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All right.
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So we're going to continue building our calculator but, firstly, make sure that you've deleted that
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Test.swift file if you are following along with me and you created it as well.
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We don't need it and we definitely don't want it
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modifying our isFinishedTypingNumber variable.
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So, currently, our app works, namely, that it can accumulate numbers, but it doesn't actually do anything,
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right?
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When I say something like, you know, 12+3 =
Nothing happens. It doesn't do any calculations
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yet because we haven't told it to.
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So let's go ahead and add in that logic
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and that functionality to make our calculator work.
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So if I pull up the calculator that's on my Mac, you can see that we've got three buttons up here.
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And the first button simply clears the board.
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It turns everything to zero.
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The second button will turn whatever it is that I've got in the display to the negative version of it.
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So if it was 2, then it becomes minus 2.
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But if it was minus 2, then it becomes 2.
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And the third one turns everything into a percentage,
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so it divides it by 100.
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So 90 will become 0.9, et cetera.
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So these are a little bit simpler because they only need one number to be able to execute the functionality,
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whereas these four are a little bit more complicated because we need to be able to say 5, then we hold
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that number in memory, and we press one of these buttons 5+
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then now we've got somewhere which has got five stored.
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We've got the operation plus stored and we now have to add another number 2, and then we press the equal
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sign, and it does all of that calculation.
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So these four are a little bit more complicated than these three.
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So let's first tackle these three.
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So the place that we want to do this is inside our calcButtonPressed IBAction because what we want
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to happen is to, say, for example, if I typed 10, and then I pressed the percentage sign, then this IBAction
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will get triggered, and I want to be able to turn that 10 into 0.1, so divided by 100.
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So first things first, I need to get the value that's inside my label here as a number, as a double, ideally,
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because we'll be working with decimal places later on as well.
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So let's say let number = displayLabel.text,
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so now I'm taking whatever is currently in the display label
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when I pressed one of these buttons and I'm saving it to a constant code number.
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Now, this number is a local variable at the moment already.
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So it's only accessible inside the scope of the calcButtonPressed,
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so I don't have to also add a private in front of it.
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However, if I was creating this in the global scope,
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so if you're confused about global and local, make sure you have a look back at the video on scope.
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We spoke about this in more detail. But, essentially, we don't have to turn any local variables into private
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vars because it's already limited in scope and only accessible inside the block where it was declared.
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So, now that I've got that number saved, I want to be able to turn it into a double. Because, currently,
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if I hold down alt and click on number, you can see it's just a string,
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right?
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Because it's set to the text that's inside the displayLabel.
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So this will be currently equal to, essentially, the string 3
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and I want to turn it into the double 3.0. So I can do that by just doing a simple cast. So I can say Double
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and then I can wrap whatever it is that I want to cast into a double inside some parentheses.
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So, now if I check number, again, you can see it's now a Double?
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But it's not just a regular double, it's actually an optional double.
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And the reason for that is because if I'm turning text into number, there's a lot of cases where that's
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not possible.
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So, for example, if I tried to turn the string "A" into a Double, then what is that going to be?
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I have no idea and nor does the computer.
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So that's a case where this could fail this casting process.
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So that's why when you cast a string to a double, it will always produce a optional Double.
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Now, the opposite case,
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Let's say, we try to create a string, right?
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And we set that to equal String casting, something that was a number, say, 12, this is going to produce
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a normal String, a bog-standard string because when you cast from number to string, it's always possible.
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All it does is add some quotation marks around it and, bam, you've got a String.
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But the opposite fails occasionally.
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So, now we have this optional number which makes it very difficult to work with.
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So we want to either unwrap it or we have to do something with it, right?
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Now, one option is to simply unwrap this by putting a exclamation mark at the end of this group.
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So, now what this says is that I'm definitely sure that there is text inside the displayLabe, and that
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makes sense,
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right?
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Because even if we don't put in numbers, it's still got a zero in there.
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So that's a valid unwrap.
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So next, we're turning it into a double and we're saying that it's always going to work, namely whatever
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is in here is always going to be able to be turned into a double.
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And this is also a valid assertion because where does the number here come from,
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well, it comes from whatever I do in here.
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And all of this code only tries to put the numValue which is the current title of the sender,
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so one of the number buttons, any of these ones into here. And they can all be turned into a number.
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So if you were to force unwrap this, it's also not exactly wrong, because the logic is kind of valid.
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Now, the only problem is if you screw up on your linkages,
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so your IBOutlets and your IBActions.
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So there's many ways in which this can actually fail.
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Now, failing and crashing your app is not necessarily a bad thing because it allows you to figure out
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what other things are unexpected, and for you to test your app, and also to address all of these unexpected
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scenarios.
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Now, what you don't want is to, say, use an "if let" and just not catch this error
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if it does happen because it'll skip the code instead.
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So let's say that if instead of linking all of the number buttons to this numButtonPressed, I accidentally
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also linked one of the text buttons to it.
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So let's just delete the current linkage to calcButtonPressed and let's add in a new link to
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numButtonPressed instead.
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And now this IBAction is linked to all of the numbers as well as this button.
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So that means that when I press this button, it's going to try and put that into the displayLabel, and
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then, later on, it's going to try and turn that text into a Double which will fail.
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So let me show you what will happen.
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So at this stage when I type numbers and I press any of these calc buttons, nothing happens.
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It's all working fine. But if I press this button and the string gets put into the display label, when
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I press any of these calc buttons, our app is going to crash and it's going to show us that the line
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that caused this crash is this one,
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and the problem is a "Fatal error: Unexpectedly found nil while unwrapping an optional."
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Now, the optional here is this one Double question mark, and it's unwrapped, because we assumed that it
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would always be possible to turn this into a Double.
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Now, because of what I've just done just now, my little mishap,
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well, that's not possible anymore.
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Plus, minus does not become a number.
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So you might think, well, you know that means that we should never force unwrap anything because it's
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always bad,
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and instead, we should use maybe something like optional binding, right? Use an "if let."
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Well, I would argue that actually using an "if let" is not always the best option
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and sometimes you do want your code to crash, especially when you're testing it and when you're developing
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it.
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The reason for that is because when it crashes, it alerts you and you are forced to figure out why it
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crash and what caused it.
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Now, let's say I didn't want to force unwrap this.
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What are the other options that I have in my arsenal?
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Well, let's see.
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Let's try and use this optional, right?
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Let's try number + 2
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and I'm going to use command B to build.
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Now, I'm going to get a build failed and if I click on this little button here, it tells me that value
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of optional type 'Double?' must be unwrapped to a normal double
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in order to use it, for example, in this calculation.
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Now, there's two options that it's giving me.
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One is to force unwrap using the exclamation mark which is what we did previously,
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and if you click on this fix, you can see that it will force unwrap this number, or you can also force
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on unwrap over here.
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Now, the other option that is giving me is to use what's called a nil-coalescing operator using two
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question marks to provide a default when the optional value contains nil.
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So in this case, what I can do is if I can turn this string into a Double, then I can put two question
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marks and say, well, in that case, instead of giving back nil, let's just give 0.
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So now let's retest our app.
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And, again, I'm going to put the +/- sign into the displayLabel.
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I'm going to press one of the calculation buttons and you can see nothing happens.
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My app doesn't crash and it just continues, and it just continues along as if nothing happened because
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I managed to set that number to zero.
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So I would argue that this is actually worse than if you actually let your app crash when it's doing
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something unexpected.
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Now, when we leave this as a forced unwrap, it can be a little bit difficult to figure out why it crashed
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when our string can't be converted into a double.
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So an alternative here is to instead use a "guard."
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So we're saying guard let number = Double.
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And if this conversion process from string to double can't happen, then we're going to say, else throw
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a fatalError.
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And there's two types of fatalError where you've got the simple fatalError and it stops your app from
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executing, namely crashes your app,
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or you can have the one where you can put in your own message.
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So that's what we're going to use.
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So if we can't convert our displayLabel.text to a Double successfully and turn this into
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a nonoptional Double, then we're going to say, "Cannot convert display label text to a Double."
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So, now when our app crashes, we will get a far more explicit error message,
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and this means that three months down the line when we can't actually remember when anything does in our
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app any more, then if we typed this into the displayLabel and tried to convert it into a double, then
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our app crashes,
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and also we get a "Fatal error: Cannot convert display label text to a Double"
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which should hint to us that, oh, this must be related to this part, right?
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So, now that we've tested this and we fixed it,
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let's make sure that we don't persist this error.
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So let's delete that connection to numButtonPressed, and instead, make sure that we connect it again
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by holding down the control, and clicking and dragging, and we're going to connect it to calcButtonPressed,
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so that it'll be addressed in the calcButtonPressed IBAction.