Data commit

Task/Letter-frequency/ACL2/letter-frequency.acl2

@@ -0,0 +1,16 @@
+(defun increment-alist (tbl key)
+   (cond ((endp tbl) (list (cons key 1)))
+         ((eql (car (first tbl)) key)
+          (cons (cons key (1+ (cdr (first tbl))))
+                (rest tbl)))
+         (t (cons (first tbl)
+                  (increment-alist (rest tbl) key)))))
+
+(defun freq-table (xs)
+   (if (endp xs)
+       nil
+       (increment-alist (freq-table (rest xs))
+                        (first xs))))
+
+(defun letter-freq (str)
+   (freq-table (coerce str 'list)))