WEBVTT 0 00:00.170 --> 00:07.760 In previous lessons, we learned about using the if and else statements to check whether if somebody 1 00:07.760 --> 00:16.850 is over 120cm or not, and allow them to actually purchase a ticket if they are over a certain height. 2 00:17.030 --> 00:23.810 Now, in addition to the height, there's another condition that we need to check for, namely their 3 00:23.810 --> 00:24.530 age. 4 00:24.560 --> 00:31.280 If somebody is over 18 years old, then they should be paying the adult price, which is let's say $12, 5 00:31.280 --> 00:36.470 but if they are 18 or under, then they should only be paying $7. 6 00:37.280 --> 00:42.980 So how can we represent this extra condition that we need to check for in our code? 7 00:43.250 --> 00:48.080 Well, we could use something called a nested if else statement. 8 00:48.260 --> 00:53.780 We've seen our if else statements look like this, where it's only got two choices, 9 00:53.780 --> 00:57.710 if this condition is true, do this, otherwise do that. 10 00:57.710 --> 01:06.260 But in a nested if statement, once the first condition has passed, we can check for another condition, 11 01:06.530 --> 01:13.340 and then we can have another if else statement inside this if condition. 12 01:13.430 --> 01:17.990 In order for this thing to happen, this has to be true, 13 01:17.990 --> 01:23.420 and this also has to be true. In order for this to happen, this condition has to be true, 14 01:23.420 --> 01:25.790 but this condition has to be false. 15 01:26.450 --> 01:33.170 So essentially the computer first looks at the larger picture, which is this first condition, and 16 01:33.170 --> 01:40.280 decides on whether if it should go into the else block here or if it should go into the nested block 17 01:40.280 --> 01:42.440 inside the if statement. 18 01:42.800 --> 01:47.720 So now this is what our flowchart looks like, in the first if statement, 19 01:47.720 --> 01:51.680 we check whether if their height is over 120cm, 20 01:51.680 --> 01:54.560 if no, then the if statements all end. 21 01:54.560 --> 01:56.780 You can't write, you can't buy a ticket. 22 01:56.780 --> 02:03.110 But if yes, we actually take them to yet another if statement where we check their age. 23 02:03.110 --> 02:08.360 If their age is 18 or under, then we give them a $7 ticket. 24 02:08.540 --> 02:12.050 If they're over 18, then they have to pay $12. 25 02:12.710 --> 02:17.180 So the place where we're going to nest, our if statement is inside here. 26 02:17.210 --> 02:19.190 Notice how it's indented. 27 02:19.190 --> 02:22.520 So it's already inside this if block. 28 02:22.520 --> 02:25.160 And this already has to be true. 29 02:25.460 --> 02:30.200 Now here we're going to create another if and else statement. 30 02:30.290 --> 02:33.110 And the condition checks for their age. 31 02:33.110 --> 02:35.450 So we better ask them for an age. 32 02:35.450 --> 02:40.520 Let's say age equals, convert the input to an int. 33 02:40.520 --> 02:42.710 And, "What is your age?" 34 02:42.740 --> 02:50.150 So now that we've gotten hold of their age, we can see if their age is less than or equal to 18. 35 02:50.180 --> 02:54.680 Well in this case we're going to give them the $7 ticket. 36 02:56.510 --> 03:04.910 But else namely if this is not true, if their age is over 18, well, in that case, we're going to 37 03:04.910 --> 03:06.980 give them the $12 ticket. 38 03:07.880 --> 03:18.110 So now we have a nested if statement because this if and else statement lives inside this if statement. 39 03:18.110 --> 03:23.570 So this condition will only be checked if this is already deemed to be true. 40 03:23.600 --> 03:27.140 Now let's say our situation got a little bit more complex. 41 03:27.140 --> 03:31.280 The boss comes over, checks our code and says, wait, wait, wait, wait. 42 03:31.280 --> 03:34.010 There's actually more price tiers than that. 43 03:34.040 --> 03:38.720 In fact, if you're less than 12 years old, you pay $5. 44 03:38.840 --> 03:42.590 If you're between 12 and 18, you pay $7. 45 03:42.620 --> 03:47.900 And if you're over 18, then you pay the full adult price, which is $12. 46 03:48.170 --> 03:51.200 Now there are three possibilities. 47 03:51.200 --> 03:54.200 So how do we represent this in our if statement? 48 03:54.890 --> 03:57.920 Well, we could use something called the elif. 49 03:58.190 --> 04:05.540 Instead of having a simple if else statement where there's only one condition, if it's true, do this, 50 04:05.540 --> 04:07.190 otherwise do that. 51 04:07.460 --> 04:14.210 You can add as many elif conditions as you want. So we can check for condition1, 52 04:14.210 --> 04:16.430 if that's true, then do A, 53 04:16.430 --> 04:21.830 but if that's not true, then we can continue and check for condition2. 54 04:21.860 --> 04:25.700 If condition2 is true, well then we can do B. 55 04:25.700 --> 04:31.250 And finally, if none of those conditions were true, we can do this final thing. 56 04:31.400 --> 04:34.550 Our flowchart now looks something like this, 57 04:34.550 --> 04:37.400 and this is the logic that we're trying to program. 58 04:37.820 --> 04:44.720 Once we're inside this nested if statement, we're going to check if their age is under 12, in which 59 04:44.720 --> 04:46.800 case they should pay $5. 60 04:46.800 --> 04:50.970 If they're between 12 and 18, then they should pay $7. 61 04:50.970 --> 04:55.080 And finally, if they're over 18, then they should pay $12. 62 04:55.590 --> 05:05.250 Now, the first thing I'm going to check is if the age is less than 12 under this condition, they should 63 05:05.250 --> 05:06.990 pay $5. 64 05:06.990 --> 05:09.090 So let's change that to 5. 65 05:09.630 --> 05:15.680 Now the next condition should be created using an elif, which stands for else-if. 66 05:15.680 --> 05:17.450 So, it means, 67 05:17.450 --> 05:20.240 if this is not true, else if, 68 05:20.240 --> 05:21.980 can you check if this is true? 69 05:22.010 --> 05:24.380 Well, in that case then we should do this. 70 05:24.740 --> 05:33.320 For example, if the age is not less than 12, so they're over 12, then are they under 18? 71 05:33.320 --> 05:39.770 Well then this condition basically catches everybody who's between 12 and 18. 72 05:40.070 --> 05:47.720 And finally, if they're not less than or equal to 18 and they're not less than 12, then that's everybody 73 05:47.720 --> 05:49.580 else who is over 18. 74 05:50.840 --> 05:56.750 Now remember that we can use as many elif conditions between the if and else as we like. 75 05:56.750 --> 06:04.100 So I could add another elif that checks whether if the age is less than, say, 22. 76 06:04.130 --> 06:10.040 Well, in this case, do something else, and then I can keep going with these elif's until I'm done 77 06:10.040 --> 06:11.570 with all my conditions. 78 06:12.080 --> 06:14.330 So have a play around with this code. 79 06:14.330 --> 06:21.170 Write it yourself, see if it makes sense, and then mess around with the elif"s so that it does what 80 06:21.170 --> 06:22.640 you expect it to do. 81 06:22.640 --> 06:27.530 And then once you're ready, go ahead and head over to the next lesson where I've got a coding challenge 82 06:27.530 --> 06:27.950 for you.