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In the last lesson, we started working with UISliders.
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In this lesson, we'll use the slider values to calculate the BMI.
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What if we wanted to get a hold of our slider values outside of these IBActions where we have access
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to the sender where we're getting that sender.value from,
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how might we do that? So if I go ahead and create IBAction from this calculate button and put
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it right at the bottom of my View Controller,
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so let's call that the calculatePressed, and change the type to UIButton, click Connect.
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Now, I've got this IBAction which gets triggered whenever the user taps on the calculate button.
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Now, how can we get hold of the weight and height inside these curly braces? Because this sender is, of
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course, the button that triggered the IBAction, whereas this sender is the slider that triggered this
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IBAction, and they are separate from each other. One way of doing this, which is probably the easiest
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way, is to simply create some IBOutlets from these sliders as well.
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We've got our IBActions which gets triggered when these are moved, but we can also create a IBOutlet
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from the height slider which I'll simply just call heightSlider, and another IBOutlet from our weight
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slider which is gonna be called weightSlider. And now because we have these variables to play with, our
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heightSlider and our weightSlider which are linked to the Interface Builder Sliders. Well, inside our
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calculatePressed, we could quite easily get hold of the heighSlider.value and the weightSlider.value
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And if we run our app, you can see that when I move these sliders and click calculate, you
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can see the original floating point values from each of these sliders being printed.
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So now that we have the height and the weight, let's set this to a height constant, and down here, let's
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create a a weight constant.
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How can we use these two pieces of data and calculate the body mass index? Here's the equation for calculating
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the body mass index or BMI.
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We take the weight in kilograms and we divide it by the height in meters squared, so height times height.
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Now, I want you to try and be able to print out the BMI based on the weight and height that we got from
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the sliders, and calculate it and print it into the console
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based on this equation. So pause the video and try to complete this challenge.
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Okay, so we have our height and we have our weight,
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we just need to implement the equation, so that we can create the BMI.
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So I'm going to create a constant called BMI and it's going to be equal to the weight divided by the
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height squared.
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Now, there's two ways of doing this.
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You can either do the simple way where we just write height times height because after all, that is what
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height squared does at the end of the day.
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But the other way that you could do this is use a custom Swift method.
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So you might have come across this
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if you Google for "How to square a number using Swift." But there is a function called the power function
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which takes two inputs. The first, the number that you want to raise to a particular power.
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And the second, the actual exponent.
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So in this case, if we wanted to get the height squared, we would write "pow,"
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and then the first input would be the height which is the value we want to raise to a power,
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and then the second value is the power that we want to raise it to which is 2.
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So this line of code is effectively exactly the same as this part of our equation where we raise our
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height to the power of 2. And now we can print our BMI into the console. And you can see, if I put in my
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height: 1.8, and my weight: 63, and hit calculate,
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you can see my body mass index printed here.
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The thing that you have to be careful about is that in programming, as with maths, it follows something
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called BODMAS which defines the order that the calculations are completed.
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So the first thing that happens are anything that's in a bracket and then anything that's an exponent
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or a root,
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and then we have our division or multiplication,
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and finally, our addition or subtraction. So you might have written the code as a weight divided by height
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times height.
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Now, in this case, what happens is that the first calculation: weight divided by height, actually gets carried
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out first, instead of what we need which is the height squared,
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so height times height. And if you run the code as it is right now, you'll see that if I put in my height
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and weight again,
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instead of getting something around 19, I'm actually getting 62 which is extremely, morbidly obese
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which would shock somebody if they used your app that was using this incorrect equation.
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So we have to put in those brackets around the height times height if we want to write our equation
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like this, so that the brackets are carried out first before any of the other operations. Now that we've
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managed to get our slider to work to be able to put input into the slider, to change and update our label,
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and use it to calculate the BMI,
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well, what else is there left in this module?
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Don't worry there's still plenty of things coming up. And in the next lesson, we're going to learn about
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a really exciting topic which are classes. For all of that and more,
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I'll see there.