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So let's look at how we control flow in Swift now, flow is one of those kind of nebulous terms that



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describes how your application flows as you go from, say, data input to displaying data on the front



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end to the user.



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That's just one example of flow.



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But the concepts we're going to look at are things that allow us to partition that flow, to send it



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down one route or another route, depending on what kind of variables we impose on the flow.



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So the very first thing we're going to look at is the four in loop.



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So previously we've looked at arrays a couple of lessons ago and we would define an array as, let's



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say, an array of integers.



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So whatever it is.



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We have that now, you'll recall that one of the homeworks I set you was to add everything in an array



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and in that array there were only three numbers, but you had to type out the following.



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You had to go array element, not waps element.



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Element, not plus.



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Array, element one, etc, etc., to get your final answer, and I said, well, there's two major



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problems with that.



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The first problem, of course, is that your statements are going to get longer and longer, the larger



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the array, and you don't want to be typing that out manually.



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Secondly, arrays are sometimes of an unknown length.



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In fact, most of the time they are of an unknown length.



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If I get some return values from a server, for example, I might not know how many music tracks I need



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to display to the user.



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And so it's impossible to code in that way.



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I just showed you the solution is for the four in loop, right?



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So we go for a in array.



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What this does is go through the array iteratively starting at Element zero and working its way up one,



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two, three until it hits the end of the array.



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In that case, we can print a.



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So let's run that every time we go round and round, this loop is going to print a and every time we



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go round the loop, it moves from six to seven to three to seventy seven.



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And you can see all those results down here in the console.



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So that's the four in very, very, very useful, right?



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So what else we got here?



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Uh, what's this?



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And he paced this in.



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OK, so here we have an array with some key values.



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OK, so we have a spider which has eight legs and ant which is six and a cat which has four.



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Now, we can separate out each of these things in the four in a row.



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If I drop this in here for animal name and leg count.



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So we're dealing with tuples here in a way in a number of legs.



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So what it does is it goes its a number of legs.



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It looks at the first element, which is this, it assigns spider to animal name and the number to leg



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count and these two items here.



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We can set to whatever we like, whatever we need, we just giving names to the variables so that when



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we're working inside the for loop, we know what we're referring to.



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So if I have print animal name s have X legs, then when I fill in these variables, I know what I'm



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talking about.



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The second thing you'll notice about this little statement is I've gone away from saying animal name



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plus open quote, space, et cetera.



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Instead, we're using this backslash character, which allows us to grab a variable in brackets and



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print that directly in the string so we don't have to had all the convoluted pluses anymore.



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So if I hit play on that.



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Ants have six legs, spiders have eight and cats have four legs.



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OK, so that's simple enough.



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That's how you name variables in the for loop and how we interpolate our strings.



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What's next?



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Now, this is the for loop without in, OK?



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Well, actually, it is within my bed for index, so what's the index index is just a name we give to



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a variable and then we say to the loop, I want you to go from number one to number five.



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And what the fallout will do is automatically fill in index equals one, index equals two, etc. until



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it gets to five.



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Once it's reached five, it'll stop the loop and then carry on to the next line of code.



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So if we played that, we're going to get one times five is five times five is ten, etc. until we get



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five times five and then the loop ends.



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So there's something important to note here, about four loops.



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They have an inherent ending, the ending of the array.



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One is the length of the array.



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The ending of the number of legs is the number of elements in that array.



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And the ending in this four index, one to five is the number five.



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Once it hits five loop has finished.



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So these are pretty safe loops as they go.



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And as we go on through this course, I'm going to show you what unsafe loops look like, including



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one that [REMOVED] me in this code as I was preparing it.



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And I could believe such a silly error got me who's been programming for donkey's years.



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But it does.



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It happens to all of us.



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OK, what have we got next up?



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What happens if we don't actually want to print index?



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So let's say we want to save a little bit of memory and we don't want to store that value in index.



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While we can use this underscore notation, which is a general notation across SWIFT, it's not just



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for for loops.



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And what we'll do is have no variable that gets carried through to the loop.



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So that makes it probably a touch quicker, in my opinion.



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It's not going to make it that much quicker.



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There are lots of other areas that you need to address as a developer that will speed up your app and



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things like this, I don't think make that much difference.



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Now, again, if you're making a financial app and you are trading on the markets within nanoseconds,



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this is something you have to do, something you need to be aware of.



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Although you probably wouldn't be using Swift, you'd be using something low level like C to do that



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for just for the utter speed that you can get out of it.



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But for us regular developers, no problems whatsoever.



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So this fall, it goes one to five in the same way as the previous one without carrying a variable through



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to the loop itself.



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OK, so that's all about the fallout.



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I've got a bit of homework here.



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Make a set of four string variables.



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Make three unique and one the same as another and print them out.



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OK, now notice.



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This magic word set, I know it's not part of this lesson, but it was part of a previous lesson, wasn't



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it?



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A set is not an array.



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Hmm.



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So if you don't remember what that means, go back and look it up.



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Make for variables of which three are unique and one is the same as one of those three and then print



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all of them out using a for loop.



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Good luck.