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These are the user uploaded subtitles that are being translated: 1 00:00:00,100 --> 00:00:01,420 Now in the last lesson, 2 00:00:01,450 --> 00:00:06,450 we saw how we could use if, elif, else to check for multiple conditions. 3 00:00:06,970 --> 00:00:11,970 But in this case we're only checking one condition even though we have multiple, 4 00:00:12,460 --> 00:00:15,610 because if this first condition is true, 5 00:00:15,910 --> 00:00:20,350 then we would do whatever it is we need to do and then we would bypass 6 00:00:20,440 --> 00:00:21,460 everything else. 7 00:00:22,030 --> 00:00:25,810 Now what if you were in a situation where you need to check for multiple 8 00:00:25,810 --> 00:00:30,010 conditions even if the previous one was already true? 9 00:00:31,180 --> 00:00:33,910 Coming back to our rollercoaster ticketing problem, 10 00:00:34,420 --> 00:00:36,400 if you're going on a good rollercoaster ride, 11 00:00:36,550 --> 00:00:41,020 you'd probably want to keep a picture for the memories, right? 12 00:00:41,800 --> 00:00:44,020 And our rollercoaster is no exception. 13 00:00:44,470 --> 00:00:49,470 We want to be able to charge users an extra $3 if they want to purchase a ticket 14 00:00:49,690 --> 00:00:52,120 that includes a photo. Now, 15 00:00:52,150 --> 00:00:57,150 this is quite interesting because this is completely independent of their age or 16 00:00:57,910 --> 00:00:58,743 their height. 17 00:00:58,960 --> 00:01:02,500 Even if we've already gotten their age and height and determined their ticket 18 00:01:02,500 --> 00:01:07,450 price, this is an extra question. Do you want a photo or not? 19 00:01:07,480 --> 00:01:09,670 Yes or no. If you do, 20 00:01:09,700 --> 00:01:13,750 then we're going to add $3 to your existing ticket price. 21 00:01:14,650 --> 00:01:18,460 To do this, we would write multiple if conditions. 22 00:01:19,030 --> 00:01:21,940 If condition 1 is true, then do a, 23 00:01:22,360 --> 00:01:27,070 but then the code is going to go to the next case and check if condition 2 is 24 00:01:27,070 --> 00:01:27,903 also true, 25 00:01:28,180 --> 00:01:32,320 in which case it will do B and if the final condition is also true, 26 00:01:32,440 --> 00:01:35,680 it's going to do C. Whereas on the example on the left 27 00:01:35,680 --> 00:01:40,270 here, only one of these things, A, B, or C will be carried out. 28 00:01:40,690 --> 00:01:44,230 Comparing the example on the left where we're using 29 00:01:44,260 --> 00:01:48,370 if, elif, else, only one of these things A, 30 00:01:48,370 --> 00:01:52,480 B or C will be carried out. But on the right hand side, 31 00:01:52,900 --> 00:01:57,070 all three conditions are checked and if it's so happens that all three 32 00:01:57,070 --> 00:01:59,470 conditions are true, then A, 33 00:01:59,590 --> 00:02:02,860 B and C will all be executed. 34 00:02:03,520 --> 00:02:08,320 Currently, our code operates like this. We check for their height. 35 00:02:08,950 --> 00:02:11,020 If they're over 120 centimeters, 36 00:02:11,350 --> 00:02:13,990 we check their age and depending on their age, 37 00:02:14,380 --> 00:02:16,990 we give them a different price ticket. 38 00:02:18,130 --> 00:02:23,130 What we now want is even after we've checked for their ticket price, 39 00:02:24,040 --> 00:02:28,510 we want to ask them a question, do you want photos with your ticket? 40 00:02:29,200 --> 00:02:32,800 And if the answer is yes, then we're going to add $3 to that bill, 41 00:02:33,340 --> 00:02:35,500 no matter which type of ticket they've got. 42 00:02:36,220 --> 00:02:39,490 And finally we give them the total bill. If they say no, 43 00:02:39,550 --> 00:02:43,090 then we jump straight to the total bill and just tell them the price of their 44 00:02:43,090 --> 00:02:47,230 ticket. So how do we implement this in our code? 45 00:02:48,970 --> 00:02:52,240 Well, first let's change these print statements. 46 00:02:52,270 --> 00:02:55,420 Instead of giving them the bill at each of these steps, 47 00:02:55,750 --> 00:02:58,930 I'm going to tell them which type of ticket they're eligible for. 48 00:03:02,560 --> 00:03:07,450 So the next thing to do is to ask them whether if they want a photo or not. 49 00:03:07,870 --> 00:03:12,730 So I'm going to need to use an input, but the question is where do I put that? 50 00:03:13,450 --> 00:03:18,450 Now the correct answer is that it has to be at the same indentation level as 51 00:03:20,080 --> 00:03:23,860 this if block. So you can imagine this whole thing with the, 52 00:03:23,860 --> 00:03:27,640 if elif and else as sort of belonging together. 53 00:03:28,420 --> 00:03:30,400 Because they all relate to one thing: 54 00:03:30,760 --> 00:03:35,590 what is their age and which type of ticket are they eligible for. Now, 55 00:03:35,620 --> 00:03:37,540 outside of that if statement, 56 00:03:37,780 --> 00:03:41,650 I'm going to create an input and ask the user, 57 00:03:42,370 --> 00:03:46,360 do you want a photo taken? Type Y for yes or N for no. 58 00:03:47,110 --> 00:03:52,110 And then I'm going to save their input inside a variable called wants_photo. 59 00:03:55,120 --> 00:04:00,120 So now I'm going to use an if statement to check what their answer was. 60 00:04:00,700 --> 00:04:04,240 If it was true, then I'm going to add $3 and if it was false, 61 00:04:04,300 --> 00:04:07,120 I'm just going to skip to giving them their ticket price. 62 00:04:07,840 --> 00:04:10,810 So here is where I write my if statement. 63 00:04:11,440 --> 00:04:16,440 Notice how it's at the same indentation level as this previous set of if 64 00:04:16,630 --> 00:04:17,463 statements. 65 00:04:17,920 --> 00:04:22,920 But it's not at the same indentation level as this set of if statement. 66 00:04:24,220 --> 00:04:27,760 So essentially what's happening is once I've checked their age, 67 00:04:28,270 --> 00:04:31,720 then I'm going to check whether if they want a photo or not. 68 00:04:32,050 --> 00:04:35,710 And this is going to apply to everybody no matter their age. 69 00:04:36,490 --> 00:04:40,930 So if wants_photo is equal to Y, 70 00:04:41,680 --> 00:04:46,680 well in this case I'm going to go and add $3 to their bill. 71 00:04:48,970 --> 00:04:53,970 But how can I add $3 the bill when I don't have a variable to vary? 72 00:04:55,480 --> 00:04:57,730 So instead of using these print statements, 73 00:04:58,450 --> 00:05:01,120 I'm going to create a variable up here called bill and 74 00:05:01,460 --> 00:05:03,400 I'm going to set it to equal zero. 75 00:05:04,600 --> 00:05:09,010 Now in addition to printing to the user how much their ticket is, 76 00:05:09,340 --> 00:05:13,450 I'm going to set the bill to the price that they're supposed to pay. 77 00:05:13,930 --> 00:05:18,930 So if age is less than 12 bill is equal to $5, if age is less than 18 then bill 78 00:05:19,750 --> 00:05:23,170 is equal to $7, and finally for everybody else, 79 00:05:23,200 --> 00:05:28,200 the bill is equal to $12. So now depending on these conditions, 80 00:05:30,460 --> 00:05:34,780 the variable bill is going to be changed to a different number. 81 00:05:35,470 --> 00:05:38,260 But once we land in this if statement, 82 00:05:38,560 --> 00:05:42,400 I'm going to have to add $3 to their bill 83 00:05:42,700 --> 00:05:45,340 no matter which value it is at the moment. 84 00:05:45,880 --> 00:05:50,880 So effectively, what I want to do is bill equals the current value of bill plus 85 00:05:51,700 --> 00:05:54,130 $3. So if bill was $7, 86 00:05:54,160 --> 00:05:59,160 then this new value bill should be $10. If bill was $12 then it should now be $15. 87 00:06:00,530 --> 00:06:05,240 Now in Python as well as many other languages, there's actually a slightly shorter way 88 00:06:05,240 --> 00:06:06,260 of writing this. 89 00:06:06,740 --> 00:06:11,690 When you want to increase the current value that's held in a variable and you 90 00:06:11,690 --> 00:06:13,610 wanna save it back into the variable, 91 00:06:13,970 --> 00:06:17,240 you can simply write plus equals. 92 00:06:17,810 --> 00:06:22,040 So bill += 3 is the same as bill = bill + 3. 93 00:06:22,790 --> 00:06:27,680 Now, no matter what the value of bill is before it reached this if statement, 94 00:06:28,040 --> 00:06:32,930 I'm still going to add $3 to it. Now after this 95 00:06:32,930 --> 00:06:34,580 if statement is completed, 96 00:06:34,880 --> 00:06:39,880 I don't actually have to write a companion else statement because in this case, 97 00:06:40,940 --> 00:06:45,080 if the answer is no, then we're simply going to do nothing. 98 00:06:45,080 --> 00:06:48,380 We're not going to do anything to the bill. Instead, 99 00:06:48,410 --> 00:06:52,610 I'm just going to skip ahead and print to the user their final bill. 100 00:06:55,370 --> 00:07:00,370 And I'm going to use fstrings to insert the value that the bill variable has 101 00:07:02,990 --> 00:07:07,820 into this print statement. Now for your code to work, 102 00:07:08,000 --> 00:07:13,000 the indentation matters a huge deal because the computer will think you want 103 00:07:14,750 --> 00:07:18,320 it to do different things depending on the indentation. 104 00:07:19,040 --> 00:07:24,040 So notice how this indentation shows that this bill plus three is to be executed 105 00:07:26,510 --> 00:07:27,890 when this condition is true, 106 00:07:28,580 --> 00:07:33,580 but this print statement is not indented to the same level and effectively it's 107 00:07:34,220 --> 00:07:38,300 going to happen after this if statement has been executed. 108 00:07:39,320 --> 00:07:44,320 So you can play around with indenting this print statements, un-indenting it 109 00:07:45,230 --> 00:07:46,730 and see what the difference is. 110 00:07:47,810 --> 00:07:51,770 But now I'm going to go ahead and hit command 111 00:07:51,800 --> 00:07:56,800 + enter or control + enter on Windows to run my code and I'm going to go ahead and 112 00:07:58,070 --> 00:07:59,330 try and get a ticket. 113 00:08:00,050 --> 00:08:03,530 So let's say that I am 21 years old, 114 00:08:04,730 --> 00:08:08,150 tells me the adult tickets at $12 and then it asked me, 115 00:08:08,150 --> 00:08:11,810 do you want a photo taken? So yeah, I would love to have a photo taken. 116 00:08:12,560 --> 00:08:15,530 And it tells me that my final bill is 15, 117 00:08:16,010 --> 00:08:18,170 so I forgot a dollar sign here. 118 00:08:18,710 --> 00:08:23,710 But effectively it's now added the $3 to the $12 because of this additional if 119 00:08:26,510 --> 00:08:31,460 statement. So if this concept is still a little bit confusing to you, 120 00:08:31,880 --> 00:08:36,880 then I want you to really study this flow chart and just look at the difference, 121 00:08:37,460 --> 00:08:40,550 um, and visualize it because the, 122 00:08:40,670 --> 00:08:45,670 if elif else statement is only ever going to be true in one of these arrows, 123 00:08:47,450 --> 00:08:51,410 right? You can't be less than 12 and 18 and over. 124 00:08:52,280 --> 00:08:53,030 And similarly, 125 00:08:53,030 --> 00:08:56,460 it doesn't make sense for you to have two different ticket prices. 126 00:08:57,270 --> 00:08:59,250 But with the want photos, 127 00:08:59,550 --> 00:09:04,550 this condition has to be asked no matter whichever branch they landed on. 128 00:09:05,160 --> 00:09:08,130 So it's a completely separate from this if statement. 129 00:09:09,030 --> 00:09:11,910 And once you've had a look at this flow chart, 130 00:09:12,120 --> 00:09:16,980 then compare it against the code and the way that the code works should be a lot 131 00:09:16,980 --> 00:09:21,360 more obvious. And once you're happy with the code and this concept, 132 00:09:21,690 --> 00:09:25,440 then I want you to head over to the next lesson where I've got a code challenge 133 00:09:25,440 --> 00:09:29,340 for you. So for all of that, and more, I'll see you there. 13020