1 00:00:00,570 --> 00:00:04,740 Head over to day 5.1, the average height exercise. 2 00:00:06,000 --> 00:00:09,300 Now the instructions are that you're going to write a program 3 00:00:09,390 --> 00:00:13,470 that's going to calculate the average student height from a list of heights. 4 00:00:13,830 --> 00:00:14,880 So for example, 5 00:00:14,940 --> 00:00:18,420 let's say we had a list of student heights that looks something like this, 6 00:00:18,780 --> 00:00:21,150 then you're going to be using a for loop, 7 00:00:21,420 --> 00:00:24,030 hopefully, to figure out the average. 8 00:00:24,690 --> 00:00:29,160 And the average can be calculated by adding up everybody's height and then dividing 9 00:00:29,160 --> 00:00:31,770 it by the number of students inside the list. 10 00:00:32,370 --> 00:00:35,730 And then I want you to round it to the nearest whole number. 11 00:00:37,620 --> 00:00:38,010 Now, 12 00:00:38,010 --> 00:00:40,350 notice how we've got some starting code here. 13 00:00:40,590 --> 00:00:45,590 And this is just for us to be able to input a list of student heights for testing 14 00:00:46,200 --> 00:00:50,160 and see if your code actually works. So if you go ahead and run the code, 15 00:00:50,430 --> 00:00:54,180 then you'll notice that it asks you for a list of student heights. 16 00:00:54,660 --> 00:00:57,410 And the input is going to look something like this. 17 00:00:57,420 --> 00:01:00,810 They're just going to be a bunch of numbers separated by a space. 18 00:01:01,140 --> 00:01:04,560 So if I go ahead and paste that in there, then I hit enter, 19 00:01:04,890 --> 00:01:09,890 then notice how this line on line five is going to print it out just as a normal 20 00:01:10,380 --> 00:01:13,290 list full of numbers. Now, 21 00:01:13,320 --> 00:01:17,040 Python has some really useful functions as we've seen. 22 00:01:17,100 --> 00:01:22,100 We've seen that we could use the len function to calculate the size of a 23 00:01:22,770 --> 00:01:23,760 particular list. 24 00:01:24,180 --> 00:01:28,440 So if I go ahead and print my length of student heights, 25 00:01:29,490 --> 00:01:32,160 and then I paste my student heights in here, 26 00:01:32,490 --> 00:01:37,080 then you can see that I get five printed by calculating the number of items in 27 00:01:37,080 --> 00:01:41,310 here. Now, Python also has something called sum, 28 00:01:41,820 --> 00:01:46,820 and this gives us the total of all the items from a list. 29 00:01:48,330 --> 00:01:51,840 So in this case, all of these added up would be 857. 30 00:01:52,590 --> 00:01:53,760 Now your job though, 31 00:01:53,790 --> 00:01:58,140 is to figure out how these functions actually work under the hood. 32 00:01:58,680 --> 00:02:01,020 In order to fully complete this challenge, 33 00:02:01,440 --> 00:02:06,180 you can't use the len function or the sum function. 34 00:02:06,510 --> 00:02:11,190 Instead, you have to figure out another way through the use of for loops 35 00:02:11,580 --> 00:02:15,720 where you can add each of the items inside the list together, 36 00:02:16,320 --> 00:02:21,320 and you can use a for loop to figure out how many items there are in this list 37 00:02:22,470 --> 00:02:27,470 and use that to calculate the average. So you can't use either of these 38 00:02:27,510 --> 00:02:28,343 functions 39 00:02:28,560 --> 00:02:32,130 and instead, you have to rely on what you learned in the last lesson to complete 40 00:02:32,130 --> 00:02:36,360 this challenge. So have a think about it, and once you are ready, give that a go. 41 00:02:42,980 --> 00:02:43,250 Now, 42 00:02:43,250 --> 00:02:46,460 we know that we can't use either of these functions, 43 00:02:46,520 --> 00:02:50,330 the sum function or the length function, because if we could, 44 00:02:50,360 --> 00:02:54,620 then this challenge would be super easy and we would be able to calculate the 45 00:02:54,620 --> 00:02:58,910 average by getting the total height, by getting the 46 00:02:59,890 --> 00:03:03,100 sum of all of the items inside student heights. 47 00:03:03,610 --> 00:03:06,610 And then we can calculate the number of students 48 00:03:09,040 --> 00:03:10,930 by using the length function 49 00:03:10,990 --> 00:03:14,620 to calculate the number of items inside the list. 50 00:03:15,070 --> 00:03:16,810 And then we would get the average height 51 00:03:19,000 --> 00:03:22,900 by dividing the total height by the numbers of students. 52 00:03:23,230 --> 00:03:28,230 And then we would be able to round this value off and print it out. 53 00:03:30,520 --> 00:03:34,390 But this, as you noticed, doesn't use a single for loop. 54 00:03:34,750 --> 00:03:37,990 And the last thing we learned about was a for loop. 55 00:03:38,080 --> 00:03:43,080 So this is definitely not the right way to solve the challenge. It works, 56 00:03:43,360 --> 00:03:48,360 but what I wanted you to do is to figure out how the sum and the len functions 57 00:03:48,940 --> 00:03:52,960 work under the hood. How is it actually implemented? 58 00:03:53,560 --> 00:03:56,530 And to do that, we have to use a for loop. 59 00:03:56,920 --> 00:04:00,340 So let's have a think about how we would first tackle this one, 60 00:04:00,940 --> 00:04:05,940 where we get the total of all the items inside student heights. 61 00:04:06,970 --> 00:04:11,970 So we basically want to add each of them together until we get the total. 62 00:04:12,730 --> 00:04:16,840 Now, we can do this by simply using a for loop as you've learned in the last 63 00:04:16,840 --> 00:04:20,650 lesson. So if we start off total_height as zero, 64 00:04:21,160 --> 00:04:26,160 and then we write a for loop and we say for height in student_heights, 65 00:04:29,350 --> 00:04:30,850 and then we add a colon, 66 00:04:31,540 --> 00:04:35,350 then now we're going to loop through each of the heights inside this list 67 00:04:35,620 --> 00:04:37,930 and we're going to add it to our total height. 68 00:04:38,200 --> 00:04:43,200 So we can say total_height equals previous value of total_height plus height. 69 00:04:44,170 --> 00:04:45,730 Or as you might remember, 70 00:04:45,760 --> 00:04:50,740 the shortened version of this is just total height plus equals each of the 71 00:04:50,740 --> 00:04:51,573 heights. 72 00:04:51,790 --> 00:04:56,790 So now we can go ahead and check whether if this works by printing out the final 73 00:04:57,970 --> 00:05:02,410 value of total_height. And notice how I'm doing it outside the loop 74 00:05:02,530 --> 00:05:05,500 by making sure that it's not indented like this. 75 00:05:05,980 --> 00:05:10,980 So let's run our code and I'm going to paste in those numbers as the demo 76 00:05:12,310 --> 00:05:15,340 numbers. And then we get the value 77 00:05:15,340 --> 00:05:20,230 total_height is 857, which if you take a calculator, 78 00:05:20,560 --> 00:05:23,170 you'll be able to verify its exactly right. 79 00:05:23,980 --> 00:05:27,940 So now we've basically replicated how the sum function works. 80 00:05:28,450 --> 00:05:31,900 And now that we can try and replicate how the len function works. 81 00:05:32,380 --> 00:05:32,920 We know that 82 00:05:32,920 --> 00:05:37,630 basically the len function figures out how many items are in a particular list. 83 00:05:38,200 --> 00:05:42,490 How can we do this using a for loop instead? Well, 84 00:05:42,490 --> 00:05:46,360 what if we do something similar to what we did above where we start off the 85 00:05:46,360 --> 00:05:48,340 numbers of students as zero, 86 00:05:48,820 --> 00:05:53,820 and then we write a for loop for students in student heights, and inside our for 87 00:05:55,450 --> 00:05:55,990 loop 88 00:05:55,990 --> 00:06:00,990 all that we do is that we add one to our numbers of students each time 89 00:06:02,600 --> 00:06:06,440 our for loop runs. And if you remember from the last lesson, 90 00:06:06,440 --> 00:06:11,440 I said that the for loop will run for as many times as there are items inside 91 00:06:12,320 --> 00:06:15,440 the list. If there are five items inside the list, 92 00:06:15,740 --> 00:06:18,530 then the for loop is going to run five times, 93 00:06:18,770 --> 00:06:22,220 which means this line of code is going to be called five times, 94 00:06:22,460 --> 00:06:26,450 which means we add five to our numbers of students 95 00:06:26,750 --> 00:06:30,140 and we end up with the total_length of the list. 96 00:06:31,070 --> 00:06:36,070 So now let's go ahead and print this number of students out and run our code. 97 00:06:38,300 --> 00:06:40,310 And I'm going to paste in the same 98 00:06:40,310 --> 00:06:43,580 five numbers in the example input. 99 00:06:44,180 --> 00:06:49,180 And you can see that the first value that comes out is my total height, 100 00:06:49,760 --> 00:06:54,050 the second value is the length of the list, the number of items in the list 101 00:06:54,080 --> 00:06:58,550 which is five, we can verify that. And the final one is the average 102 00:06:58,610 --> 00:07:00,740 which matches up with the example output. 103 00:07:01,310 --> 00:07:05,720 So now let's get rid of our print statements by commenting them out. 104 00:07:08,210 --> 00:07:08,510 Now, 105 00:07:08,510 --> 00:07:12,290 one of the things I want to point out to you that you might've noticed is that 106 00:07:12,620 --> 00:07:17,150 in the first for loop, even though we're looping through the same list, 107 00:07:17,600 --> 00:07:20,150 I've called each of the items height. 108 00:07:20,600 --> 00:07:24,140 And in the second for-loop I've called them students. Now, 109 00:07:24,140 --> 00:07:29,030 whatever we name each of the items inside the list, it's totally up to you. 110 00:07:29,030 --> 00:07:32,240 If you could call it h, you could call it x, 111 00:07:32,270 --> 00:07:34,700 you could call it basically anything you want 112 00:07:35,030 --> 00:07:38,780 as long as you are consistent with using it inside the for loop. 113 00:07:39,260 --> 00:07:42,200 So if you're calling each of the items inside the list 114 00:07:42,320 --> 00:07:45,530 h, then when you're using it inside the for loop, 115 00:07:45,530 --> 00:07:47,660 when you're modifying it or doing something with it, 116 00:07:47,960 --> 00:07:50,870 then you are going to refer to it by the same name. Now, 117 00:07:51,020 --> 00:07:54,380 usually it's a good idea to give it the name 118 00:07:54,620 --> 00:07:59,180 which is the singular form of each of the items in the list. 119 00:07:59,540 --> 00:08:04,520 For example, previously, we had a list of fruits. So each item was called fruit, 120 00:08:04,580 --> 00:08:08,480 the singular form. And in this case, we have a bunch of heights, 121 00:08:08,750 --> 00:08:12,560 so each of them I've called height. In the second for loop 122 00:08:12,740 --> 00:08:17,030 I've decided to call each of the items a student. Now, in this case, 123 00:08:17,360 --> 00:08:21,380 even though you have two for loops, you can call them the same thing. 124 00:08:21,950 --> 00:08:26,360 But sometimes I find it a little bit confusing to have the same name in two 125 00:08:26,360 --> 00:08:29,780 for loops. I prefer it this way, but it's totally up to you. 126 00:08:30,050 --> 00:08:34,850 Just follow the good practice of using a singular form of whatever is contained 127 00:08:34,850 --> 00:08:37,550 in a list when you write a for loop like this, 128 00:08:37,730 --> 00:08:39,320 and you'll have less trouble down the line.