Let's walk through a complete example, from problem description, through problem solving to code and testing.

Just read along and understand. Do not $"$do$"$ any of this until you are asked. Read it carefully and make sure you understand.

Problem Description:

Firstly, we understand that for some people, things like body image, weight and health can be sensitive issues.

There is no intention for the following scenario to be taken personally or thought of in any way that might cause concern.

It is a useful scenario for the purpose of breaking a problem into functions, which is why it is used.

According to cancer.org:

BMI is used to broadly define different weight groups in adults $20$ years old or older. The same groups apply to both men and women.

BMI $($Body Mass Index$)$ can be calculated by the formula:

weight in kg $/($height in m $**2)$

Remember $"**2"$ means $"$to the power of $2"$

The rough weight group categories defined by BMI are:

Underweight: BMI is less than $18\mathrm{.}5$

Normal weight: BMI is $18\mathrm{.}5$ to $24\mathrm{.}9$

Overweight: BMI is $25$ to $29\mathrm{.}9$

Obese: BMI is $30$ or more

This list of categories was taken from the cancer.org website, but we understand boundary conditions and should write our conditions in a better way. E$.$g$.,$ what category should a BMI of $29\mathrm{.}95$ be$?$

The client wants a program that will ask for a person's height and weight, then tell them their BMI and weight category.

Algorithm

When we look at the problem, we can break it up into a number of main sections. This is decomposition.

get inputs $($making sure they're valid$)$

calculate BMI

determine weight category

Each of these can be implemented using functions.

Our main program algorithm will call the other functions.

We should notice that getting the weight and getting the height are very similar tasks that could be done with almost no difference, so this is a good situation for a reusable function.

Like we did in the lecture with get$\_$valid$\_$age, we'll pass in parameters to define the low and high bounds of the input.

Unlike that function, we'll also pass in the prompt so we can customise the function's print$/$output$.$

function main$()$

height $=$ get$\_$valid$\_$number$("$Height$",0,3)$

weight $=$ get$\_$valid$\_$number$("$Weight$",0,300)$

bmi $=$ calculate$\_$bmi$($height$,$ weight$)$

category $=$ determine$\_$weight$\_$category$($bmi$)$

print bmi, category

At this point, we have an algorithm that represents the whole program, but does not define any of the details. This is useful, but incomplete.

Let's define the algorithms for each of the functions now.

For the get$\_$valid$\_$number function, we should see that this uses the normal error$-$checking while loop pattern.

When a valid number has been entered, we "return" it$.$

function get$\_$valid$\_$number$($prompt$,$ low, high$)$

print prompt

get number

while number $<$ low or number $>$ high

print error, prompt

get number

return number

For the calculate$\_$bmi function, we should see that this is a simple calculation$/$processing step.

When a result has been calculated, we return it$.$

function calculate$\_$bmi$($height$,$ weight$)$

return weight $/($height $**2)$

The job of the determine$\_$weight$\_$category function is not to print, but to return the category that has been determined by the function.

Upgraded pattern with functions

For a situation like this, we want a single, mutually exclusive result, so it seems fine to use the usual if$-$elif$-$else decision structure:

function determine$\_$weight$\_$category$($bmi$)$

if bmi $<18\mathrm{.}5$

return underweight

else if bmi $<25$

return normal

else if bmi $<30$

return overweight

else

return obese

However, since we are now using a function, the "else" is redundant. If any one of these conditions is True, the function returns, so we know it will only get to the next condition if the previous one was False.

If we run pylint to check the code for the above algorithm, we would get the message:

Unnecessary "else" after "return", remove the "else" and de$-$indent the code inside it $($no$-$else$-$return$)$

So$,$ let's "upgrade" this pattern, noting that this ONLY makes sense for use within a function because of how return works:

function determine$\_$weight$\_$category$($bmi$)$

if bmi $<18\mathrm{.}5$

return underweight

if bmi $<25$

return normal

if bmi $<30$

return overweight

return obese

We could "desk check" each of these pseudocode functions.

E$.$g$.,$ if we put a BMI value of $27$ into the determine$\_$weight$\_$category function, we should get "overweight".

Code

When it comes to coding functions, you can either write main first and then each function $($top$-$down$),$ or write each function and then put it all together in main $($bottom$-$up$).$

There's no right answer, but we're going to do it bottom$-$up and test our functions one at a time before putting them all together.

Look at how very similar the Python code is to the algorithm for the first function we'll write. This would be ne