CS50: Introduction to Computer Science I (Week 9/Mon)

>> Unknown: All right, welcome to CS50. This is the start of week 9. So one announcement, if you haven't RSVP'd already and would like to join us, Wednesday, Mather [assumed spelling], 6 pm. [Inaudible] just RSVP. One new handout today and this is a bug of sort or a feature if you will, that's actually been circulated a bit on various blogs this past week. So it's been around for some time. I thought I'd replicate it here with this website called, Google. I was wanting to do some math the other day and it turns out Google does math. For instance, if I want to know what 3 - 2 is, hit enter, well we'll actually do that kind of math for you. But that I can do in my head, but I can't really do in my head very well something like this, this, that's a big number and then let's just minus, actually let's try to be creative here. Let's put almost the same number but 998. OK, so for anyone who's taken like Math 55, what's the answer?

[ laughter ] Seven. Excellent. So it's pretty close, it should be 1 right. Pretty big numbers, but one's just 1 bigger than the other, but if you do a little Google search for this, the Google fails us. So.

[ booing ] I know.

[ laughter ] So this went around a while. So this isn't a bug per say because otherwise someone at Google would have addressed it. Because it's been blogged about at nauseum. So what might the explanation be? What's that? So rounding or there's some kind of imprecision here. So even though we talk mostly about this in the context of floating point values, so even with integers, we've seen in C at least, that there are upper bounds on the kinds of math that we can actually compute. So if you like this kind of thing, so there's really no big lesson here, this is in fact a feature of the way math is implemented at least by Google here, but there's actually a neat article that's been recirculated. It's called, Why Computers Suck at Math. It's an interesting article and it discusses this and other such things. So we've linked that on the course's website if you would like. Also on the course's website now, we've replaced the old big board with the new big board. The old big board in still linked there. You'll see that your classmate Charles has already figured out how to exploit financial opportunity perhaps, more likely bugs or weaknesses in our own implementation of this big board. This is sort of this funny thing, especially about teaching a Harvard class right, you know, you just want to do something nice and fun that gets the students excited about this and that. It's completely ancillary to the piece and then there's always one or more people who decide, let's see what we can break. And of course, there are weaknesses in this and that's fine, Charles is up 4000% since like 9 am this morning. If you actually look at his history by clicking his name, you'll see exactly how he did this and what he bought and things we probably shouldn't have let him buy. And again, you can essentially see into the future if you simply watch your own eTrade account or CNN.com and then make your purchases within the 15 minute window that is the delay that Yahoo Finance imposes. But that's OK. Why don't we just throw down the gauntlet and say that Charles is now the person to be. So 433,000 is currently the number one position. I think I'm doing rather the opposite. Oh actually, we're only down 1%. But one takeaway that's worth nothing here is and you may have glimpsed it really fast just now, is this is actually an Ajax implementation of this big board.