recursive,
you could look at the Common Lisp section of
http://rosettacode.org/wiki/Power_set#Common_Lisp
and find the following short iterative version:

(defun powerset (xs)
  (loop for i below (expt 2 (length xs)) collect
       (loop for j below i for x in xs if (logbitp j i) collect x)))

There is no `logbitp' function in Emacs, but you could use a replacement
like this:

(defun logbitp (j i) (> (logand i (lsh 1 j)) 0))