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Kristian Rother
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add crypto task to builtin functions
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‎first_steps/builtin_functions.rst‎

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Substitution Cipher
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===================
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In this chapter you learn:
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--------------------------
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==== ==============================================
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area topic
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==== ==============================================
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🚀 encrypt text with a substitution cipher
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⚙ convert strings to lists and back
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⚙ use a random seed
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💡 use the ``random`` module
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💡 use the ``string`` module
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💡 use the ``zip`` function
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🔀 use the ``dict(zip())`` pattern
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🐞 fix IndexErrors
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==== ==============================================
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Exercise 1: Random string
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-------------------------
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The following code creates a random chiffre that we will use for encryption.
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Insert ``alphabet``, ``chars``, ``join`` and ``random`` into the code:
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.. code:: python3
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import random
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alphabet = "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
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___ = list(___)
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___.shuffle(chars)
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chiffre = "".___(chars)
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print(chiffre)
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Run the program a couple of times.
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Exercise 2: Random seed
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-----------------------
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Add the following line after the import:
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.. code:: python3
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random.seed(77)
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Run the program a couple of times.
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What changes?
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Also try running the program with different numbers.
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Exercise 3: Chiffre indices
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---------------------------
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Output each character from the chiffre and its index
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Simplify the following code using the function ``enumerate()``:
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.. code:: python3
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i = 0
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for char in chiffre:
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print(i, char)
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i += 1
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Exercise 4: Lookup dictionary
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-----------------------------
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Create a dictionary that maps unencrypted to encrypted characters:
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.. code:: python3
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lookup = {}
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i = 0
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while i < len(alphabet):
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char = alphabet[i]
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lookup[char] = chiffre[i]
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i += 1
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print(lookup)
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Execute the program slowly on paper to make sure you understand the index operations.
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Exercise 5: Zip
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---------------
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Use the following code to make the program above simpler:
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.. code:: python3
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for char, chif in zip(alphabet, chiffre)
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print(char, chif)
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Once you understand what happens, also try:
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.. code:: python3
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lookup = dict(zip(alphabet, chiffer))
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Exercise 6: Encrypt
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-------------------
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Use the lookup dictionary to encrypt a message.
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Use the following lines as a starting point:
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.. code:: python3
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message = ...
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result = ""
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for char in message:
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result += ...
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-
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Exercise 7: Debugging
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---------------------
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What happens when you enter lowercase letters or special characters in the message?
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Try the expression:
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.. code:: python3
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print(lookup.get(";", "X"))
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Fix the problem.
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Exercise 7: Decrypt
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-------------------
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Decrypt a message when all you know is the encrypted message and the random seed.
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Reflection Questions
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--------------------
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- what does the ``enumerate()`` function do?
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- what does the ``zip()`` function do?
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- is a substitution cipher rather safe or unsafe?
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- what is **symmetric encryption**?
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.. seealso::
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**Further Reading**
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- `Kerckhoffs Principle <https://en.wikipedia.org/wiki/Kerckhoffs%27s_principle>`__

‎first_steps/dictionaries.rst‎

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print("You have reached your destination")
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Exercise 6: Zip
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---------------
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Simplify the following code using the function ``zip()``:
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.. code:: python3
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fruits = ["apple", "banana", "orange", "cherries"]
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prices = [0.5, 1.0, 1.5, 3.0]
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table = []
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i = 0
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while i < len(fruits):
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row = (fruits[i], prices[i])
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table.append(row)
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i += 1
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print(table)
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Try the expression:
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.. code:: python3
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for a, b in zip(fruits, prices):
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...
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Reflection Questions
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--------------------
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‎first_steps/indexing.rst‎

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numbers[2:-2]
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numbers[::2]
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Exercise 3: Ranges
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------------------
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Use ``list(range())`` to create the following lists:
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.. code:: python3
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[4, 7, 9, 12, 15]
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[10, 20, 30, 40, 50, 60]
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[33, 32, 31, 30]
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Exercise 3: Decypher
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Exercise 4: Decypher
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The following text contains an encrypted word:
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Print every second character, starting with the 2nds.
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Exercise 4: Slicing puzzle
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Exercise 5: Slicing puzzle
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--------------------------
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Use the expressions to modify the list as indicated. Use each expression
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:alt: list funcs exercise1
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Exercise 5: Blocks
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Exercise 6: Blocks
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------------------
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The following code is creating the first two blocks for a `transposition cipher <https://en.wikipedia.org/wiki/Transposition_cipher>`__ .
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encrypted = block1 + block2 + block3 + block4
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Exercise 6: Transposition Cipher
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Exercise 7: Transposition Cipher
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Complete the program that encrypts a text using a transposition cipher:
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for start in range(4):
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___
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Exercise 7: Decrypt
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Exercise 8: Decrypt
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Write a program to decrypt an encrypted message again.
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Exercise 8: Encryption Key
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Exercise 9: Encryption Key
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Use an encryption key like ``2031`` that specifies a new order for the blocks.

‎first_steps/statistics.rst‎

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⚙ implement a function
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⚙ call a function you defined
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💡 use the ``math`` module
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💡 use the ``sum`` function
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🔀 use a recursive function
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==== ==============================================
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Don't forget about the ``return`` statement.
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Exercise 3: Standard Deviation
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Exercise 3: Shortcut
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--------------------
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Simplify the mean function using the function ``sum()``.
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Use your own or the builtin ``sum()`` function.
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Exercise 4: Standard Deviation
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The following program calculates the standard deviation from a list of
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print(f"Standard Deviation: {stdev:8.2f}")
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Exercise 4: Optional Parameters
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Exercise 5: Optional Parameters
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-------------------------------
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Explain the program:
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print(add(b=4))
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Exercise 5: Recursion
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Exercise 6: Recursion
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Explain the code:

‎index.rst‎

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first_steps/dictionaries.md
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first_steps/indexing.rst
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first_steps/nested_lists.rst
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first_steps/builtin_functions.rst
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first_steps/slideshow.rst
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first_steps/babynames.rst
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--------------
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Program Structures
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------------------
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.. toctree::
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:maxdepth: 1
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structure/functions.md
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first_steps/statistics.rst
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debugging/README.rst
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structure/builtin_functions.md
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structure/modules.md
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structure/refactoring.rst
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refactoring/refactoring.rst
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challenges/text_adventure.rst
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Challenges
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==========
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Coding Challenges
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-----------------
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.. toctree::
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:maxdepth: 1
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challenges/ctree.rst
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challenges/pyramid.rst
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challenges/fizzbuzz.rst
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challenges/tictac.rst
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challenges/quiz.rst
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challenges/checker.rst
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challenges/baby_name_generator.rst
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challenges/palindrome.rst
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challenges/anagrams.rst
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challenges/spiral.rst
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challenges/roman.rst
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challenges/encrypt.rst
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challenges/tictac.rst
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challenges/minesweeper.rst
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challenges/gcd.rst
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challenges/fibonacci.rst
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challenges/querprodukt.rst
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challenges/birthdays.rst
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challenges/seven_peaks.rst
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challenges/misty_mountains.rst
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challenges/count_words.rst
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challenges/postcard.rst
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challenges/text_adventure.rst
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challenges/spiral.rst
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reference/while.rst
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reference/builtin_functions.rst
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reference/functions.rst
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reference/introspection.rst
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Appendix

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