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I n this exercise,

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we're going to be writing a program

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that calculates somebody's Body Mass Index

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from a user's weight and height.

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So if you look in the description pane,

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you'll see a link to the Wikipedia page for BMI.

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And if you click on that, you can learn a little bit more

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about what the Body mass index is and how it's used.

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But essentially, it's a way for us

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to measure someone's weight

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taking into account their height.

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So for example, if a tall person

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and a short person both weighed the same,

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then in most cases, the shorter person is more overweight.

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The formula for calculating BMI

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is dividing the person's weight, in kilograms,

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by the square of their height, in meters.

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This is height multiplied by height,

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and then taking that value

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and dividing the weight by that value.

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Now, if you're not familiar with mathematical notation

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for how the BMI is calculated in the formula

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in the description pane, then feel free to look up in Google

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for alternative ways of representing this formula.

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But it's very important

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that the inputs are in the correct units.

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So in this case, it's metric units, kilograms and meters.

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And the goal is to be able to print out the BMI

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using any potential inputs,

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where the first input is the height

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and the second input is the weight,

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in meters and in kilograms.

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Now, one thing to note

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is that in our tests for this exercise,

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we're expecting the BMI to be printed out as a whole number.

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So think about what you need to do

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in order to make that happen.

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Otherwise, when you click SUBMIT,

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your answer might not match the expected output.

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See if you can create this program.

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You can always Google for a Body Mass Index Calculator

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and check your result against their result

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to figure out if you went wrong anywhere

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or if you need to change your code.

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So good luck, and give that a go.

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We'll go through the solution together on the next slide.

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Now, the first thing we want to do is to convert the numbers

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that we're getting through the inputs

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into numbers that we can work with

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and do mathematical operations on.

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So that means on line 4, you'll see

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that I've created the weight_as_integer variable

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to store the weight input as a whole number.

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So I'm converting, in this case, in the example input 69

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into a number that I can multiply or add and use later on.

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On line 5, I'm converting the height that is coming in

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with height in meters.

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And because none of us have one meter or two meter,

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we're usually somewhere in between the two.

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This is usually represented as a floating point number.

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So there's a decimal point in there.

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For example, I am 1 meter 79,

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and that will be 1.79 meters.

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In this case, we're going convert that input

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into a different data type,

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one that can store decimal places,

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and that is the "float data type".

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So we wrap the float around the height variable,

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and we can turn that input 1.63 in the sample input

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into a floating point number,

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which again can be used in our calculations later on.

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We can calculate the BMI using various methods.

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One method and probably the shortest method,

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but this would involve a little bit of Googling around,

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is to figure out how to use the exponent operator (**) in Python.

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Now, we haven't talked about this,

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but it's good to get used to the idea

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that there are multiple ways of doing everything.

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And just because you did it differently

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doesn't mean it's wrong.

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So I wanted to present you with many ways.

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Now, the exponent operator is equivalent to a math

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where you see that super scripted number,

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so the little number at the top.

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In this case, we need to square the height,

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so that means multiplying the height by itself,

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or in math, this is called raising the height

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to a power of two.

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And we can do that in Python

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using the double asterisk operator.

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So height as a float, two asterisks,

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and then the number two (height_as_float **2) will square the height.

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And then all we have to do

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is do weight divided by height squared,

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and we end up with our BMI.

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An alternative way of doing this is by using parentheses.

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You might remember from high school maths,

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in the US it's called PEDMAS

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and in the UK it's called BODMAS,

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but brackets always come before any other operation.

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In this case, we can't simply just write

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weight divided by height times height,

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because the first operation that's going to be calculated

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is weight divided by height,

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and it's going to give us erroneous answers.

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But by wrapping height times height in a set of parentheses,

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we ensure that part of the operation gets carried out first,

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and then we take the result

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and then we divide weight by the result.

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This is more likely the way that you might have solved this,

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but you might have had an error

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because you forgot about the parentheses.

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So this is something to note.

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Finally, we can convert the BMI into a whole number,

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so rounding it if necessary,

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and then we print it out as the solution.

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And this will allow us to pass the tests

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in the previous slide.