new tasks

Task/Caesar_cipher/Racket/caesar_cipher-2.rkt

@@ -0,0 +1,5 @@
+> (define s (encrypt "The five boxing wizards jump quickly."))
+> s
+"Uif gjwf cpyjoh xjabset kvnq rvjdlmz."
+> (decrypt s)
+"The five boxing wizards jump quickly."

Task/Caesar_cipher/Racket/caesar_cipher.rkt

@@ -0,0 +1,19 @@
+#lang racket
+
+(define A (char->integer #\A))
+(define Z (char->integer #\Z))
+(define a (char->integer #\a))
+(define z (char->integer #\z))
+
+(define (rotate c n)
+  (define cnum (char->integer c))
+  (define (shift base) (integer->char (+ base (modulo (+ n (- cnum base)) 26))))
+  (cond [(<= A cnum Z) (shift A)]
+        [(<= a cnum z) (shift a)]
+        [else c]))
+
+(define (caesar s n)
+  (list->string (for/list ([c (in-string s)]) (rotate c n))))
+
+(define (encrypt s) (caesar s 1))
+(define (decrypt s) (caesar s -1))