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OK.

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Welcome back, everybody.

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So in this lecture, we are going to be discussing a new topic which is nested loops, so nested loops

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are pretty much just the idea of having a loop or more inside of a another loop as many layers as you

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could think of.

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So having a loop inside of a loop or a loop inside of a loop, inside of a loop, inside of a loop,

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you know, continuing.

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This is the idea of nested loops in this lecture.

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We're just solely going to be covering a singly nested loop, which is just a loop inside of a loop.

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No more layers than that.

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And this can initially be quite a confusing topic, but an important thing to remember is that you can

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always go back to the execution flow of this and draw it out step by step so you can go through a loop

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and its inner loop and keep track of the variables that are changing for four loops and stuff like that.

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And it'll help you kind of figure out what is going on.

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So if it gets confusing, pull out a pen and paper kind of draw out a table, keep track of the values,

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make some drawings step by step to the code, and that'll be a good way to remind yourself of what's

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going on.

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If, after this lecture, you forget and become a little confused at what's really going on inside these

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nested loops loops.

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So one thing we do have to become comfortable with is the idea of for each time something happens.

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Another thing will happen multiple times.

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So when I say for each time, something happens that something is the outer loop and the another thing

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that happens multiple times is the inner loop.

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So I think the best way to explain this is to use an analogy.

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And in this analogy, what I am using is a sandwich shop, so you can imagine something like Subway.

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If you've ever been there, that's an international chain.

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So maybe it's a good idea to use that.

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And at this particular specific sandwich shop, we only have one employee working right now.

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So we have a line of clients here, and each client basically needs to be able to go through the sandwich

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bar and choose each one of the toppings and then pay before the next client is allowed to start that

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process.

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So this is the type of live control structure I'm talking about the turquoise or teal, kind of.

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Circle here is representing the outer loop, so everything that's in between.

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So I should say everything that is actually inside of this turquoise shape, but outside of the purple

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shape.

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So in this region, I'm actually going to trace it.

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So this looks this here and this area in this dollar sign over here and all of this that is considered

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the scope of the outer loop.

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Specifically, just the outer loop.

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Although the inner loop is still inside of the scope of the outer loop, the inner loop is represented

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by the purple.

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So everything inside the purple, which is the same which path that is the inner loop.

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So what we're going to do is have one client go into the inner loop and choose each one of these topping

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categories, which is a b for bread and for meat, and B for veggies.

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So they will iterate over each one of these and then they'll go back out into the outer loop and they

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will have to pay and then they'll be done and we can then start processing the next client.

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So let's go ahead and walk through this first client gets processed here in the outer loop and we then

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head into the inner loop with that client.

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This client chooses a bread, so that's one iteration of the inner loop.

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Then they go through and they choose the meat that is, say, next iterations of the second iteration

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of the Loop.

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And then they choose a type of veggies or multiple veggies, and that is the third iteration of the

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inner loop.

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They then exit the inner loop and go back into the outer loop to do one last thing, which is pay.

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And then they are done and we move.

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We process the second client and then bring them into the inner loop.

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They choose their bread, they choose their meat, they choose their veggies.

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They go out of that in a loop and they then pay the register.

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We then process the third client.

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This client chooses bread and then meat and then veggies, and then they go to the register and then

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we do the last client.

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They come into the inner loop and they iterate over bread, meat and then veggies and then pay.

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And then we are done.

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So I am now going to go over this with a code example that is pretty much the same as what we drew right

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here.

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So we're going to chase this code example of the sandwich bar here in code just to give you an overview.

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I start out by defining, declaring, initializing a variable here called line length.

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I say equal to four.

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I then have an array of type char that size three that's called toppings, and it has those symbols

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right B, M and V for bread, meat and vegetables, respectively.

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So then we start our loops, and this is where it gets interesting, so.

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We're starting out with a person here, and we're saying that our person starts out at person zero.

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We want to go as long as the person number is less than the line length.

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So that will be zero one to three.

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That is four times one for each of the four clientele of the four clients.

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We do a person plus plus because we're going to increment from one client to the other, right?

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So I'm going to move to this next line and I'm going to kind of now from this point on, keep track

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of these really important variables.

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So we are on person.

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Zero person equals zero to start with.

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We have now hit the line of our inner loop here, where we have top in position is equal to zero.

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You notice that we're also doing a tapping position less than three top position plus plus because there

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are three toppings and we care about going from zero one two, but not to three because that's sufficient.

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Zero one two two cycle across our top end positions.

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So now that we've had this line and we have started out with Typekit position on zero, we can go to

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this inner line inside the inner loop.

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We say we print out person, what is person zero?

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So Person Zero chose topping, then we see this array.

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We ask ourselves what is top position that is zero, so what is at top position index zero?

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It is B.

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Therefore, this whole sentence will be person zero chose topping B.

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Right, and then a space we now have finished with this iteration of the inner loop, so we go back

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up to the top and now we do a top in position plus plus, which brings us from top position was zero

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to top in position is now one.

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We go back into the loop and we say person zero, because person has not changed, we're still on person.

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The first person which is represented as persons zero, right, and that's what we started out on.

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This is still the first client in terms of just ordering, but the computer science position, you know,

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is zero because we're starting zero.

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So we say Person Zero chose topping, but now we're on one with position one inside here it's M.

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So we're saying person zero choice topping M and then a space.

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We go back up here now it's two.

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So we can say Person zero now chose topping V.

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It's the final topping right.

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We go back up here, we increment in position to three.

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But what we said is that we're not going to run the code inside this loop.

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If top position is not less than three, because this was our loop running condition, it has to be

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less than three since that condition is not satisfied.

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We are now finished with our loop.

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We will hit this bracket here and continue on to the code.

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Here we are now going to print out a new line character.

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So if you haven't seen this yet, it's similar to end line.

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It just prints out a new line so we can move on to a next to the next line of the console.

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And then we say Person zero.

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Paid and then we print out and in line, right?

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So they paid.

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Now we have finished with the outer loop because we hit this close bracket here, right?

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So we're done with everything inside the outer loop, so we go back to the top for the next iteration

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of the outer.

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Now that we're back up here, we can increment the person so person goes from zero to one.

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This condition says that will go as long as a person is less than line length, line length is four,

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one is less than four.

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We continue, of course, so we did that person plus plus.

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So we're going to go into here now top in position.

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You notice it just was a three.

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It gets reset to zero.

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Top position is now zero again and we can start cycling through our toppings for this person, which

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is person one.

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Technically, the second person, right?

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Because we already did the first person, that person was considered person zero.

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So we cycled through the toppings again, incrementing as we go, we say person one chose topping B

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because topping B is the position zero right.

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Increment it to the next topping now.

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Person one just topping M now person one chose topping we we go up here.

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Three is not less than three.

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So now we're done with the inner loop.

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We go to this last line of the outer loop and we say Person one has paid.

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Now we go to back to the outer loop, we go to the next person.

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We are now on person number two, which we consider the third person, right because we did zero and

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one already.

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And now we're on the third person, but we're calling it person two because it started zero.

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We go here and start doing the toppings for person two, so we reset top in position to zero.

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Person to choose this top and be person two to chose topping in person, to chose to be done with this

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loop.

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Person to paid person three.

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We go in here person three.

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So we in Typekit position back to zero.

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We say person three chose topping be person three chose topping in person.

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Three chose topping the.

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We're done with this person to be paid.

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We go back up here.

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We say person is for him is for less than four line like this.

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Four is for less than four.

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No, it's not.

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We're now done with our outer loop and we now go past this closing bracket and we just do it here.

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Return zero.

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We are now finished.

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So I'm going to escape out of here, go into the visual studio where I have this code.

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I am going to compile this code

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and I'm just going to say

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NFL easy.

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I will now run an AFL AFC and we can check out the results, so let's bring this up.

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I'm going to scroll up a bit and I bring this down just a bit.

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So as you can see here, oops, sorry about that.

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You can see that we said.

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Person zero showstopping, be person zero, just tapping in person, zero showstopping V. I didn't put

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a space between those, I should probably have done that.

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That would be a lot nicer.

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So let's say.

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We do that, and then we put like a comma or something like that.

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Let's go ahead and say this and make the output nicer.

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So I'm going to compile again and run again.

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All right.

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That looks a little nicer.

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Actually, person zero, let's put a space here as well, because that looks still looks kind of gross.

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All right.

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Here we go, so person zero, just tapping the person's, you know, just tapping in person through

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just tapping we and we say persons who are still not a space there should put a space there, right?

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So.

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Let's go ahead and do that, make all of our little corrections, so the outfit looks nice and then

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you notice that says each person chooses these toppings, then they pay.

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Then the next person chooses their toppings, then they pay, then the next person chooses their toppings

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and they pay and so on.

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Until all four people starting at zero going to one and two, then three, they all get to choose their

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toppings and pay in the correct order in which they should be moving through the sandwich shop.

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Right?

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OK, so pretty neat concept.

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You can also loop some other things, though, so we can have a for loop inside of a while loop and

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a while loop inside of a while loop.

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So let's go ahead and check out some of those examples, too.

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So I'm going to go up here and I'm going to go ahead and just delete this.

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And then I'm going to say I'm going to put a while loop.

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So I'll just say, let's say and.

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The number of people equals four, and I will just say that we will do some sort of while loop where

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we can continue getting some input or something like that.

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So I could say, um, let's say.

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Uh.

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A name, right, let's do like a string.

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Name equals.

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We'll just say a string name, right?

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No industry I'll get and then what we will say after that.

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We all see in to name.

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And then what we will do is we will say.

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While name not equal to quit and what we will do is.

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We will add the name, let's see, let's see, we'll just see out.

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So we could say see out name

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and then we'll just say.

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For.

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And equals zero, I is less than and we will just say a number of people plus plus and we will just

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say in here that this is just kind of some random thing, you know, some hypothetical situation, but

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we'll just say our name.

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Helps person

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I.

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How about that?

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And then we will do a space and something like that, and then we will just see in and we will get another

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name right?

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So this is just someone that maybe works somewhere.

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Maybe the person at the shop counter what he does in this time, though, is it's just a bunch of people

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coming by.

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Let's just say that this is like people coming to get vaccinated for COVID 19.

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It seemed kind of appropriate for this point in time.

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Where am making this course?

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At least, you know, it is, as you can see, June and twenty twenty one.

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So we still have a little bit of a COVID problem now.

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So let's just say that there's this guy or there's multiple people and they help four people each.

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So we have some qualified people giving vaccinations and each one of these people, we're going to enter

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their name and they're going to help their assigned people, which will be like person one person to

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person to person or.

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So let's go ahead and compile this and run this, it's still called nested for.

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But we can, of course, uh, rename that.

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We can do actually do something in here while I can say move nested for.

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And I will now call it moving it.

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You can actually change the name or you can move it to a different location.

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But I'm going to do is change the name in this same directory, and I'm just going to call this nested.

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I call it Nest Wild for CBD.

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So now I move that it says that this is deleted here.

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But if but we could totally open it up and it would be there.

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So I'm scrolling through all of the gross stuff that I have in this single directory, which should

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be organized.

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You should organize it in a nice way and keep it like me.

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So now I open this and it's totally fine and I actually can compile this and, you know, just allow

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for show.

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Well, for Dot C or easy so I can compile that.

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Oops.

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We get a lot of.

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Errors here, so.

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Let's see what the problem is.

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See, that name helps, person.

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It's not derived.

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From we put the wrong

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operators here.

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So I actually put the wrong operators throughout this entire expression.

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So let's go ahead and change that.

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And it needs to be changed here as well.

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So I put the input operator four instead of the the output one.

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So now that is changed, I'll go ahead and save this and that should fix it.

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All right, that fixes it, so I'm going to go ahead and run it now, so you notice nothing prints out

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because we're just asking for the name without printing anything out.

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The initial time.

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So I'm going to go ahead and put a name, and I'm just going to say Bob.

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And so you notice that it prints out Bob.

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So I entered Bob and it says, OK, well, I'm Bob.

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Bob helps person zero.

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Bob helps person one.

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Bob helps person to bob.

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That was person three.

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I did not do another in line after this, so it's kind of gross, but now I can say, you know, Mary

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or something like that, and it says Mary Mary helps all of these people.

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So you notice that what we're doing is it's waiting for us to enter a new name and it's going to keep

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asking for a name over and over until we do quit.

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And each time we enter a name, it is thing going to run this inner for loop, and it's going to say

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that that person helps each of these other people.

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You know, a number of people person, zero person, one person, two in person, three.

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Right.

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So each of those four people.

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So I see Jeff now and it says, OK, we're on Jeff.

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Jeff helps person zero.

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Jeff helps person one.

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And it's just going to keep going until we Typekit, in which case the program quits, right?

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So just wanted to give another example there that you can put the four inside of the wall and you can

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also put a while inside of me while here as well, I don't think that I need to demonstrate that.

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I think you can figure that out.

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You just simply put another well in there and you should be able to trace it on your own.

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But I think that I'm going to leave off the lecture at this point because in the next lecture, we're

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going to have to get into looking into.

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Not just a raise, but we're going to look into matrices which are two dimensional arrays.

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At least we're going to be looking at two dimensions, and they're not going to be looking at anything

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more than two dimensions.

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But we're going to need to use nested loops to be able to iterate over these two dimensional arrays,

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and so I wanted to introduce the concept of nested loops so you could solidify that.

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And then afterwards, you would understand more when we're introducing the matrices.

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So with that, I will leave you to it, go ahead and practice some of this stuff here.

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Once we get into the matrices, I will have some more practice problems for you to get used to all this.

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But until then, I will see you in the next lecture.