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.

ThinkPython: The way of the program

1. With +, the strings get added together, and with * we get an error.
2. With + we get an error, and with * the string is repeated multiple times.
3. Python calculates the result and echoes it.

Using if, else and elif

1. Just ask the word and print word * 1000.

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

2. Add a space to the word before printing.

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

   We 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. If we really want to get rid of the space, we 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)

4. You can compare the word against an empty string ("" or '') to check if it's empty. 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.")

5. 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!
   >>> 
   

Loops

1. For loop over the users, each user is a list that contains a username and a password.

   users = [
       ['foo', 'biz'],
       ['bar', 'baz'],
   ]
   
   username = input("Username: ")
   password = input("Password: ")
   
   logged_in = False
   for user in users:
       if username == user[0] and password == user[1]:
           logged_in = True
           break
   
   if logged_in:
       print("Welcome, " + username + "!")
   else:
       print("Wrong username or password.")

2. Just put the whole thing in a while True:. Remember that a break will always break the innermost loop it's in.

   users = [
       ['foo', 'biz'],
       ['bar', 'baz'],
   ]
   
   while True:
       username = input("Username: ")
       password = input("Password: ")
   
       logged_in = False
       for user in users:
           if username == user[0] and password == user[1]:
               logged_in = True
               break   # break the for loop
   
       if logged_in:
           print("Welcome, " + username + "!")
           break   # break the while loop
       else:
           print("Wrong username or password.")

3. Add a for loop that works as an attempt counter.

   users = [
       ['foo', 'biz'],
       ['bar', 'baz'],
   ]
   
   for attempts_left in [3, 2, 1, 0]:
       if attempts_left == 0:
           print("No attempts left!")
           break   # break the outer for loop
   
       print(attempts_left, "attempts left.")
       username = input("Username: ")
       password = input("Password: ")
   
       logged_in = False
       for user in users:
           if username == user[0] and password == user[1]:
               logged_in = True
               break   # break the inner for loop
   
       if logged_in:
           print("Welcome, " + username + "!")
           break   # break the outer for loop
       else:
           print("Wrong username or password.")

Defining functions

1. Use -value to get tha negative in the abs function, and for loops in the other two functions.

   def my_abs(value):
       if value < 0:
           return -value
       # actually, this else is useless because returning ends the
       # function anyway
       else:
           value
   
   def my_any(a_list):  # don't call this "list", see summary in the Lists chapter
       for item in a_list:
           if item:
               return True    # ends the function
       return True
   
   def my_all(a_list):
       for item in a_list:
           if not item:
               return False
       return True

2. Like this:

   def box(message, character='*'):
       number_of_characters = len(message) + 4
       print(character * number_of_characters)
       print(message)
       print(character * number_of_characters)

You may use this tutorial freely at your own risk. See LICENSE.

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