[Courses] [python] Lesson 2: Loops, if, and beer

[Sondra Schreibman]

*Hi!

My name is Sondra. I'm running Python 2.7.1 on Ubuntu Linux 11.04. *

1. Your first programming assignment!
  Write a program for the old "bottles of beer" song. It should print
  something like:
  99 bottles of beer on the wall
  98 bottles of beer on the wall
  97 bottles of beer on the wall
  ... all the way down to 1.

  Extra credit: make it print the number twice, e.g.
  99 bottles of beer on the wall, 99 bottles of beer

*for i in range(99,1,-1):
    print i, "Bottles of beer on the wall,", i, "bottles of beer"
print 1, "Bottle of beer on the wall,", 1, "bottle of beer"*

2. What does range(0, 10, -1) give?  (Try it.)
  Any idea why?

*It returns an empty set of brackets: [ ]
I think it happens because it's impossibly to count backwards from 0 to 10.
There isn't anything in that range.
*
3. How was the length of this lesson? Too much, too little or just right?

*I think it was perfect. Thanks for writing it!*

I don't know how to complete #4. I'm looking forward to seeing how others do
it!


On Fri, Jun 24, 2011 at 1:31 PM, Akkana Peck <akkana at shallowsky.com> wrote:

> Today we're going to do some real programming:
> "for" loops and variables.
>
> ================= A simple loop =========================
>
> A loop is a simple concept: something that gets executed over and over.
>
> There are lots of types of loops. Let's start with a numeric loop that
> lets you do something a certain number of times.
>
> Suppose you want to print "Hello, world!" five times instead of once.
> Here's the easiest way to do that in Python:
>
> for i in range(1, 6) :
>    print "Hello, world!"
>
> First of all, what's i?
>
> It's a variable, like name was in lesson 1, only this time it's going
> to store a number. That number is going to be 1 the first time through,
> then 2, then 3, then 4, then 5 ... then it will stop before it gets to
> 6, the second number you specified in range(1, 6).
>
> It's very common when you're looping over numbers to call your loop
> variable i, but you don't have to. It would work just as well if you said
>
> for hippopotamus in range(1, 6) :
>
> Note the indentation of the print line. I've indented four spaces, but
> you can use two, or three, or whatever you prefer. But it's very
> important that you be consistent about your indentation. Python
> cares about that, and if you forget to indent to the right level,
> your program won't work right.
>
> That sounds like kind of a pain, but it's actually a good thing. In
> languages that let you be sloppy about indentation, it's easy to write
> code that seems fine but has bugs you don't immediately notice, and it
> can be a lot harder to read other people's code.
>
> The colon at the end of the for line is important too: it tells Python
> this is a statement that takes multiple lines. A for loop makes no
> sense unless you do something inside the loop; the colon tells it to
> expect those lines, and the indentation tells it which lines are
> inside the loop.
>
> You don't have to put a space before the colon. A lot of Python
> programmers do, but a lot of others don't. It's just a style choice.
>
> So each time Python sets i to a new value, it will print "Hello,
> world!" one more time.
>
> So what about that range() part?
>
> range(1, 6) gives you a list of numbers, starting at 1 and stopping
> just before getting to 6: [1, 2, 3, 4, 5].
>
> So what this loop says is:
>
> For every number in the list [1, 2, 3, 4, 5] :
>    print "Hello, world!"
>
> If you want to keep tabs on what your variable is, you can print it:
>
> for i in range(1, 6) :
>    print "Hello, world! i =", i
>
> ========== Python console, and more on range() ============
>
> If you're confused about range, an easy way to get more comfortable
> with it is to type it into Python directly.
>
> Run python -- just that, without a filename or any other arguments.
> Then when you see the >>> prompt, type:
>
> range(1, 6)
>
> and see what it prints out.
>
> The Python console is handy any time you have a few simple commands to
> test, and you don't want to make a new file just to test them.
> You can even type your whole loop into it: just don't forget to
> indent the lines inside your loop!
>
> range can do more than count by ones. If you ask for range(0, 10, 2)
> it will count two at a time.
>
> At the Python >>> prompt, try typing:
> range(10, 2)
> [2, 4, 6, 8]
>
> You can even tell range to count backward. Try this:
>
> range(5, 0, -1)
>
> See what it's doing? To get each new number, it's taking the current
> number and adding -1. Then if it's gotten to the end point, 0, it stops
> and doesn't include that.
>
> ================= if statements =======================
>
> One more thing for today's lesson: if statements.
>
> for i in range(1, 6) :
>    if i == 3 :
>        print "Hello, world! i =", i
>
> If statements are fairly straightforward. If the variable i is equal to 3,
> do whatever's inside the if (any lines that are indented).
>
> The only tricky part is that double equals sign. Why double? A single
> equals is for when you're setting a variable, like in lesson 1:
> name = "Amelia Earhart"
>
> So you have to use == when you're comparing two values (in this case,
> i and 3), or else Python will give you a syntax error. You'll find
> == in most modern languages, so it's a good rule to be familiar with.
>
> You can stack up a bunch of conditions by "elif", meaning "else if".
>
> for i in range(1, 6) :
>    if i == 3 :
>        print "i is three!"
>    elif i == 4 :
>        print "now it's four!"
>    else :
>        print "Hello, world! i =", i
>
> ===================== Homework =======================
>
> 1. Your first programming assignment!
>   Write a program for the old "bottles of beer" song. It should print
>   something like:
>   99 bottles of beer on the wall
>   98 bottles of beer on the wall
>   97 bottles of beer on the wall
>   ... all the way down to 1.
>
>   Extra credit: make it print the number twice, e.g.
>   99 bottles of beer on the wall, 99 bottles of beer
>
> 2. What does range(0, 10, -1) give?  (Try it.)
>   Any idea why?
>
> 3. How was the length of this lesson? Too much, too little or just right?
>
> Problem 4 is optional -- it's a little harder, but give it a try,
> and if you don't get it, take a look at the answers other students
> post. But try it on your own first, if you can.
>
> 4. Write a loop using range() that prints out the first five numbers
>   as words:
>   one
>   two
>   three
>   four
>   five
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