WEBVTT 0 00:00.410 --> 00:07.250 Here's a question, have you ever sat in your bath and wondered, why is it that no matter how forgetful 1 00:07.250 --> 00:10.550 I am, my bath never overflows like this? 2 00:10.550 --> 00:15.710 This is something I was thinking about the other day, and I realized that it's not because I'm particularly 3 00:15.710 --> 00:16.280 diligent, 4 00:16.280 --> 00:18.620 I forget stuff all the time. 5 00:18.620 --> 00:20.480 I mean pizza, anyone? 6 00:21.290 --> 00:23.570 It's what my pizzas usually look like. 7 00:23.750 --> 00:31.960 But the reason why our bathtub or the sink doesn't overflow is because of this fantastic piece 8 00:31.960 --> 00:34.450 of engineering, the overflow. 9 00:34.480 --> 00:43.180 So this means that whenever the water reaches beyond a certain level about here, the water starts overflowing 10 00:43.180 --> 00:47.830 so that the bathtub doesn't overflow and annoy your neighbors downstairs. 11 00:48.280 --> 00:53.350 In fact, we could represent this mechanism with a conditional statement. 12 00:53.410 --> 01:00.790 When the water level is, say, greater than 80cm, then it should drain the water. 13 01:00.790 --> 01:07.480 But if the water level is not greater than 80cm, it should continue filling up the tub. 14 01:07.510 --> 01:12.790 This type of conditional statement is known as an if / else statement. 15 01:12.790 --> 01:21.290 Depending on a particular condition, we would do either A or B, and when we want to write Python code 16 01:21.290 --> 01:24.050 to represent this, it looks something like this. 17 01:24.050 --> 01:29.180 There's the keyword, if, and then the condition that we're testing for, 18 01:29.180 --> 01:31.100 and then a colon. 19 01:31.100 --> 01:38.270 And after the colon we've got an indented block of code which should be executed if this condition is 20 01:38.270 --> 01:38.570 met, 21 01:38.570 --> 01:46.180 if it's true, but if it's not true, then we will skip to the else block and it's just the else keyword 22 01:46.180 --> 01:47.410 with a colon. 23 01:47.410 --> 01:52.330 And then this code block would execute if the condition is false. 24 01:52.540 --> 01:58.060 So we could represent that previous bathtub situation with code that looks a bit like this. 25 01:58.060 --> 02:00.940 Let's say our water level is at 50cm, 26 02:00.940 --> 02:06.010 and then we would test if the water level is greater than 80cm. 27 02:06.010 --> 02:10.060 Well, if that is the case, then we should drain the water, 28 02:10.060 --> 02:13.060 but if it's not greater than 80cm, 29 02:13.060 --> 02:14.440 in other words, else. 30 02:14.440 --> 02:18.040 Well, in this case, we should just continue filling up the bathtub. 31 02:18.040 --> 02:21.670 Let's put this into practice with a real life problem. 32 02:21.670 --> 02:28.210 Now, let's say that you've gotten a job at a theme park, and your first job of the day is to write 33 02:28.210 --> 02:31.490 some code that replaces the ticket box. 34 02:31.700 --> 02:34.700 Now there's a couple of things that you'll need to think about. 35 02:34.700 --> 02:41.180 Firstly, in order for somebody to actually purchase a ticket to go on the roller coaster ride, they 36 02:41.180 --> 02:44.240 will need to be over 120cm. 37 02:44.330 --> 02:50.240 So we have to check what their height is, because if they're too short, then we won't be able to sell 38 02:50.240 --> 02:51.740 them a ticket anyways. 39 02:52.040 --> 02:59.560 In the Course resources I've included a link to this flow chart that I've created on Draw.io. 40 02:59.590 --> 03:05.290 This is a really, really useful tool for creating any sort of flow charts or diagrams, and it's really 41 03:05.290 --> 03:06.430 easy to use. 42 03:06.610 --> 03:13.990 Now, if we take a look at this flow chart, this is basically the logic that we have to program using 43 03:13.990 --> 03:16.030 our if and else statements. 44 03:16.060 --> 03:25.440 If the person who is trying to purchase a ticket is not over 120cm, then they can't ride on the roller 45 03:25.440 --> 03:26.220 coaster, 46 03:26.220 --> 03:30.690 but if their height is greater than 120cm, then they can ride. 47 03:30.690 --> 03:32.460 So let's try it out. 48 03:32.460 --> 03:38.820 All that I've got here is a print statement that says, "Welcome to the roller coaster!" as well as an 49 03:38.820 --> 03:46.430 input asking the user for their height in centimeters and then converting the string into a whole number, 50 03:46.430 --> 03:50.750 an integer, and then I'm storing it inside this variable called height. 51 03:50.960 --> 03:57.560 Now we're going to check whether if the height that the user is typed in is greater than 120. 52 03:57.560 --> 04:04.430 So we use the keyword if and then we check if the height is greater than 120, 53 04:04.430 --> 04:06.560 and then we add the colon. 54 04:06.680 --> 04:13.930 And now when I hit Enter, you'll notice that the code editor has automatically indented me slightly 55 04:13.930 --> 04:14.530 over. 56 04:14.530 --> 04:22.390 I'm not over here, because in Python the spacing and indentation is really, really important. 57 04:22.390 --> 04:29.650 It tells the computer that the code that I'm about to write is what should be executed when this condition 58 04:29.650 --> 04:30.670 is met. 59 04:30.790 --> 04:34.650 So what should happen if your height is over 120? 60 04:34.680 --> 04:36.570 Then we will just print, 61 04:36.570 --> 04:38.820 "You can ride the roller coaster." 62 04:38.970 --> 04:44.070 But what should we print if height is not greater than 120? 63 04:44.220 --> 04:50.250 Well, in this case we would use the else statement to catch when that happens. 64 04:50.250 --> 04:57.410 And it's really important that you don't write the else here because this is indented over. 65 04:57.560 --> 05:03.380 Instead, you want it to be at the same indentation level as the if statement. 66 05:03.410 --> 05:06.680 These two are essentially a pair if and else. 67 05:06.770 --> 05:12.170 Now after the else keyword, we again add a colon, and then we hit enter. 68 05:12.170 --> 05:13.640 And we're now indented again. 69 05:13.640 --> 05:19.790 And here we can write the code that should happen if this condition is false. 70 05:20.410 --> 05:21.010 "Sorry, 71 05:21.010 --> 05:22.870 you have to grow taller before you can ride." 72 05:23.410 --> 05:31.510 So the really important things here are the condition which we're testing for, is the value of height 73 05:31.510 --> 05:33.610 greater than 120. 74 05:33.790 --> 05:36.130 The syntax of this code. 75 05:36.130 --> 05:42.850 So the keywords if and else, as well as the colons that come after each of these lines. 76 05:43.240 --> 05:46.360 And finally also the indentation. 77 05:46.420 --> 05:52.420 Everything that is indented after the if is a block of code. 78 05:52.510 --> 05:59.920 So this is indented and it's effectively inside this if. 79 06:00.040 --> 06:04.630 So this line of code lives inside the else statement. 80 06:04.630 --> 06:08.620 This block of code lives inside the if statement. 81 06:09.030 --> 06:15.630 and if you mess up on the indentation, then you're probably going to get an IndentationError telling 82 06:15.630 --> 06:20.310 you that this line five probably should be indented. 83 06:20.310 --> 06:28.140 But when we do actually correct it and we hit Run, let's say that our height is 130cm, 84 06:28.140 --> 06:29.580 then we get back, 85 06:29.580 --> 06:38.000 "You can ride the roller coaster." But if our height was, say, 90cm, then we get a different outcome. 86 06:38.000 --> 06:40.910 We get, "Sorry you have to grow taller before you can ride." 87 06:41.000 --> 06:48.140 So by using if and else statements, we're able to get our code to do different things, either printing 88 06:48.140 --> 06:54.080 this line or printing this line depending on a condition that we're testing for. 89 06:54.230 --> 07:01.960 Now, when we use this greater than sign (>) effectively, what we're saying is that is the height greater 90 07:01.960 --> 07:07.840 than 120, which means that it does not include 120. 91 07:07.990 --> 07:15.520 In fact, if I run this code and if I type in my height as 120, then it actually goes into the else 92 07:15.520 --> 07:17.710 block and prints this. 93 07:17.920 --> 07:27.690 So if we wanted to include 120cm so that all the people who are exactly 120cm can ride the roller coaster, 94 07:27.690 --> 07:33.240 then instead of just using a greater than symbol, we have to write greater than or equals (>=). 95 07:33.240 --> 07:36.390 So these two symbols have to be next to each other. 96 07:37.020 --> 07:44.790 And now when I run my code and I write 120, you'll see that it's now falling into this block of code. 97 07:44.790 --> 07:47.360 And it's telling me that I can ride the roller coaster. 98 07:47.900 --> 07:52.940 Now, these are called comparison operators, and we've already seen Greater Than. 99 07:52.940 --> 07:59.810 So Lesser than is pretty self-explanatory, but we've also seen greater than or equal to and lesser 100 07:59.810 --> 08:00.980 than or equal to. 101 08:01.010 --> 08:07.400 So if you want to include a particular number or particular value when you do these comparisons, you 102 08:07.400 --> 08:09.320 would be using these instead. 103 08:09.470 --> 08:14.570 Now in this table there's also the equal to and not equal to. 104 08:14.990 --> 08:22.820 For example, if you wanted to check if somebody's height is equal to precisely 120, then you would 105 08:22.820 --> 08:25.130 use two equal signs. 106 08:25.130 --> 08:30.770 And very often it gets a little bit confusing, especially if you're new to programming when you're 107 08:30.770 --> 08:32.120 typing equal signs. 108 08:32.120 --> 08:36.030 Because sometimes we're typing one and other times we're typing two. 109 08:36.060 --> 08:42.540 It's important to remember that when you have one equal sign, it means that you're assigning this value 110 08:42.540 --> 08:44.460 to this variable. 111 08:44.460 --> 08:50.610 But when you have two equal signs, you're checking to see if the value on the left is equal to the 112 08:50.610 --> 08:52.020 value on the right. 113 08:52.020 --> 08:53.850 And they're completely different. 114 08:53.880 --> 09:00.440 Now, the good thing is that when you get it wrong, usually you'll get enough clues in the error to 115 09:00.440 --> 09:05.150 actually hint to you, hey, maybe there's something that's wrong here, right? 116 09:05.150 --> 09:07.280 Because the syntax doesn't look right. 117 09:07.280 --> 09:12.680 For this to be a condition, it has to be something that evaluates to true or to false. 118 09:12.680 --> 09:16.490 And height equals 120 does not evaluate to either. 119 09:16.490 --> 09:23.860 So by changing it to this we're saying if height is equal to 120 then we'll execute this line of code 120 09:23.920 --> 09:27.100 or any other lines which are in the same block of code. 121 09:27.100 --> 09:33.340 But if the height is not equal to 120, then we're going to execute this block of code. 122 09:33.370 --> 09:36.850 So if I write 120 then that works. 123 09:36.850 --> 09:42.040 But if I write 121 then it does not work. 124 09:42.490 --> 09:47.710 Similarly, you can also check for not equals to which just flips it around. 125 09:47.710 --> 09:56.290 But in our case it makes sense to say if the height is greater than or equals to 120, then you can 126 09:56.290 --> 10:04.330 ride the roller coaster, but otherwise, you cannot ride the roller coaster. By using if and else statements, 127 10:04.330 --> 10:11.530 we can get our computer to do different things and respond differently depending on different conditions. 128 10:11.530 --> 10:15.200 So in the next lesson, I've got a coding exercise for you, 129 10:15.230 --> 10:16.250 I'll see you there.