Re: RECURSION PROBLEM

[Jon S Anthony <jsa@synquiry.com>]

Robert Byrne <byrner@db.erau.edu> writes:

> 
> I need to use recursion to display all pairs (combinations) of
> characters from a set of characters read from a file given on the

I note that you say _combinations_, not permutations.  The following
is a simple solution in Lisp.  Translate into Ada and you are done...

(defun combos (letters i)
  (labels ((a-set (cur letters next)
		  (if (= next (length letters)) nil
		    (cons (cons cur (aref letters next))
			  (a-set cur letters (1+ next))))))
	  (if (= i (length letters))
	      nil
	    (append
	     (a-set (aref letters i) letters (1+ i))
	     (combos letters (1+ i))))))

COMBOS
* (combos "abcdef12345" 0)

((#\a . #\b) (#\a . #\c) (#\a . #\d) (#\a . #\e) (#\a . #\f) (#\a . #\1)
 (#\a . #\2) (#\a . #\3) (#\a . #\4) (#\a . #\5) (#\b . #\c) (#\b . #\d)
 (#\b . #\e) (#\b . #\f) (#\b . #\1) (#\b . #\2) (#\b . #\3) (#\b . #\4)
 (#\b . #\5) (#\c . #\d) (#\c . #\e) (#\c . #\f) (#\c . #\1) (#\c . #\2)
 (#\c . #\3) (#\c . #\4) (#\c . #\5) (#\d . #\e) (#\d . #\f) (#\d . #\1)
 (#\d . #\2) (#\d . #\3) (#\d . #\4) (#\d . #\5) (#\e . #\f) (#\e . #\1)
 (#\e . #\2) (#\e . #\3) (#\e . #\4) (#\e . #\5) (#\f . #\1) (#\f . #\2)
 (#\f . #\3) (#\f . #\4) (#\f . #\5) (#\1 . #\2) (#\1 . #\3) (#\1 . #\4)
 (#\1 . #\5) (#\2 . #\3) (#\2 . #\4) (#\2 . #\5) (#\3 . #\4) (#\3 . #\5)
 (#\4 . #\5))
*

/Jon

-- 
Jon Anthony
Synquiry Technologies, Ltd., Belmont, MA 02178, 617.484.3383
"Nightmares - Ha!  The way my life's been going lately,
 Who'd notice?"  -- Londo Mollari