VeereshShringari
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@@ -31,6 +31,7 @@ it.**
 6. [Using functions](using-functions.md)
 7. [If, else and elif](if.md)
 8. [ThinkPython: Lists](lists.md)
+9. [Loops](loops.md)
 
 Other things this tutorial comes with:
 
 
@@ -110,6 +110,87 @@ These are answers for exercises in the chapters. In programming, there's always
     >>> 
     ```
 
+## Loops
+
+1.
+
+    ```py
+    users = [
+        ['foo', 'bar'],
+        ['biz', '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.
+
+    ```py
+    users = [
+        ['foo', 'bar'],
+        ['biz', '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.
+
+    ```py
+    users = [
+        ['foo', 'bar'],
+        ['biz', 'baz'],
+    ]
+
+    for attempts_left in [3, 2, 1, 0]:
+        if attempts_left == 0:
+            print("No attempts left!")
+            break
+
+        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.")
+    ```
+
 ***
 
 You may use this tutorial freely at your own risk. See [LICENSE](LICENSE).
 
@@ -10,17 +10,20 @@ Everything in this tutorial contains lots of examples, and the tutorial
 should be easy to follow. But what should you do if you don't understand
 something the first time you read it?
 
-1. Read the example code and the explanation for it again.
-2. Try the example yourself.
-3. If you still don't understand, please [tell me](contact-me.md). I
-    want to improve the tutorial so other readers won't have the same
-    problem you had.
-
-One of the most important things with learning to program is to
-**not fear mistakes**. If you make a mistake, your computer will not
-break in any way. You'll get an error message that tells you what's
-wrong and where. Even professional programmers do mistakes and get error
-messages all the time, and there's nothing wrong with it.
+- Read the example code and the explanation for it again.
+- Try the example yourself.
+- If there's something you haven't seen before in the tutorial and it's
+    not explained, try to find it from the previous chapters.
+- If you still have trouble understanding the tutorial or any other
+    problems with the tutorial, please [tell me](contact-me.md). I want
+    to improve the tutorial so other readers won't have the same
+    problems as you have.
+
+One of the most important things with learning to program is to **not
+fear mistakes**. If you make a mistake, your computer will not break in
+any way. You'll get an error message that tells you what's wrong and
+where. Even professional programmers do mistakes and get error messages
+all the time, and there's nothing wrong with it.
 
 Even though a good tutorial is an important part about learning to
 program, you also need to learn to make your own things. Use what you
 
@@ -0,0 +1,267 @@
+# Loops
+
+In programming, a **loop** means repeating something multiple times.
+There are different kinds of loops:
+
+- **While loops** repeat something while a condition is true.
+- **Until loops** repeat something until a condition is true.
+- **For loops** repeat something for each element of a sequence.
+
+We'll talk about all of these in this tutorial.
+
+## While loops
+
+Now we know how if statements work.
+
+```py
+its_raining = True
+if its_raining:
+    print("Oh crap, it's raining!")
+```
+
+While loops are really similar to if statements. 
+
+```py
+its_raining = True
+while its_raining:
+    print("Oh crap, it's raining!")
+    # we'll jump back to the line with the word "while" from here
+print("It's not raining anymore.")
+```
+
+If you're not familiar with while loops, the program's output may be a
+bit surprising:
+
+```
+Oh crap, it's raining!
+Oh crap, it's raining!
+Oh crap, it's raining!
+Oh crap, it's raining!
+Oh crap, it's raining!
+Oh crap, it's raining!
+Oh crap, it's raining!
+Oh crap, it's raining!
+Oh crap, it's raining!
+Oh crap, it's raining!
+Oh crap, it's raining!
+Oh crap, it's raining!
+Oh crap, it's raining!
+Oh crap, it's raining!
+Oh crap, it's raining!
+(much more raining)
+```
+
+Again, this program does not break your computer. It just prints the
+same thing multiple times. We can interrupt it by pressing Ctrl+C.
+
+In this example, `its_raining` was the **condition**. If something in
+the while loop would have set `its_raining` to False, the loop would
+have ended and the program would have printed `It's not raining anymore`.
+
+Let's actually create a program that does just that:
+
+```py
+its_raining = True
+while its_raining:
+    print("It's raining!")
+    answer = input("Or is it? (y=yes, n=no) ")
+    if answer == 'y':
+        print("Oh well...")
+    elif answer == 'n':
+        its_raining = False     # end the while loop
+    else:
+        print("Enter y or n next time.")
+print("It's not raining anymore.")
+```
+
+Running the program may look like this:
+
+```
+It's raining!
+Or is it? (y=yes, n=no) i dunno
+Enter y or n next time.
+It's raining!
+Or is it? (y=yes, n=no) y
+Oh well...
+It's raining!
+Or is it? (y=yes, n=no) n
+It's not raining anymore.
+```
+
+We can also interrupt a loop even if the condition is still true using
+the `break` keyword. In this case, we'll set condition to True and rely
+on nothing but `break` to end the loop.
+
+```py
+while True:
+    answer = input("Is it raining? (y=yes, n=no) ")
+    if answer == 'y':
+        print("It's raining!")
+    elif answer == 'n':
+        print("It's not raining anymore.")
+        break   # end the loop
+    else:
+        print("Enter y or n.")
+```
+
+The program works like this:
+
+```py
+Is it raining? (y=yes, n=no) who knows              
+Enter y or n.
+Is it raining? (y=yes, n=no) y
+It's raining!
+Is it raining? (y=yes, n=no) n
+It's not raining anymore.
+```
+
+## Until loops
+
+Python doesn't have until loops. If you need an until loop, use
+`while not`:
+
+```py
+raining = False
+while not raining:
+    print("It's not raining.")
+    if input("Is it raining? (y/n) ") == 'y':
+        raining = True
+print("It's raining!")
+```
+
+## For loops
+
+Let's say we have a list of things we want to print. To print each item
+in stuff, we could just do a bunch of prints:
+
+```py
+stuff = ['hello', 'hi', 'how are you doing', 'im fine', 'how about you']
+
+print(stuff[0])
+print(stuff[1])
+print(stuff[2])
+print(stuff[3])
+print(stuff[4])
+```
+
+The output of the program is like this:
+
+```py
+hello
+hi
+how are you doing
+im fine
+how about you
+```
+
+But this is only going to print five items, so if we add something to
+stuff, it's not going to be printed. Or if we remove something from
+stuff, we'll get an error saying "list index out of range".
+
+We could also create an index variable, and use a while loop:
+
+```py
+>>> length_of_stuff = len(stuff)        # len(x) is the length of x
+>>> index = 0
+>>> while index < length_of_stuff:
+...     print(stuff[index])
+...     index += 1
+... 
+hello
+hi
+how are you doing
+im fine
+how about you
+>>> 
+```
+
+But there's `len()` and an index variable we need to increment. That's
+a lot of stuff to worry about for just printing each item.
+
+This is when for loops come in:
+
+```py
+>>> stuff = ['hello', 'hi', 'how are you doing', 'im fine', 'how about you']
+>>> for thing in stuff:
+...     # this is repeated for each element of stuff, that is, first
+...     # for stuff[0], then for stuff[1], etc.
+...     print(thing)
+... 
+hello
+hi
+how are you doing
+im fine
+how about you
+>>> 
+```
+
+Without the comments, that's only two simple lines, and one variable.
+Much better than anything else we tried before.
+
+```py
+>>> for thing in stuff:
+...     print(thing)
+... 
+hello
+hi
+how are you doing
+im fine
+how about you
+>>> 
+```
+
+There's only one big limitation with for looping over lists. You
+shouldn't modify the list in the for loop. If you do, the results can
+be surprising:
+
+```py
+>>> stuff = ['hello', 'hi', 'how are you doing', 'im fine', 'how about you']
+>>> for thing in stuff:
+...     stuff.remove(thing)
+... 
+>>> stuff
+['hi', 'im fine']
+>>> 
+```
+
+Instead, you can create a copy of stuff and loop over it.
+
+```py
+>>> stuff = ['hello', 'hi', 'how are you doing', 'im fine', 'how about you']
+>>> for thing in stuff.copy():
+...     stuff.remove(thing)
+... 
+>>> stuff
+[]
+>>> 
+```
+
+Or if you want to clear a list, just use the `.clear()` list method.
+
+```py
+>>> stuff = ['hello', 'hi', 'how are you doing', 'im fine', 'how about you']
+>>> stuff.clear()
+>>> stuff
+[]
+>>> 
+```
+
+## Excercises
+
+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 your program:
+
+    ```py
+    users = [
+        ['foo', 'bar'],
+        ['biz', 'baz'],
+    ]
+    ```
+
+    Then rewrite the rest of the program using a for loop.
+
+2. Make the program ask the username and password over and over again
+3. Can you limit the number of attempts to 3?