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For those of you who have already taken a course of this before,
you're already pros at doing concept challenges.
For those of you who are new to us,
let's talk through the details again of a concept challenge.
The idea behind a concept challenge, is for you to pause and
trace all the problems yourself.
We're gonna pose a problem and then give you an end video quiz about it.
Try to solve that problem and then after you've come up with an answer for it.
Try to discuss that with friends or colleagues that are taking the course.
This is a great time to get them all together and talk about the concepts.
If you don't have them around you can always watch the UC San Diego learners
talk about the concepts themselves.
0:45
After you've asked the question again,
you can confirm your understanding based on our explanation.
All right, so let's just dive right into it.
What we're gonna try to do is keep working with regular expressions.
So what I have here is the string below, which is splitting a string and
it's easy as one, two, 33, right?
If I try to do getTokens on that and
give it a regular expression which is [1-3].
It's gonna give me back one, two, three and three.
But what I really want back is the numbers here.
I want one, two and 33.
So your challenge here, is what regular expression should I put there?
1:52
>> Yeah, I'm kinda, hm.
I think I'm kind of leaning towards what Bree said.
That the commas might be like,
it's not a symbol like we use to separate different terms.
>> Since it's in the brackets.
Like, if you look at E, there's no brackets in there.
>> Yeah and I think I remember that the brackets kind of say that look for
a comma specifically.
Rather than separate one, two, and 33.
So I don't think it'll quite give us what were looking for.
>> Okay, and then C has numbers zero through nine, and then it has a plus sign,
which means one or more of any value from zero to nine.
I think that would work.
>> You think?
I think, hm, but won't it give us one, two, three and three?
Because it'll split up the 33, cuz it's looking for
just one digit between zero and nine.
So it'll give us one, two, three, three.
Instead of one, two, 33, right?
>> No, because the plus sign let's you have more than just one individual value.
>> So it would be like, how do you mean more than one individual value?
>> So you can have a zero, and then you can have a one, you can have a two.
You can also have a zero, zero, a one, one, a two, two, a three, three.
>> Oh okay, I see.
So, it could be like three, three, or five,
five, or six, six or things like that.
>> I think that's just more than one.
>> Oh but, would it also work for
the list that the last character we were looking for ends up, 33 would be 34.
>> So it wouldn't work for 34 or 57, or something like that.
>> Yeah, it's like, instead of like one, two, 33.
It was one, two, 34, or some letter.
>> I think it would.
>> Yeah I think it might work, because it'll just look for
any digit, like three, four.
It doesn't have to be the same one exactly.
Okay.
>> Okay.
>> Let's move on to answer d, then.
So answer d has a star, and it's asking for
characters one through three rather than a plus.
You think that changes the answer?
Cuz I'm leaning towards, if I think it'll be the same.
4:12
Where, like Bree said before, it'll search for like one or one one,
or two and two two, or three and three three.
So it'll give us this, what we have right here, one two and three, three right?
>> Yeah, except you'll also be counting for zero one.
So every time you have no ones, you're gonna get a space.
>> So, basically anytime you have an empty string.
I think it would include those empty strings.
4:41
>> Oh right, cuz it's a star.
So, star means like zero, okay.
Okay, so I guess D wouldn't work either.
So, what about E?
>> As one, or two, or 33?
>> What do you think?
>> I think it wouldn't work,
because I was thinking more about the ones of one and two and three.
But this has a line, which means or.
So I don't think it would be looking exactly for
the list that we're looking for, one, two, and 33.
>> Right, we are looking for one and two, and 33, so.
>> But the and might put them all together.
So that it's one and two and 33, all together.
So if you're evaluating- >> Right, one and two and 33,
that's our answer, right?
>> Right, but if you've got, you're scanning through each and
every single number, right.
So you're gonna look at the first one.
Is the first number a one, and a two, and a 33?
That wouldn't really make sense.
5:38
>> Okay, we should look at them separately.
>> Yeah.
>> Look at one separately, and look at two separately-
>> With the or.
>> and look at 33 separately.
And that's the point of the or.
So okay.
>> Welcome back.
I hope the UC San Diego learners video was helpful.
Somewhat similar to what they did.
What we're gonna do here is essentially walk one by one, and
talk through whether or not that approach would work or not.
So option A was bracket 1233 bracket.
The idea here is well maybe that would capture 33.
The problem here is if it's within the brackets,
all it's gonna say is include one more three, but you've already included three.
So all you get is the same thing.
In fact if you try to run this.
All you'll get back is the same thing you had before, which is just one, two,
three and three.
So this didn't quite work.
6:28
Option B tries to split this up using commas.
The problem is, just like the last example, all that adding those commas did
was to simply add comma to the group of letters that we're already looking for.
So, now we're gonna look for one, two, three, and comma.
If I run this on the code, what am I gonna get back?
We're gonna get comma, one, two, three, three.
So this didn't work either.
All right, next one, option C.
This one said in brackets [0-9]+.
Remember that plus means one or more of these elements.
7:04
If this was your answer, you were right.
What this is gonna do is look for anything between zero and nine.
If it finds at least one, look for more adjacent ones.
So what this is gonna do then is find one, two and 33,
which is exactly what we were looking for.
Let's keep looking at our other options though,
cuz this was a select all that apply.
So there may be more than one right answer.
7:36
This might look like it's gonna work, but
you have to be careful about the zero or more.
Because if you look at the letter S, are there zero or
more one through threes there?
Well, yeah, there's zero.
Oh, shoot.
That means it's gonna match everything, right?
So what you're gonna get then is you're gonna get every single letter matching and
give you back an empty string, because there isn't a one through three there.
It does, in fact, give you one, two, and 33,
but it gives you a whole bunch more garbage.
So this is not what we want either.
Now what if we just said, I want either one, or two, or 33?
Would that work?
Yeah, actually that's gonna work just fine too.
The problem is, now that we have two right answers, it'd be useful to say,
well which one's better than the other?
So, given [0-9]+.
And either one, or two, or 33.
Which do you think might be better?
Go ahead and take a few seconds and think about it.
8:35
If your answer is C, you're spot-on.
[0-9]+ is gonna capture essentially any non-negative integer.
That's fantastic.
Whereas if I try to do the same thing with E.
How am I gonna capture any non-negative integer.
I have to do something like zero, or one, or two, or three, or
four, all the way out to any possible integer.
I can capture it so concisely with option C that, that is just far more versatile.
And that's essentially the better of these two choices.
Christine AlvaradoAssociate Teaching Professor
Mia MinnesAssistant Teaching Professor
Leo PorterAssistant Teaching Professor
