These are answers for exercises in the chapters. In programming, there's always more than one way to do things, so if your solution wasn't exactly like mine it's not necessarily wrong. Some Python users say that there should be only one right way, but that goal will never be fully reached.

Chapter 1

1. 18996.20 €

   >>> 49.95 + 200
   249.95
   >>> 100 * 249.95
   24995.0
   >>> 1 - (24 / 100)
   0.76
   >>> 24995.0 * 0.76
   18996.2
   >>>

   All in one line:

   >>> 100 * (49.95 + 200) * (1 - (24 / 100))
   18996.2
   >>>

2. About 523

   >>> (4 / 3) * 3.14 * (5 * 5 * 5)
   523.3333333333334
   >>> 4 / 3 * 3.14 * 5 * 5 * 5      # parentheses aren't needed
   523.3333333333334
   >>>

   More advanced way:

   >>> from math import pi
   >>> 4 / 3 * pi * 5 ** 3
   523.5987755982989
   >>>

   Spaces don't effect the calculating order, but you can use them to make the order easier to see:

   >>> 4/3 * pi * 5**3
   523.5987755982989
   >>>

Chapter 3

1. Content of the file:

   word = input("Enter a word: ")
   print(word * 1000)

2. Content of the file:

   word = input("Enter a word: ")
   word += " "
   print(word * 1000)

   You can also add the space right away on the first line:

   word = input("Enter a word: ") + " "
   print(word * 1000)

   Of course, there are 999 spaces between 1000 words and this will print 1000 spaces instead, so there will be a useless space at the end, but it doesn't matter. To get rid of the space, you can do something like this instead:

   no_space = input("Enter a word: ")
   yes_space = no_space + " "
   print(yes_space * 999 + no_space)

3. Like this:

   first = input("Enter a word: ")
   second = input("Enter another word: ")
   words = first + " " + second + " "
   print(words * 1000)

Chapter 4

1. You can compare the word against an empty string ("" or ''). In this example, the password is "secret".

   word = input("Enter your password: ")
   
   if word == "secret":
       print("Welcome!")
   elif word == "":
       print("You didn't enter anything.")
   else:
       print("Access denied.")

2. Simply check the username first, then the password in indented blocks of code.

   username = input("Enter your username: ")
   password = input("Enter your password: ")
   
   if username == "foo":
       if password == "biz":
           print("Welcome foo!")
       else:
           print("Wrong password!")
   elif username == "bar":
       if password == "baz":
           print("Welcome bar!")
       else:
           print("Wrong password!")
   else:
       print("Wrong username.")

   Example output:

    >>> ================================ RESTART ================================
    >>>
    Enter your username: foo
    Enter your password: biz
    Welcome foo!
    >>> ================================ RESTART ================================
    >>>
    Enter your username: bar
    Enter your password: baz
    Welcome bar!
    >>> ================================ RESTART ================================
    >>>
    Enter your username: spam
    Enter your password: eggs
    Wrong username.
    >>> ================================ RESTART ================================
    >>>
    Enter your username: foo
    Enter your password: lol
    Wrong password!
    >>>
   

3. This is a great chance to use a while loop. In this example, the correct password is "secret".

   running = True
   while running:
       password = input("Enter your password: ")
       if password == "secret":
           print("Welcome!")
           running = False
       else:
           print("Wrong password.")

   Another alternative:

   while input("Enter your password: ") != "secret":
       print("Wrong password.")
   print("Welcome!")

4. One way to do this is to put the inputs directly to if and elif lines. Again, the correct password is "secret".

   if input("Enter your password: (3 attempts left) ") == "secret":
       print("Welcome!")
   elif input("Enter your password: (2 attempts left) ") == "secret":
       print("Welcome!")
   elif input("Enter your password: (last attempt) ") == "secret":
       print("Welcome!")
   else:
       print("Access denied!")

   Example output:

    >>> ================================ RESTART ================================
    >>>
    Enter your password: (3 attempts left) asdf
    Enter your password: (2 attempts left) asdf
    Enter your password: (last attempt) asdf
    Access denied!
    >>> ================================ RESTART ================================
    >>>
    Enter your password: (3 attempts left) asdf
    Enter your password: (2 attempts left) secret
    Welcome!
    >>>
   

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