add tray hunner stuff and fix stupid mistakes in answers.md

Akuli · Akuli · commit 60154fd668dc · 2016-10-01T22:08:57.000+03:00
diff --git a/README.md b/README.md
@@ -39,9 +39,10 @@ with other tutorials.
 8. [ThinkPython: Lists](lists.md)
 9. [Handy stuff with strings](handy-stuff-strings.md)
 10. [Loops](loops.md)
-11. [Files](files.md)
-12. [Defining functions](defining-functions.md)
-13. [Modules](modules.md)
+11. [Trey Hunner: zip and enumerate](trey-hunner-zip-and-enumerate.md)
+12. [Files](files.md)
+13. [Defining functions](defining-functions.md)
+14. [Modules](modules.md)
 
 Parts of this tutorial that aren't ready to be added to the rest of it
 yet:
diff --git a/answers.md b/answers.md
@@ -195,10 +195,77 @@ These are answers for exercises in the chapters. In programming, there's always
             print("Wrong username or password.")
     ```
 
+## Trey Hunner: zip and enumerate
+
+1. Read some lines with `input` and then enumerate it.
+
+    ```py
+    print("Enter something, and press Enter without typing anything",
+          "when you're done.")
+
+    lines = []
+    while True:
+        line = input('>')
+        if line == '':
+            break
+        lines.append(line)
+
+    for index, line in enumerate(lines, start=1):
+        print("Line", index, "is:", line)
+    ```
+
+2. Let's start by trying out `zip` with strings:
+
+    ```py
+    >>> for pair in zip('ABC', 'abc'):
+    ...     print(pair)
+    ... 
+    ('A', 'a')
+    ('B', 'b')
+    ('C', 'c')
+    >>> 
+    ```
+
+    Great, it works just like it works with lists. Now let's create
+    the letter printing program:
+
+    ```py
+    uppercase = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ'
+    lowercase = 'abcdefghijklmnopqrstuvwxyz'
+
+    for upper, lower in zip(uppercase, lowercase):
+        print(upper, lower)
+    ```
+
+3. This one is a bit more difficult than the other two because we
+    need to combine `zip` and `enumerate`. I would pass a `zip`
+    result to `enumerate`, like this:
+
+    ```py
+    uppercase = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ'
+    lowercase = 'abcdefghijklmnopqrstuvwxyz'
+
+    for index, letterpair in enumerate(zip(uppercase, lowercase), start=1):
+        upper, lower = letterpair
+        print(index, upper, lower)
+    ```
+
+    Another alternative is to pass an `enumerate` result to `zip`. This is
+    a bit more complicated, so I wouldn't do it this way.
+
+    ```py
+    uppercase = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ'
+    lowercase = 'abcdefghijklmnopqrstuvwxyz'
+
+    for upper, indexlowerpair in zip(uppercase, enumerate(lowercase, start=1)):
+        index, lower = indexlowerpair
+        print(index, upper, lower)
+    ```
+
 ## Defining functions
 
-1. Use `-value` to get the negative in the abs function, and for loops
-    in the other two functions.
+1. Use `-value` (it works just like `-1`) to get the negative in
+    the abs function, and for loops in the other two functions.
 
     ```py
     def my_abs(value):
@@ -213,7 +280,7 @@ These are answers for exercises in the chapters. In programming, there's always
         for item in a_list:
             if item:
                 return True    # ends the function
-        return True
+        return False
 
     def my_all(a_list):
         for item in a_list:
@@ -225,10 +292,10 @@ These are answers for exercises in the chapters. In programming, there's always
 2. Like this:
 
     ```py
-    def box(message, character='*'):
+    def print_box(message, character='*'):
         number_of_characters = len(message) + 4
         print(character * number_of_characters)
-        print(message)
+        print(character, message, character)
         print(character * number_of_characters)
     ```
 
diff --git a/trey-hunner-zip-and-enumerate.md b/trey-hunner-zip-and-enumerate.md
@@ -0,0 +1,196 @@
+# Trey Hunner: zip and enumerate
+
+Now we know how [for loops](loops.md#for-loops) work in Python. But
+for loops aren't limited to printing each item in a list, they can
+do a lot more.
+
+To be able to understand for loop tricks we need to first know
+assigning values to multiple variables at once. It works like this:
+
+```py
+>>> a, b = 1, 2
+>>> a
+1
+>>> b
+2
+>>> 
+```
+
+We can use `()` and `[]` around these values however we want and
+everything will still work the same way. `[]` creates a list, and
+`()` creates a tuple. We'll talk more about tuples later.
+
+```py
+>>> [a, b] = (1, 2)
+>>> a
+1
+>>> b
+2
+>>> 
+```
+
+We can also have `[]` or `()` on one side but not on the other
+side.
+
+```py
+>>> (a, b) = 1, 2
+>>> a
+1
+>>> b
+2
+>>> 
+```
+
+What actually happened is that Python created a tuple automatically.
+
+```py
+>>> 1, 2
+(1, 2)
+>>> 
+```
+
+If we're for looping over a list with pairs of values in it we
+could do this:
+
+```py
+>>> items = [('a', 1), ('b', 2), ('c', 3)]
+>>> for pair in items:
+...     a, b = pair
+...     print(a, b)
+... 
+a 1
+b 2
+c 3
+>>> 
+```
+
+Or we can tell the for loop to unpack it for us.
+
+```py
+>>> for a, b in items:
+...     print(a, b)
+... 
+a 1
+b 2
+c 3
+>>> 
+```
+
+Now you're ready to read [this looping
+tutorial](http://treyhunner.com/2016/04/how-to-loop-with-indexes-in-python/).
+Read it now, then read the summary and do the exercises.
+
+## Summary
+
+Assigning multiple values at once:
+
+```py
+>>> a, b, c = 1, 2, 3
+>>> a
+1
+>>> b
+2
+>>> c
+3
+>>> 
+```
+
+For looping over a list of pairs:
+
+```py
+>>> stuff = [('a', 1), ('b', 2), ('c', 3)]
+>>> for a, b in stuff:
+...     print(a, b)
+... 
+a 1
+b 2
+c 3
+>>> 
+```
+
+For looping over two lists at once:
+
+```py
+>>> colors = ['red', 'green', 'blue']
+>>> letters = ['R', 'G', 'B']
+>>> for letter, color in zip(letters, colors):
+...     print(letter, color)
+... 
+R red
+G green
+B blue
+>>> 
+```
+
+For looping over a list with indexes:
+
+```py
+>>> fillernames = ['foo', 'bar', 'biz', 'baz', 'spam', 'eggs']
+>>> for index, name in enumerate(fillernames):
+...     print(index, name)
+... 
+0 foo
+1 bar
+2 biz
+3 baz
+4 spam
+5 eggs
+>>> 
+```
+
+## Exercises
+
+1. Create a program that works like this. Here I entered everything
+    after the `>` prompt that the program displayed.
+
+    ```
+    Enter something, and press Enter without typing anything when you're done.
+    >hello there
+    >this is a test
+    >it seems to work
+    >
+    Line 1 is: hello there
+    Line 2 is: this is a test
+    Line 3 is: it seems to work
+    ```
+
+2. Create a program that prints all letters from A to Z and a to z
+    next to each other:
+
+    ```
+    A a
+    B b
+    C c
+    ...
+    X x
+    Y y
+    Z z
+    ```
+
+    Start your program like this:
+
+    ```py
+    uppercase = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ'
+    lowercase = 'abcdefghijklmnopqrstuvwxyz'
+    ```
+
+    **Hint:** how do strings behave with `zip`? Try it out on the
+    `>>>` prompt and see.
+
+3. Can you also make it print the indexes also?
+
+    ```
+    1 A a
+    2 B b
+    3 C c
+    ...
+    24 X x
+    25 Y y
+    26 Z z
+    ```
+
+***
+
+You may use this tutorial freely at your own risk. See [LICENSE](LICENSE).
+
+[Back to the list of contents](README.md)
