1 00:00:00,340 --> 00:00:04,180 Here's a question. Have you ever sat in your bath and wondered, 2 00:00:04,540 --> 00:00:07,750 why is it that no matter how forgetful I am, 3 00:00:08,140 --> 00:00:10,450 my bath never overflows like this? 4 00:00:10,690 --> 00:00:14,230 This is something I was thinking about the other day and I realized that it's 5 00:00:14,260 --> 00:00:19,180 not because I'm particularly diligent. I forget stuff all the time. I mean, 6 00:00:19,300 --> 00:00:24,190 pizza, anyone? This is what my pizzas usually look like. Um, 7 00:00:24,490 --> 00:00:29,490 but the reason why a bathtub or the sink doesn't overflow is because of this 8 00:00:30,490 --> 00:00:34,270 fantastic piece of engineering, the overflow. 9 00:00:34,810 --> 00:00:39,810 So this means that whenever the water reaches beyond a certain level about here, 10 00:00:40,810 --> 00:00:43,240 the water starts overflowing 11 00:00:43,270 --> 00:00:47,740 so the bathtub doesn't overflow and annoy your neighbors downstairs. 12 00:00:48,340 --> 00:00:53,110 In fact, we could represent this mechanism with a conditional statement. 13 00:00:53,680 --> 00:00:57,820 When the water level is say greater than 80 centimeters, 14 00:00:58,240 --> 00:01:00,670 then it should drain the water. 15 00:01:01,030 --> 00:01:04,420 But if the water level is not greater than 80 centimeters, 16 00:01:04,599 --> 00:01:07,240 it should continue filling up the tub. 17 00:01:07,660 --> 00:01:12,520 This type of conditional statement is known as an if/else statement. 18 00:01:13,000 --> 00:01:15,340 Depending on a particular condition, 19 00:01:15,580 --> 00:01:18,460 we would do either A or B. 20 00:01:18,910 --> 00:01:22,420 And when we want to write Python code to represent this, 21 00:01:22,600 --> 00:01:23,950 it looks something like this. 22 00:01:24,280 --> 00:01:29,280 There's the keyword if, and then the condition that we're testing for and then a 23 00:01:30,280 --> 00:01:35,280 colon and after the colon we've got an indented block of code which should be 24 00:01:35,800 --> 00:01:39,370 executed if this condition is met, if it's true. 25 00:01:40,120 --> 00:01:41,680 But if it's not true, 26 00:01:42,010 --> 00:01:47,010 then we will skip to the else block and it's just the else keyword with a colon 27 00:01:47,800 --> 00:01:52,090 and then this code block would execute if the condition is false. 28 00:01:52,840 --> 00:01:57,490 So we could represent that previous bathtub situation with code that looks a bit 29 00:01:57,490 --> 00:01:58,150 like this. 30 00:01:58,150 --> 00:02:03,150 Let's say our water level is at 50 centimeters and then we would test if the 31 00:02:03,220 --> 00:02:07,690 water level is greater than 80 centimeters. Well, if that is the case, 32 00:02:07,750 --> 00:02:09,910 then we should drain the water. 33 00:02:10,360 --> 00:02:13,060 But if it's not greater than 80 centimeters, 34 00:02:13,120 --> 00:02:15,400 in other words else, well in this case, 35 00:02:15,430 --> 00:02:17,830 we should just continue filling up the bathtub. 36 00:02:18,280 --> 00:02:21,370 Let's put this into practice with a real life problem. 37 00:02:21,940 --> 00:02:26,350 Now let's say that you've gotten a job at a theme park and your first job of the 38 00:02:26,350 --> 00:02:31,330 day is to write some code that replaces the ticket box. 39 00:02:31,990 --> 00:02:35,800 Now there's a couple of things that you'll need to think about. Firstly, 40 00:02:35,980 --> 00:02:40,060 in order for somebody to actually purchase a ticket to go on the rollercoaster 41 00:02:40,060 --> 00:02:44,050 ride, they will need to be over 120 centimeters. 42 00:02:44,650 --> 00:02:49,060 So we have to check what their height is because if they're too short, 43 00:02:49,120 --> 00:02:53,740 then we won't be able to sell them a ticket anyways. In the course resources, 44 00:02:53,830 --> 00:02:58,830 I've included a link to this flow chart that I've created on draw.io. 45 00:02:59,890 --> 00:03:00,700 This is a really, 46 00:03:00,700 --> 00:03:04,960 really useful tool for creating any sort of flow charts or diagrams and it's 47 00:03:04,960 --> 00:03:09,550 really easy to use. Now, if we take a look at this flow chart, 48 00:03:09,820 --> 00:03:14,260 this is basically the logic that we have to program using our, 49 00:03:14,350 --> 00:03:15,850 if and else statements. 50 00:03:16,360 --> 00:03:21,360 If the person who's trying to purchase a ticket is not over 120 centimeters, 51 00:03:23,020 --> 00:03:26,020 then they can't ride on the rollercoaster. 52 00:03:26,530 --> 00:03:30,670 But if their height is greater than 120 centimeters, then they can ride. 53 00:03:30,850 --> 00:03:34,870 So let's try it out. If you head over to the starting Repl.it, 54 00:03:35,560 --> 00:03:39,310 you'll find some starting code in there and you can go ahead and fork this 55 00:03:39,310 --> 00:03:40,143 Repl.it. 56 00:03:40,780 --> 00:03:44,470 All that I've got here is a print statement that says "Welcome to the 57 00:03:44,470 --> 00:03:49,470 rollercoaster" as well as an input asking the user for their height in 58 00:03:49,930 --> 00:03:54,930 centimeters and then converting the string into a whole number, an integer, and 59 00:03:56,020 --> 00:03:58,780 then I'm storing it inside this variable called height. 60 00:03:59,440 --> 00:04:04,330 Now we're going to check whether if the height that the user has typed in is 61 00:04:04,330 --> 00:04:09,330 greater than 120. So we use the keyword if and then we check if the height is 62 00:04:10,960 --> 00:04:15,960 greater than 120 and then we add the colon and now when I hit enter, 63 00:04:17,410 --> 00:04:22,410 you'll notice that the code editor has automatically indented me slightly over. 64 00:04:23,050 --> 00:04:28,050 I'm not over here because in Python the spacing and indentation is really, 65 00:04:29,560 --> 00:04:30,400 really important. 66 00:04:30,790 --> 00:04:35,080 It tells the computer that the code that I'm about to write is what should be 67 00:04:35,080 --> 00:04:38,620 executed when this condition is met. 68 00:04:39,340 --> 00:04:44,340 So what should happen? If your height is over 120 then we will just print 69 00:04:44,980 --> 00:04:46,720 You can ride the rollercoaster!. 70 00:04:47,470 --> 00:04:52,470 But what should we print if height is not greater than 120? Well, 71 00:04:52,930 --> 00:04:57,930 in this case, we would use the else statement to catch when that happens. 72 00:04:58,840 --> 00:05:03,840 And it's really important that you don't write the else here because this is 73 00:05:04,120 --> 00:05:06,820 indented over. Instead, 74 00:05:07,030 --> 00:05:11,470 you want it to be at the same indentation level as the if statement. 75 00:05:11,950 --> 00:05:15,610 These two are essentially a pair, if and else. Now, 76 00:05:15,700 --> 00:05:16,990 after the else keyword, 77 00:05:16,990 --> 00:05:21,820 we again add a colon and then we hit enter and we're now indented again. 78 00:05:22,180 --> 00:05:27,180 And here we can write the code that should happen if this condition is false. 79 00:05:28,930 --> 00:05:30,910 "Sorry, you have to grow taller before you can ride." 80 00:05:31,900 --> 00:05:36,900 So the really important things here are the condition which we're testing for, is 81 00:05:38,290 --> 00:05:43,290 the value of height greater than 120, the syntax of this code 82 00:05:44,470 --> 00:05:45,520 so the keywords 83 00:05:45,550 --> 00:05:50,550 if and else as well as the colons that come after each of these lines. 84 00:05:51,850 --> 00:05:54,310 And finally also the indentation. 85 00:05:54,880 --> 00:05:57,740 Everything that is indented after the 86 00:05:57,950 --> 00:06:00,440 if is a block of code. 87 00:06:01,070 --> 00:06:06,070 So this is indented and it's effectively inside this 88 00:06:07,640 --> 00:06:08,840 if. So 89 00:06:08,840 --> 00:06:12,740 this line of code lives inside the else statement. 90 00:06:13,160 --> 00:06:18,160 This block of code lives inside the if statement. And if you mess up on the 91 00:06:19,340 --> 00:06:24,140 indentation then you're probably going to get an indentation error telling you 92 00:06:24,140 --> 00:06:28,250 that this line 5 probably should be indented. 93 00:06:28,910 --> 00:06:32,180 But when we do actually correct it and we hit run, 94 00:06:32,780 --> 00:06:36,200 let's say that our height is 130 centimeters, 95 00:06:36,680 --> 00:06:39,410 then we get back, you can ride the rollercoaster. 96 00:06:40,010 --> 00:06:43,520 But if our height was say 90 centimeters, 97 00:06:43,790 --> 00:06:47,180 then we get a different outcome. We get, sorry, 98 00:06:47,180 --> 00:06:50,510 you have to grow taller before you can ride. So by using, 99 00:06:50,600 --> 00:06:55,310 if and else statements we're able to get our code to do different things, 100 00:06:55,580 --> 00:07:00,580 either printing this line or printing this line depending on a condition that 101 00:07:01,130 --> 00:07:05,660 we're testing for. Now, when we use this greater than sign, 102 00:07:05,960 --> 00:07:10,960 effectively what we're saying is that is the height greater than 120 which means 103 00:07:13,160 --> 00:07:17,030 that it does not include 120. In fact, 104 00:07:17,030 --> 00:07:22,030 if I run this code and if I type in my height as 120 then actually goes into the 105 00:07:23,420 --> 00:07:25,760 else block and prints this. 106 00:07:26,480 --> 00:07:31,480 So if we wanted to include 120 centimeters so that all the people who are 107 00:07:31,730 --> 00:07:35,810 exactly 120 centimeters can ride the rollercoaster, 108 00:07:36,230 --> 00:07:38,840 then instead of just using a greater than symbol, 109 00:07:38,960 --> 00:07:41,330 we have to write greater than or equals. 110 00:07:41,780 --> 00:07:44,570 So these two symbols have to be next to each other. 111 00:07:45,470 --> 00:07:50,470 And now when I run my code and I write 120, you'll see that it's now falling into 112 00:07:51,980 --> 00:07:56,630 this block of code and it's telling me that I can ride the roller coaster. Now, 113 00:07:56,630 --> 00:08:01,630 these are called comparison operators and we've already seen greater than, so 114 00:08:01,730 --> 00:08:04,160 lesser than is pretty self explanatory. 115 00:08:04,430 --> 00:08:09,140 But we've also seen greater than or equal to and lesser than or equal to. 116 00:08:09,470 --> 00:08:13,580 So if you want to include a particular number, a particular value, 117 00:08:13,910 --> 00:08:17,270 when you do these comparisons, you would be using these instead. 118 00:08:17,990 --> 00:08:22,580 Now in this table, there's also the equal to and not equal to. 119 00:08:23,450 --> 00:08:24,050 For example, 120 00:08:24,050 --> 00:08:29,050 if you wanted to check if somebody's height is equal to precisely 120 then you 121 00:08:30,860 --> 00:08:35,860 would use two equal signs. And very often it gets a little bit confusing, 122 00:08:36,350 --> 00:08:40,370 especially if you're new to programming when you're typing equal signs, 123 00:08:40,429 --> 00:08:44,059 because sometimes we're typing one and other times we're typing two. 124 00:08:44,630 --> 00:08:47,420 It's important to remember that when you have one equal sign, 125 00:08:47,510 --> 00:08:52,400 it means that you're assigning this value to this variable. 126 00:08:53,000 --> 00:08:54,950 But when you have two equal signs, 127 00:08:55,050 --> 00:08:59,640 you are checking to see if the value on the left is equal to the value on the 128 00:08:59,640 --> 00:09:02,790 right and they're completely different. Now, 129 00:09:02,790 --> 00:09:04,650 the good thing is that when you get it wrong, 130 00:09:04,680 --> 00:09:09,680 usually you'll get enough clues in the error to actually hint to you, 131 00:09:09,750 --> 00:09:13,380 Hey, maybe there's something that's wrong here, right? 132 00:09:13,500 --> 00:09:17,220 Cause the syntax doesn't look right. For this to be a condition, 133 00:09:17,250 --> 00:09:20,760 it has to be something that evaluates to true or to false. 134 00:09:21,180 --> 00:09:24,600 And height =120 does not evaluate to either. 135 00:09:24,990 --> 00:09:26,520 So by changing it to this, 136 00:09:26,580 --> 00:09:31,580 we're saying if height is equal to 120 then we'll execute this line of code 137 00:09:32,250 --> 00:09:35,100 or any other lines which are in the same block of code. 138 00:09:35,640 --> 00:09:40,640 But if the height is not equal to 120 then we're going to execute this block of 139 00:09:41,010 --> 00:09:45,000 code. So if I write 120 then that works. 140 00:09:45,330 --> 00:09:50,100 But if I write 121 then it does not work. 141 00:09:51,000 --> 00:09:55,770 Similarly, you can also check for not equals to, which just flips it around. 142 00:09:56,220 --> 00:10:01,220 But in our case, it makes sense to say if the height is greater than or equals 143 00:10:01,350 --> 00:10:05,700 120 then you can ride the rollercoaster. 144 00:10:06,120 --> 00:10:11,120 But otherwise you cannot ride the rollercoaster. By using, 145 00:10:11,220 --> 00:10:12,570 if and else statements, 146 00:10:12,780 --> 00:10:17,780 we can get our computer to do different things and respond differently depending 147 00:10:18,060 --> 00:10:21,330 on different conditions. So in the next lesson, 148 00:10:21,390 --> 00:10:24,420 I've got a coding exercise for you. I'll see you there.