loop stuff

Akuli · Akuli · commit 36b737f0bdb3 · 2017-01-21T12:31:22.000+02:00
diff --git a/basics/answers.md b/basics/answers.md
@@ -220,84 +220,33 @@ isn't exactly like mine but it works just fine it's ok, and you can
 
 ## Loops
 
-1. For loop over the users, each user is a list that contains a
-    username and a password.
+1. The problem is that `things` is a string because we converted it to a
+    string with `str`, so the for loop loops over the characters `[`,
+    `1`, `,` and so on. Replace `str([1, 2, 3, 4, 5])` with
+    `[1, 2, 3, 4, 5]`.
 
-    ```py
-    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.
+2. The code appends each list in `before` to `after`, so the `number`
+    variable actually pointed to a list like `[1, 2]`. An easy solution
+    is to just write two for loops inside each other:
 
     ```py
-    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.")
+    before = [[1, 2], [3, 4], [5, 6]]
+    after = []
+    for sublist in before:
+        for number in sublist:
+            after.append(number)
+    print(after)
     ```
 
-3. Add a for loop that works as an attempt counter.
+    Lists also have an extend method that appends each item from another
+    list, so we can also use that:
 
     ```py
-    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.")
+    before = [[1, 2], [3, 4], [5, 6]]
+    after = []
+    for sublist in before:
+        after.extend(sublist)
+    print(after)
     ```
 
 ## Trey Hunner: zip and enumerate
diff --git a/basics/loops.md b/basics/loops.md
@@ -238,6 +238,31 @@ with than while loops and index variables, especially in large projects.
 For looping is also a little bit faster than while looping with an index
 variable.
 
+For loops are not actually limited to lists. We can for loop over many
+other things also, including strings and
+[tuples](lists-and-tuples.md#tuples). For looping over a tuple gives us
+its items, and for looping over a string gives us its characters as
+strings of length one.
+
+```py
+>>> for short_string in 'abc':
+...     print(short_string)
+...
+a
+b
+c
+>>> for item in (1, 2, 3):
+...     print(item)
+...
+1
+2
+3
+>>>
+```
+
+If we can for loop over something, then that something is **iterable**.
+Lists, tuples and strings are all iterable.
+
 There's only one big limitation with for looping over lists. We
 shouldn't modify the list in the for loop. If we do, the results can
 be surprising:
@@ -275,10 +300,6 @@ Or if we just want to clear a list, we can use the `clear`
 >>>
 ```
 
-Later we'll learn that for loops are not actually limited to lists.
-There are many other things we can for loop over. Things that can be for
-looped over are called **iterables**.
-
 If you're using Python 3.2 or older you need to use `stuff[:]` instead
 of `stuff.copy()` and `stuff[:] = []` instead of `stuff.clear()`.
 `stuff[:]` is a slice of the whole list, just like `stuff[0:]`.
@@ -403,25 +424,25 @@ while True:
 
 ## Exercises
 
-1. Back in "Using if, else and elif" we created a program that asked
-    for username and password and checks them, and we made users "foo"
-    and "bar" with passwords "biz" and "baz". Adding a new user would
-    have required adding more code that checks the username and
-    password. Add this to the beginning of the program:
+1. This code is supposed to print each number between 1 and 5. Fix it.
 
     ```py
-    users = [
-        ['foo', 'biz'],
-        ['bar', 'baz'],
-    ]
+    things = str([1, 2, 3, 4, 5])
+    for thing in things:
+        print(thing)
     ```
 
-    Then rewrite the rest of the program using a for loop.
+2. This code is supposed to print `[1, 2, 3, 4, 5, 6]`. Fix it.
 
-2. Make the program ask the username and password over and over again
-    until the user enters them correctly.
+    ```py
+    before = [[1, 2], [3, 4], [5, 6]]
+    after = []
+    for number in before:
+        after.append(number)
+    print(after)
+    ```
 
-3. Can you limit the number of attempts to 3?
+The answers are [here](answers.md#loops)
 
 ***
 
