Original subtitles

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Head over to day 5.1, the average height exercise.

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Now the instructions are that you're going to write a program

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that's going to calculate the average student height from a list of heights.

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So for example,

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let's say we had a list of student heights that looks something like this,

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then you're going to be using a for loop,

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hopefully, to figure out the average.

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And the average can be calculated by adding up everybody's height and then dividing

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it by the number of students inside the list.

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And then I want you to round it to the nearest whole number.

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Now,

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notice how we've got some starting code here.

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And this is just for us to be able to input a list of student heights for testing

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and see if your code actually works. So if you go ahead and run the code,

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then you'll notice that it asks you for a list of student heights.

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And the input is going to look something like this.

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They're just going to be a bunch of numbers separated by a space.

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So if I go ahead and paste that in there, then I hit enter,

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then notice how this line on line five is going to print it out just as a normal

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list full of numbers. Now,

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Python has some really useful functions as we've seen.

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We've seen that we could use the len function to calculate the size of a

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particular list.

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So if I go ahead and print my length of student heights,

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and then I paste my student heights in here,

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then you can see that I get five printed by calculating the number of items in

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here. Now, Python also has something called sum,

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and this gives us the total of all the items from a list.

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So in this case, all of these added up would be 857.

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Now your job though,

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is to figure out how these functions actually work under the hood.

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In order to fully complete this challenge,

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you can't use the len function or the sum function.

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Instead, you have to figure out another way through the use of for loops

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where you can add each of the items inside the list together,

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and you can use a for loop to figure out how many items there are in this list

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and use that to calculate the average. So you can't use either of these

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functions

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and instead, you have to rely on what you learned in the last lesson to complete

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this challenge. So have a think about it, and once you are ready, give that a go.

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Now,

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we know that we can't use either of these functions,

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the sum function or the length function, because if we could,

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then this challenge would be super easy and we would be able to calculate the

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average by getting the total height, by getting the

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sum of all of the items inside student heights.

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And then we can calculate the number of students

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by using the length function

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to calculate the number of items inside the list.

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And then we would get the average height

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by dividing the total height by the numbers of students.

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And then we would be able to round this value off and print it out.

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But this, as you noticed, doesn't use a single for loop.

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And the last thing we learned about was a for loop.

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So this is definitely not the right way to solve the challenge. It works,

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but what I wanted you to do is to figure out how the sum and the len functions

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work under the hood. How is it actually implemented?

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And to do that, we have to use a for loop.

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So let's have a think about how we would first tackle this one,

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where we get the total of all the items inside student heights.

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So we basically want to add each of them together until we get the total.

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Now, we can do this by simply using a for loop as you've learned in the last

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lesson. So if we start off total_height as zero,

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and then we write a for loop and we say for height in student_heights,

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and then we add a colon,

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then now we're going to loop through each of the heights inside this list

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and we're going to add it to our total height.

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So we can say total_height equals previous value of total_height plus height.

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Or as you might remember,

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the shortened version of this is just total height plus equals each of the

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heights.

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So now we can go ahead and check whether if this works by printing out the final

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value of total_height. And notice how I'm doing it outside the loop

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by making sure that it's not indented like this.

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So let's run our code and I'm going to paste in those numbers as the demo

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numbers. And then we get the value

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total_height is 857, which if you take a calculator,

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you'll be able to verify its exactly right.

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So now we've basically replicated how the sum function works.

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And now that we can try and replicate how the len function works.

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We know that

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basically the len function figures out how many items are in a particular list.

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How can we do this using a for loop instead? Well,

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what if we do something similar to what we did above where we start off the

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numbers of students as zero,

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and then we write a for loop for students in student heights, and inside our for

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loop

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all that we do is that we add one to our numbers of students each time

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our for loop runs. And if you remember from the last lesson,

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I said that the for loop will run for as many times as there are items inside

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the list. If there are five items inside the list,

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then the for loop is going to run five times,

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which means this line of code is going to be called five times,

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which means we add five to our numbers of students

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and we end up with the total_length of the list.

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So now let's go ahead and print this number of students out and run our code.

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And I'm going to paste in the same

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five numbers in the example input.

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And you can see that the first value that comes out is my total height,

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the second value is the length of the list, the number of items in the list

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which is five, we can verify that. And the final one is the average

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which matches up with the example output.

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So now let's get rid of our print statements by commenting them out.

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Now,

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one of the things I want to point out to you that you might've noticed is that

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in the first for loop, even though we're looping through the same list,

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I've called each of the items height.

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And in the second for-loop I've called them students. Now,

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whatever we name each of the items inside the list, it's totally up to you.

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If you could call it h, you could call it x,

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you could call it basically anything you want

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as long as you are consistent with using it inside the for loop.

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So if you're calling each of the items inside the list

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h, then when you're using it inside the for loop,

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when you're modifying it or doing something with it,

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then you are going to refer to it by the same name. Now,

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usually it's a good idea to give it the name

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which is the singular form of each of the items in the list.

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For example, previously, we had a list of fruits. So each item was called fruit,

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the singular form. And in this case, we have a bunch of heights,

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so each of them I've called height. In the second for loop

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I've decided to call each of the items a student. Now, in this case,

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even though you have two for loops, you can call them the same thing.

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But sometimes I find it a little bit confusing to have the same name in two

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for loops. I prefer it this way, but it's totally up to you.

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Just follow the good practice of using a singular form of whatever is contained

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in a list when you write a for loop like this,

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and you'll have less trouble down the line.