lisp problem 26/99

regtests/099-lisp-problems.lisp

@@ -442,16 +442,28 @@
 ;; ((A B C) (A B D) (A B E) ... )
 
 (defun combination (n l)
-  (let ((len (length l))
-        acc newlist)
-    (dotimes (i (- len (1- n)))
-      (let* ((rest (nthcdr i l))
-             ((resta . restd) rest))
-        (dolist (combi (combination (1- n) restd))
-          (push (cons resta combi) acc))))
-    acc))
-                    
-      
-    
+  "Return all combinations of N elements of list L, leftmost first.
+If N ist smaller than the length of L, return L."
+  (let ((len (length l)))
+    (if (<= len n)
+        (list l)
+      (if (zerop n)
+          (list nil)
+        (let (((first . rest) l))
+          (nconc (map (lambda (lrest) (cons first lrest))
+                      (combination (1- n) rest))
+                 (combination n rest)))))))
+
+(test-is "lisp problem 26/99" (combination 3 '(a b c d e f))
+         '((a b c) (a b d) (a b e) (a b f)
+           (a c d) (a c e) (a c f)
+           (a d e) (a d f)
+           (a e f)
+           (b c d) (b c e) (b c f)
+           (b d e) (b d f)
+           (b e f)
+           (c d e) (c d f)
+           (c e f)
+           (d e f)))
 
 (done-testing)