add cypher exercise

Author: Kristian Rother

data_structures/strings.md

@@ -0,0 +1,132 @@
+Cesar Cypher
+============
+
+.. image:: enigma.jpg
+
+Photo by `Christian Lendl on unsplash.com <https://unsplash.com/@dchris?utm_content=creditCopyText&utm_medium=referral&utm_source=unsplash>`__
+
+In this chapter you will:
+-------------------------
+
+======= ====================================
+area    topic
+======= ====================================
+🚀      encrypt and decrypt text
+⚙       index the positions of a string
+⚙       loop over a string
+💡      use the ``find`` method of the ``string`` data type
+🔀      build a longer string by concatenation
+🔀      use the same index for two strings
+🐞      fix index errors
+======= ====================================
+
+
+Exercise 1: Cesar Cypher
+------------------------
+
+Execute the following code calculating a **Cesar Cypher**:
+
+.. code:: python3
+
+   plain = "ABCDEFGHIJKLMNOPQRSTUVWXYZ "
+   encrypted = "DEFGHIJKLMNOPQRSTUVWXYZABC "
+
+   for i in range(26):
+       print(plain[i], " -> ", encrypted[i])
+
+Explain what the code does.
+
+
+Exercise 2: String methods
+--------------------------
+
+Find out what the expressions do to the string in the middle.
+
+.. figure:: strings.png
+
+Exercise 3: Loop over a string
+------------------------------
+
+The following program should print the position in the alphabet of every character.
+Complete program by inserting ``char``, ``message``, ``plain`` and ``position``:
+
+.. code:: python3
+
+   plain = "ABCDEFGHIJKLMNOPQRSTUVWXYZ "
+   message = "MY SECRET MESSAGE"
+
+   for char in ___:
+       position = ___.find(___)
+       print(char, "is in position", ___)
+
+Why is the space at the end of ``plain`` important?
+
+Exercise 4: String concatenation
+--------------------------------
+
+Explain the following code:
+
+.. code:: python3
+
+   encrypted = "DEFGHIJKLMNOPQRSTUVWXYZABC "
+
+   s = ""
+   s += encrypted[4]
+   s += encrypted[1]
+   s += encrypted[8]
+   s += encrypted[8]
+   s += encrypted[11]
+   print(s)
+
+Exercise 5: Encryption
+----------------------
+
+Write a program that:
+
+1. defines a plain and encrypted alphabet as a 26-character string
+2. reads a message from the keyboard
+3. defines an empty result string
+3. goes through each character of the message
+4. finds the position in the plain text alphabet
+5. looks up that position in the encrypted alphabet
+6. adds the encrypted character to the result string
+7. when all characters have been processed, output the result
+
+.. hint::
+
+   After each step, you should be able to run the program
+   and see what it already does.
+
+Exercise 6
+----------
+
+Explain why the following code does the same as in exercise 1:
+
+.. code:: python3
+
+   plain = "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
+   for i in range(26):
+       print(plain[i], " -> ", plain[(i + 3) % 26])
+
+Can you use it to make your encryption program shorter?
+
+.. hint::
+
+   If you are not sure what happens, print the value of ``i`` inside the loop.
+
+Exercise 7
+----------
+
+Also write a program for **decryption**.
+
+.. hint::
+
+   How could you make sure that the input string is in upper case?
+
+Reflection questions
+--------------------
+
+-  what happens when a string is followed by square brackets?
+-  describe two ways to loop over the characters of a string?
+-  what does the ``str.find()`` method do?
+-  how can you add characters to a string?

first_steps/cypher.rst

@@ -35,6 +35,7 @@ First Steps
    first_steps/for.rst
    first_steps/guess_the_number.rst
    first_steps/slideshow.rst
+   data_structures/cypher.rst
 
 --------------
 
@@ -44,10 +45,8 @@ Data Structures
 .. toctree:: 
    :maxdepth: 1
 
-   data_structures/strings.md
    data_structures/lists.md
    data_structures/dictionaries.md
-   data_structures/tables.md
 
 --------------
 
@@ -57,6 +56,7 @@ Tabular Data
 .. toctree:: 
    :maxdepth: 1
 
+   data_structures/tables.md
    tabular_data/readfile.md
    tabular_data/parsing.md
    tabular_data/writing_files.md