Lecture 2

[winterthunder]

All right! Hopefully we've all successfully completed Problem Set 0 and are ready to tackle Lecture 2. Note that there is a handout associated with this lecture, and having it printed out before starting is a really helpful thing. Not that I had to stop and print it in the middle of lecture or anything...

Also, since our problem set numbers started with 0, I'm going to put up a post for PS1 this Friday. From that point on, problem sets will go up every other week.

I had one problem with this lecture, and I'm hoping you all can facilitate a light bulb moment. The code example with x, y and z, the fourth one down the page... I can't figure out what the bug was. Is it just because the statements aren't indented, or is there something else?

Comments

[zulu]

I'm not sure where the handout is, so this is just a guess, but maybe the bug is that there's no colon after the first instruction? For instance, this is correnct:

if x > y:
     print x

But this is not correct:

if x > y
     print x

[winterthunder]

Oh, the handout link would have been good to include, wouldn't it? It's here.

But no, that's not it, there's a colon...

[afuna]

Oh goodness, thanks for that. I've been having a hard time making out some of the code examples; forgot it was in the handouts. This should make things easier :)

[winterthunder]

Yeah, it's a lot easier when you can actually see what they're doing. I've started blowing up the videos to full screen, too, and that helps sometimes. :)

[jetamors]

C&Ping the code to make it clearer:

x = 15
y = 5
z = 11
print x,y,z
#Is this right?
if x < y:
    if x < z: print 'x is least'
    else: print 'z is least'
else: print 'y is least'

The problem with this code is that it doesn't consider different situations. This example compares x and y, and x and z, but it does not compare y and z.

So for example, if we say x = 15, y = 13, z = 11.
The code will look and say, okay, x is not less than y. It'll skip down to the else statement, and print 'y is least'.
In actuality, z is least, but the program never compares y and z.

(Reposted to fix a small mistake.)

[winterthunder]

Ok... rephrasing this to make sure I understand. The code sets up a situation in which the else statement can be triggered without considering all possibilities. So the code is doing what it says, because x is not less than y, but the coder neglected to account for other possibilities in the scenario.

Right?

[jetamors]

That's correct. This isn't a bug in the sense that the program won't run. The problem is that the code doesn't do what we want it to do: return the smallest number under every possible condition.

Consider a case where our intent is different: let's say we decide that if x is not less than y, then we don't care what z is. In that case, this code would be perfectly fine.

[winterthunder]

Thank you! :)

[dodificus]

In the conditional statements readings there are some exercises. Do you know if they've got the solutions somewhere?

[winterthunder]

I don't off the top of my head, but I'm going to be doing the readings tomorrow night, so I'll take a look around then and let you know what I find. :)

[aranthe]

If you don't find the solutions, I can post mine. I've tested them to be sure they work.

[winterthunder]

That's a good idea, why don't you go ahead and post them behind a cut with the january start tag. I don't see solutions anywhere in the reading or class website.

Thanks!

[aranthe]

Okay. I'll annotate them and try to get them posted sometime tomorrow (Friday).

ionabdRgPETKlanJ

[(Anonymous)]

I hate my life but at least this makes it baearble.

[aranthe]

Oops—forgot to ask: Do I put them under this thread or start another?

[winterthunder]

Awesome! Go ahead and start another so it won't be hidden in the comments. :)

McPLHfDdvRaKImSAU

[(Anonymous)]

You've impressed us all with that postnig!

gZFJGTwPZPOEkpw

[(Anonymous)]

Enlightening the world, one helpful arctile at a time.

[winterthunder]

There don't appear to be solutions, but aranthe has volunteered to put up her answers. Look for a new post to go up soon!