Combinations and permutations: Difference between revisions

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 <lang Lisp>(defun combinations (n k)
+  (cond ((or (< n k) (< k 0) (< n 0)) 0)
-  (let ((num 1)
-        (den 1) )
+	((= k 0) 1)
-    (dotimes (i k (/ num den))
+	(t (do* ((i 1 (1+ i))
+		 (m n (1- m))
-      (setq num (* num (- n i)) den (* den (- k i))) )))
+		 (a m (* a m))
+		 (b i (* b i)))
+	       ((= i k) (/ a b))))))
 (defun permutations (n k)
-  (let ((p 1))
+  (cond ((or (< n k) (< k 0) (< n 0)) 0)
-    (dotimes (i k p)
+	((= k 0) 1)
-      (setq p (* p (- n i))) )))
+	(t (do* ((i 1 (1+ i))
+		 (m n (1- m))
+		 (a m (* a m)))
+	       ((= i k) a)))))
 </lang>