WEBVTT 00:00.710 --> 00:01.460 Hello. 00:01.670 --> 00:04.340 Welcome to another lecture of our python course. 00:04.340 --> 00:05.850 And my name is Stephan. 00:05.870 --> 00:12.950 In this lecture you will learn about conditions, but you will also learn if Elif and else chain. 00:12.950 --> 00:17.420 So often you will need to test more than just two possible situations. 00:17.420 --> 00:23.240 And to evaluate these you can use python's if Elif else syntax. 00:23.240 --> 00:31.760 So Python executes only one block in an if Elif else chain, so it runs each conditional test in order 00:31.760 --> 00:33.590 until one passes. 00:33.590 --> 00:40.280 So when a test passes, the code following that test is executed and Python skips the rest of the tests. 00:40.280 --> 00:44.180 So many real world situations involve more than two possible conditions. 00:44.180 --> 00:51.350 For example, consider a bus ticket, a mechanism bus ticket technology that charges different rates 00:51.350 --> 00:53.180 for different age groups. 00:53.180 --> 00:59.930 For example, bus tickets for anyone under age eight is free bus ticket for anyone between the ages 00:59.930 --> 01:02.880 of eight and 18 is $50. 01:02.880 --> 01:08.730 Bus ticket from anyone age 18 or older is $90. 01:08.730 --> 01:15.240 So how can we use an If statement to determine a person's bus ticket rate? 01:15.240 --> 01:18.270 So this code we will write here. 01:18.270 --> 01:24.060 For example, let's make the my age here, for example. 01:24.060 --> 01:31.710 My age is 25 for and we will change this age to test different conditions. 01:32.250 --> 01:40.320 If our age is here, let's actually let's actually write the first statement here. 01:41.400 --> 01:46.920 So if a ticket for anyone, a bus ticket for anyone under age is free. 01:47.040 --> 01:54.240 And so if age is less than eight, then we will print. 01:55.020 --> 01:55.950 Your. 01:58.470 --> 02:00.750 You need to pay. 02:02.320 --> 02:03.370 $0. 02:05.360 --> 02:05.600 Or. 02:05.600 --> 02:11.930 Yeah, let's actually, instead of writing that your ticket is free here. 02:12.460 --> 02:13.210 And. 02:13.830 --> 02:17.090 Here of my age. 02:17.100 --> 02:27.300 And after that we will add else if I will explain all of this here else if Elif here, Elif my age and 02:27.300 --> 02:29.970 here, let's let's actually take this. 02:31.810 --> 02:32.170 Here. 02:32.290 --> 02:34.090 So we did this. 02:34.090 --> 02:34.600 Now. 02:34.600 --> 02:35.500 Now we are. 02:35.920 --> 02:37.570 We need to do the second here. 02:37.570 --> 02:44.140 So bus ticket for anyone between the ages of eight and 18 is $50. 02:44.170 --> 02:45.550 Now we will write that. 02:45.550 --> 02:50.620 So if my age is less than. 02:56.720 --> 02:57.260 Here. 02:57.290 --> 03:03.350 Anyone between the ages of four and 18 is $50. 03:08.340 --> 03:11.410 And here we will write 18. 03:12.660 --> 03:13.320 My age. 03:13.320 --> 03:13.680 18. 03:13.680 --> 03:15.450 So you will explain here. 03:15.450 --> 03:20.610 You might be wondering why we didn't included eight here, which I will explain now after writing this 03:20.610 --> 03:29.010 code and you will understand because here print we will use your ticket. 03:29.130 --> 03:30.870 Ticket is. 03:33.700 --> 03:36.850 Bicycle, $50, $50. 03:37.270 --> 03:42.700 And after that, we will also add clouds here and here. 03:43.580 --> 03:44.210 Print. 03:45.130 --> 03:49.750 So your ticket is $100. 03:52.940 --> 03:55.500 So also we did this section. 03:55.520 --> 03:56.990 Mark This one. 03:58.600 --> 03:59.160 That's it. 03:59.170 --> 04:07.720 So firstly, the if test checks whether a person is under eight years old. 04:07.720 --> 04:15.820 So when test passes, an appropriate message is printed and Python skips the rest of the tests. 04:15.820 --> 04:19.780 And here, let's try this for example, if my age was five. 04:20.300 --> 04:28.280 And here, as you can see, your ticket is free and the life line is really another if test so which 04:28.280 --> 04:32.630 runs only if this previous test fails. 04:32.630 --> 04:36.980 So in this case it didn't fail here and this code didn't run. 04:37.990 --> 04:44.200 So at this point in the chain, we know that the person is at least eight years old because the first 04:44.200 --> 04:44.890 test. 04:45.960 --> 04:48.840 Uh, is let's actually, uh, make here. 04:50.750 --> 04:56.810 Now, in this case, we know the person is at least eight years old because the first test didn't run 04:56.810 --> 04:57.380 right. 04:57.770 --> 05:08.780 So if the person is under 18, an appropriate message is printed and Python will not execute the else 05:08.780 --> 05:09.140 block. 05:09.140 --> 05:19.910 Now, Python will now execute because our Elif here, our Elif is executed, but also our if didn't 05:19.910 --> 05:26.630 execute because it didn't meet meet the conditions here because my age was ten. 05:27.440 --> 05:28.160 And. 05:29.080 --> 05:36.790 Here if my age under eight, then it didn't print because my age is not under eight. 05:36.940 --> 05:43.990 You learned that in expressions previous lectures, and here we have Elif, which is my age, is under 05:43.990 --> 05:47.740 18 and here we have ten here. 05:47.830 --> 05:56.260 So in this example here, the if test, let's actually yes, here in this example, the if test evaluates 05:56.260 --> 05:59.740 false so it's code block is not executed. 05:59.740 --> 06:06.400 However, the Elif test evaluates true which is the ten is less than 18. 06:06.400 --> 06:06.640 Right. 06:06.640 --> 06:08.920 So its code is executed. 06:08.920 --> 06:16.420 So the output is one sentence here informing the user that admission cost is $50. 06:16.420 --> 06:18.280 So your ticket is $50. 06:18.280 --> 06:27.700 Any age greater than 17 would cause the first the if and Elif tests to fail in this situations here, 06:27.700 --> 06:35.030 let's actually make it 20 In this situations the else block will be executed here and as you can see 06:35.030 --> 06:37.010 here else block is executed. 06:37.010 --> 06:44.030 This block is in the if and Elif didn't execute because they didn't meet the condition here and here 06:44.030 --> 06:47.180 we have the else block executed here. 06:48.010 --> 06:51.250 And we printed this message here. 06:52.760 --> 06:59.570 So rather than the printing the ticket price with the if else block, it would be more concise to just 06:59.570 --> 07:04.160 set the price inside the if else chain. 07:04.160 --> 07:09.300 And then we have a single print call that runs after the chain has been evaluated. 07:09.320 --> 07:14.320 So now we will let's delete this and we will do another example with that. 07:14.330 --> 07:20.390 So here, if my age is 20, remember that the. 07:21.910 --> 07:26.610 For under a under age of eight, it will be free for bus ticket. 07:26.620 --> 07:33.850 So my age will be or and we will also add price here. 07:34.650 --> 07:36.240 Price is going to. 07:37.360 --> 07:42.950 Zero and my age eight while my age is other than 18. 07:42.970 --> 07:45.610 Then the price is going to be price. 07:46.300 --> 07:57.730 Price is going to be 25, the 50 and else if our age is greater than 18, we will praise we will use 07:57.730 --> 08:00.400 the price 19 here. 08:01.100 --> 08:06.770 And after that we will print here we will use format. 08:06.800 --> 08:14.690 Your bus ticket cost is here and we will use this price. 08:14.690 --> 08:15.590 That's it. 08:15.740 --> 08:18.110 So here, let's run this. 08:18.110 --> 08:19.570 And here we are. 08:19.570 --> 08:24.320 Our age is 20 and our else block is executed here. 08:24.320 --> 08:32.930 So the indented lines here you see here, these are the indented lines set the value of price according 08:32.960 --> 08:36.230 to the person's age, as in the previous example. 08:36.410 --> 08:48.470 So after the price is set by the if here, if Elif and else conditions as separate on indented print 08:48.470 --> 08:52.250 call uses this value the price value. 08:52.970 --> 08:54.290 Um, two. 08:54.920 --> 08:56.870 So price value. 08:58.110 --> 08:59.850 To display this message, right? 09:02.250 --> 09:08.690 And so this here, this code produces the same output as the previous example. 09:08.700 --> 09:14.550 But the purpose of the if Elif else chain is narrower. 09:14.550 --> 09:21.870 So instead of determining a price and displaying a message, it simply determines the ticket price. 09:22.020 --> 09:29.220 And in addition to being more efficient, the reveals this code here is easier to modify than the original 09:29.220 --> 09:33.620 approach that we did in earlier in this lecture. 09:33.630 --> 09:39.960 So to change the text of the ultimate message, you will need to change only this print call rather 09:39.960 --> 09:44.760 than three separate print calls that was here before.