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In the last lesson, we managed to create a single array which contained three questions and we were able
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to show it on screen and show each of the questions.
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Now, in this lesson, we're going to try and figure out how to be able to check to see if they got the
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answer correct,
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so whether if they chose the right answer to each question. And in the process, we'll also learn how to
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work with a two dimensional array in Swift.
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Now, in order to do this, we need to somehow associate an answer with each question, right?
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We need to be able to say that, well, maybe this one should be true and this one should be True,
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but this one should be False.
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So how can we associate two pieces of data together even though our quiz lives inside an array?
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Well, one way of doing this might be through the use of an array within an array or what you might call
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a 2D Array.
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So far, the arrays that we've been creating look something like this.
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We have as many items as we need or separated by a comma and placed inside a single array.
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So this is our collection of items.
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Now, we can also create a 2D Array where we have individual arrays. So there's three arrays, in this
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case, each with two items all packaged inside a larger array.
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So how would you retrieve a single entry out of a two-dimensional array? When we had a one-dimensional
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array, retrieving items was pretty straightforward. In order to retrieve items out of our array,
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we simply added a set of square brackets and a number after the name of the array. And that number,
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in this case, "i," refers to the position of the item that we want. Now, if we had a 2D Array on the other
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hand, in addition to adding a first set of square brackets where we're picking through the outer layer
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of arrays.
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Now, that we've picked out the item that is at this position, let's say, this one which is position 2, well,
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now we have access to yet another array.
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And in order to pick from this inner array, we have to tag on another set of square brackets in order
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to pick between these two items in the inner array.
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So here's a question. If we had an array that was a 2D Array that looks like this,
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so we have 1, 2, 3 inside its own array, 4, 5, 6 in its own array, 7, 8, 9, and then these three
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arrays are packaged into another array.
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Well, how do you get hold of the number 9?
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Pause the video,
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have a think about that.
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What would your code look like?
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Well, in this case, the positions of the outer array are 0, 1, and 2, and then the positions inside the array
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are also at 0, 1, and 2.
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So in order to get hold of this number 9, we first have to tap into our array, and then specify that
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we want the array that at position 2.
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So it's this one, 7, 8, 9.
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And then within this array of 7, 8, 9, we then specify the position of the item we want which is, again,
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at 0, 1, 2.
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So now we have array square brackets 2, square brackets 2 in order to get that number 9 out of this
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2D Array. Coming back to all code here,
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we currently have a 1D Array, and we can turn it into a 2D Array by simply making each of the items
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a array itself.
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So we now have a question and a answer, and I'm keeping it simple by making the array keep the same type
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of data.
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So they're both gonna be strings, right?
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So let's go ahead and modify the rest of these. So we now have three arrays within an array. And if we
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take a look at the data type of our quiz constant, you can see it's now a 2D Array of strings. So it's
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got two brackets around each side.
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And now if we wanted to pick out the first question out of our quiz array, then instead of simply specifying
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quiz[question number,] we also have to tag on a zero at the end because now it's going to pull out the
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first array using the question number which starts out being equal to zero, and then within that array,
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we want to get the first item,
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so the one at position 0.
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So this would be all code overall. So now that we have the quiz in a 2D Array,
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well, let's check the answer that we have in our 2D Array against the answer that the user chose.
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So, firstly, we have to figure out what answer they chose.
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So right here, I'm going to create a new constant called userAnswer and I'm gonna set it to equal the
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sender.currentTitle. So whichever title they chose from the two buttons are linked to my IBAction,
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so True or False.
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Now, this userAnswer is going to be equal to the string true or the string false.
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So we can now use it to check against the actual answer.
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Now, the actual answer is going to be equal to the quiz array at position 1,
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right?
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So similar to how we got our quizQuestion,
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we're going to set this equal to the quiz array and then the first layer is going to be questionNumber.
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So it's going to be one of these arrays depending on which question we are on.
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And then the second one is going to be position 1 because that's where the answers live. They're all at position
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one inside the array.
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So now in order to check to see if the user got the answer right, all we have to do is create an if statement
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and check to see if the userAnswer is equal to, so double equals, to the actual answer,
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well, then in that case, that means they got it right,
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right?
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So we'll just print "Right!."
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Otherwise, namely "else," well, they probably got it wrong because their answer doesn't match the actual
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answer,
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so we'll right wrong.
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Now, if we run our app, we should be able to see that question checking mechanism in action.
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So if we run our app, you can see that if I press Four plus Two is equal to Six,
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yeah, that sounds right,
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So let's press True.
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We got it right.
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Five-Tthree is greater than One,
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And then finally, Three + Eight is less than Ten.
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That's definitely False.
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So now we get our last answer, but then our app crashes because we, again, fall out of the range of our
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quiz array.
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We don't have a fourth question.
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So we've got a working quiz app now, although it's a pretty buggy one.
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And your app crashes on a user's phone,
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they won't know what's happened,
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they won't know that the index is out of range.
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They'll just think that you've made a terrible app.
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So let's fix this and help you maintain your reputation as an app developer.
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So the first thing that we need is a safety check.
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So inside the answer button pressed,
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before we increase the questionNumber, let's first check to see if we're actually at the end of the quiz.
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And I'd like to pose this as a challenge to you.
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Can you use an "if" statement and only increase the questionNumber as long as we're not yet at the end
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of the quiz.
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I'll give you a few seconds to pause the video before I show you my solution.
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Okay.
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Here's my sample solution.
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So in our case, we want to prevent the questionNumber from being increased passed number 2. You might think
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that you can simply write the following: If questionNumber is less than 3, then increased 
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questionNumber by 1, or you might have written it like this, if questionNumber is less than quiz.count,
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so the length of our array, then increase questionNumber by 1.
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But let's double-check this by running our app again. Even though we think we might have fixed the problem,
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you'll notice something quite peculiar.
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So if I press on the third button, my app still crashes,
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what's going on?
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Well, the line that's being highlighted is right here.
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So this is, obviously, the line that's crashing my app.
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And the reason is because the index is out of range.
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So the only thing that has an index that changes is my questionNumber.
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And we can see down here that the questionNumber was, again, allowed to go to 3, even though we said
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don't increase the questionNumber, unless the questionNumber is less than the quiz count.
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So what's happening here?
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Well, here's what's happening. When the questionNumber is equal to 2, 2 is less than quiz.count which
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which is 3,
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well, now in this case, questionNumber increases by 1.
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So questionNumber is now 3. And then it goes into the updateUI and it tries to update the UI by getting
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the item at position 3 out of our quiz which is what crashes our app.
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So instead of simply checking if the quizNumber is less than quiz.count, we actually have to increase
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this by 1.
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So now when it gets to questionNumber 2, then it's going to add 1 to it,
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so we never go beyond 2.
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And now if we run our app again,
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you'll see that we've solved our bug and it no longer progresses beyond the last question.
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Here's another challenge for you.
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See if you can change the code to make the quiz restart from the beginning when it reaches the end,
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so you should end up with a never-ending quiz, and it'll look something like this.
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So here we've got our first question,
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our second question,
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third question,
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back to the first question. So pause the video and try and see if you can get this functionality working
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in your app
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All right.
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So how do we achieve this?
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Well, the place where we reach the end of the quiz is, of course, when the questionNumber plus 1 is
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actually equal to the quiz count.
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So how do we catch that scenario?
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Well, we can simply add an "else" statement.
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So if it's gone beyond the number of items we have in our quiz,
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well, then we're simply going to set the questionNumber back down to zero.
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So the next time we update our UI, we fetch the first question out of our quiz again, and that means our
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quiz will loop on and on endlessly at least until the user gets bored. At this point,
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we're showing new quiz questions.
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We're checking the user's answer and our app is no longer crashing.
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So now is a good time to restructure and reorganize our code.
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Currently, one of the biggest eyesores in our code is this 2D Array.
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It's hard to read.
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It's hard to understand, not just for you, for any programmer. So how can we make this better?
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Well, we could use a Swift structure to create a question structure that contains a title property,
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so the title of the question, and then a answer property,
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so the answer of each question.
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So if you know all about Swift structures and you can create a structure to capture our quiz array so
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that we get rid of our 2D Array, and instead have an array of quiz items each with a title property
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and an answer property, then you can probably skip the next Swift Deep Dive.
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But if you've never heard of structures, then I recommend heading over to the next lesson
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where we're gonna do a Deep Dive on that, and more. So the choice is yours and I'll see you on the next
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lesson you choose.