Add Chapter 9 exercises

thinkcspy/Chapter-9-Exercises/Python-files/Problem21.py

@@ -0,0 +1,32 @@
+# Runestone.Academy thinkcspy course
+# Chapter 9
+# Problem 21
+
+import string
+
+def rot13(mess):
+    # Your code here
+    alpha = string.ascii_lowercase
+    newStr = ""
+    pos = 0
+    mess = mess.lower()
+    
+    for char in mess:
+        if char in alpha:
+            pos = alpha.index(char)
+            
+            if pos <= 12:
+                pos += 13
+            else:
+                pos = pos % 13
+           
+            newStr += alpha[pos]
+            
+        else:
+            newStr += char    # no rules apply so keep the character
+        
+    return newStr
+
+print(rot13('abcde'))
+print(rot13('nopqr'))
+print(rot13(rot13('Since rot13 is symmetric you should see this message')))