WEBVTT 00:00.080 --> 00:06.020 So for now, you have seen that all the instructions you write in a Python program will be executed 00:06.020 --> 00:07.880 one by one. 00:07.880 --> 00:12.770 But as you progress, you will want to make your programs more dynamic, and you will want to be able 00:12.770 --> 00:17.930 to execute only some instructions depending on some specific conditions. 00:17.930 --> 00:24.440 For example, you might do a certain action if a button is pressed, and if it's not pressed, then 00:24.440 --> 00:26.420 you will do another action okay. 00:26.450 --> 00:33.770 So in your code some instructions might be executed and some other instructions might not be executed. 00:33.770 --> 00:39.350 And let's write our first condition in Python I will start by creating a variable. 00:39.350 --> 00:45.770 Let's let's name it temperature is equal to for example 21. 00:45.770 --> 00:50.930 So even though float numbers would be better for temperatures, I'm just going to use an integer here 00:50.930 --> 00:53.240 to keep it simple for this example okay. 00:53.270 --> 00:54.830 So we have our variable. 00:54.830 --> 00:58.610 And I'm going to write a condition here how to write a condition. 00:58.610 --> 01:01.220 Well you have the if keyword. 01:01.220 --> 01:03.860 As you can see we have the syntax Highlighting on. 01:03.890 --> 01:10.910 If so, in Python, you start with if then we are going to test a condition and this condition is going 01:10.940 --> 01:13.580 to be evaluated to true or false. 01:13.580 --> 01:16.430 So let me write something first and then I will explain it to you. 01:16.430 --> 01:24.770 So if temperature I use this angle bracket like this which means greater than let's say 20. 01:24.800 --> 01:28.220 Then I add a colon and I go back to a new line. 01:28.220 --> 01:32.210 And you can see we have a new indentation of four spaces. 01:32.300 --> 01:33.800 So what does that mean? 01:33.800 --> 01:36.830 What will the if keyword we enter a condition. 01:36.830 --> 01:42.350 This here is going to be evaluated to true or to false. 01:42.350 --> 01:46.340 If this is evaluated to true then well you can see we end. 01:46.370 --> 01:53.810 If with a colon then we are going to execute whatever code is here after the if with an indentation. 01:53.810 --> 01:59.600 But if it is false, we are not going to execute this block of code here. 01:59.630 --> 01:59.960 Okay. 01:59.990 --> 02:07.770 So here we test if the temperature is greater than 20 and if the temperature is greater than 20, I'm 02:07.770 --> 02:09.120 going to write print. 02:09.150 --> 02:12.750 For example, it's warm. 02:12.780 --> 02:13.080 Okay. 02:13.110 --> 02:15.090 Here I'm using the metric system. 02:15.090 --> 02:17.370 I use the Celsius degrees okay. 02:17.370 --> 02:20.820 But it's not really going to be a problem for comprehension of conditions. 02:20.850 --> 02:21.270 Okay. 02:21.270 --> 02:23.760 So we have written our first condition. 02:23.790 --> 02:25.560 Now let's test it. 02:25.800 --> 02:27.990 And you can see we have it's warm. 02:28.020 --> 02:33.300 So what happened is that when we first create the variable temperature we assign a value to it. 02:33.330 --> 02:34.290 21. 02:34.320 --> 02:36.990 Then we test if temperature greater. 02:36.990 --> 02:39.450 And actually this is strictly greater. 02:39.480 --> 02:43.260 And I'm going to come back to conditional operators in the next lesson. 02:43.290 --> 02:43.500 Okay. 02:43.530 --> 02:46.020 For now we just focus on the condition itself. 02:46.020 --> 02:51.000 So if the temperature is strictly greater than 20 which means at least 21, then we print. 02:51.000 --> 02:53.040 It's warm because it's 21. 02:53.070 --> 02:55.260 Then we print it warm okay. 02:55.290 --> 02:59.310 If I put 41 we are also going to print. 02:59.340 --> 03:00.060 It's warm. 03:00.090 --> 03:03.180 Now let's say I put 18 degrees. 03:03.990 --> 03:08.340 Let's run the script and you can see that we don't print anything. 03:08.340 --> 03:13.050 So this line is only executed if this condition here is true. 03:13.080 --> 03:13.380 All right. 03:13.410 --> 03:16.500 So that's already making our program more dynamic. 03:16.530 --> 03:18.690 I'm going to add a new line after that. 03:18.690 --> 03:23.040 So you can see that after I press enter I'm still in this in notation. 03:23.040 --> 03:27.060 So I'm still inside the if and I can add more stuff inside the if. 03:27.090 --> 03:27.600 Okay. 03:27.630 --> 03:31.680 If I go back here I am back to the first indentation. 03:31.680 --> 03:33.960 So we are out of the if structure. 03:33.990 --> 03:35.880 I'm just going to add a space for readability. 03:35.880 --> 03:40.920 And let's print for example end of program. 03:40.920 --> 03:42.540 So let's print that. 03:43.410 --> 03:45.150 You can see we have end of program. 03:45.150 --> 03:52.800 So even though we didn't execute that line because it's in the if and this is false, we still execute 03:52.800 --> 03:56.340 this because we are after the if and this is outside of the if. 03:56.370 --> 04:02.220 Now if I for example, add an indentation here, you can see we will still be in the. 04:02.250 --> 04:05.610 If so we will not print end of program. 04:05.640 --> 04:06.240 Okay. 04:06.240 --> 04:10.290 And in this case it will only be printed if for example I put 25. 04:11.730 --> 04:12.270 then. 04:12.300 --> 04:15.270 End of program is going to be printed because it's inside the if. 04:15.300 --> 04:15.690 Okay. 04:15.720 --> 04:18.120 So pay attention to the indentation. 04:18.120 --> 04:19.470 It's very important. 04:19.500 --> 04:19.860 Great. 04:19.860 --> 04:22.680 So a condition you can see it's not very complicated. 04:22.710 --> 04:29.970 If and then condition to test colon and then a block of code to execute if it is true. 04:30.000 --> 04:31.770 Now you can add after the. 04:31.800 --> 04:36.990 If so you go back to the same indentation as the if you can add an else keyword. 04:37.140 --> 04:39.300 So you put else colon. 04:39.300 --> 04:40.470 Go back to a new line. 04:40.470 --> 04:41.850 You have an indentation. 04:41.850 --> 04:47.130 And let's say for example print not so one. 04:48.090 --> 04:49.710 What does the else do. 04:49.740 --> 04:55.050 Well if this condition is true you're going to execute this block of code and that's it. 04:55.050 --> 05:02.520 But if this condition is false you will execute what's inside the else keyword the else block here. 05:02.550 --> 05:03.150 Okay. 05:03.150 --> 05:08.010 So if the temperature is 25 you can just print it's one. 05:08.010 --> 05:16.960 And you can see this one was not executed because we only execute it It's warm, but if I put 15 in 05:16.960 --> 05:21.550 this case, you will see we have not so warm. 05:21.940 --> 05:25.090 Okay, so what happened here is that we go to the. 05:25.120 --> 05:30.070 If we evaluate this condition this is going to be false. 05:30.070 --> 05:34.750 So if this is false we go to the else block and we execute the else block. 05:34.750 --> 05:40.780 And then of course we also execute end of program because it's out of the if and on top of if and else. 05:40.780 --> 05:51.160 You can also add in between if you want to else if and actually in Python it's not else if it's been 05:51.160 --> 05:54.010 condensed into Elif okay. 05:54.010 --> 05:55.540 So Elif means else. 05:55.570 --> 05:58.990 If so you have the if Elif and else keywords. 05:58.990 --> 06:01.600 With Elif, you can test another condition. 06:01.600 --> 06:05.110 So, for example, if temperature is greater than 20, it's one. 06:05.140 --> 06:06.580 But then you can test. 06:06.580 --> 06:12.820 So if this is false, we're going to test if temperature is greater than ten. 06:14.020 --> 06:15.770 And you can see we have a new edition. 06:15.770 --> 06:16.790 Let's print. 06:16.820 --> 06:17.750 Well, let's print. 06:17.780 --> 06:19.310 Not so warm here. 06:22.610 --> 06:24.080 Then I'm going to go back to a new line. 06:24.080 --> 06:27.350 And you can see I removed the indentation and I can add another Elif. 06:27.380 --> 06:29.840 Okay, I can add as many Elif as I want. 06:29.870 --> 06:33.560 Let's say temperature greater than zero. 06:34.880 --> 06:35.960 Print. 06:36.380 --> 06:40.760 It's cold and I'm going to change the else to. 06:40.790 --> 06:46.580 It's freezing, so it's freezing. 06:46.580 --> 06:48.200 So what's happening now? 06:48.200 --> 06:54.530 Well, let's make a test with 15, actually, and let's see what's happening and understand this flow 06:54.530 --> 06:55.340 here. 06:55.340 --> 06:56.960 So we have not so warm. 06:56.960 --> 06:57.980 So what happened. 06:57.980 --> 07:00.470 We have this entire if structure. 07:00.500 --> 07:02.210 We start at the first. 07:02.240 --> 07:06.530 If we check if this is true then we're going to execute. 07:06.530 --> 07:07.790 It's warm and that's it. 07:07.820 --> 07:09.650 We don't execute any of the other. 07:09.650 --> 07:13.010 We don't even test the other else if other else okay. 07:13.040 --> 07:16.190 But in this case with temperature 15 well this is false. 07:16.190 --> 07:17.930 So if this is false we're going to check. 07:17.950 --> 07:19.210 Do we have another leaf? 07:19.240 --> 07:19.900 Yes. 07:19.900 --> 07:24.190 So we're going to test this one if temperature is greater than ten. 07:24.190 --> 07:25.210 And this is true. 07:25.210 --> 07:27.640 So we are going to print not so warm. 07:27.640 --> 07:31.840 And then we go out of the if we don't even test the next ones. 07:31.840 --> 07:34.660 So now I can make another example with five. 07:36.010 --> 07:40.210 And you can see now we have it's cold because this condition was false. 07:40.210 --> 07:41.770 So then we try the next one. 07:41.770 --> 07:43.690 This one is false. 07:43.690 --> 07:44.830 We try the next one. 07:44.830 --> 07:45.970 This one is true. 07:45.970 --> 07:47.770 So we print it's cold. 07:47.770 --> 07:50.170 And finally let's say minus five. 07:51.550 --> 07:54.970 Then we're going to have it's freezing okay. 07:55.000 --> 07:56.860 Because this one is going to be false. 07:56.860 --> 07:58.510 And then this one is going to be false. 07:58.510 --> 07:59.770 This one is going to be false. 07:59.770 --> 08:01.330 Finally we go to the else. 08:01.360 --> 08:01.720 All right. 08:01.750 --> 08:06.820 So if you have any temperature that's greater than 20 we're going to get here. 08:07.060 --> 08:11.440 But then if the temperature is greater than ten here actually you know that temperature is greater than 08:11.440 --> 08:16.240 ten, but also that the temperature is lower than 20. 08:16.270 --> 08:21.110 Because if it was greater than 20, you would have stayed here. 08:21.140 --> 08:25.820 Okay, so when you arrive in this leaf, actually, you know that the temperature is kind of between 08:25.820 --> 08:26.900 10 and 20. 08:26.930 --> 08:27.860 Same thing here. 08:27.860 --> 08:33.290 You know, the temperature is going to be between 0 and 10, because if it was greater than ten you 08:33.290 --> 08:36.260 would be in this block okay. 08:36.290 --> 08:41.060 And finally with the else you know that it's freezing because well, negative temperature is freezing. 08:41.060 --> 08:45.680 And you know that if the temperature is not greater than zero, then it's freezing. 08:45.680 --> 08:52.160 So you can see with this the order of the conditions is quite important. 08:52.160 --> 08:59.210 And you can create if blocks as simple as just one, if you can add as many Elif as you want and only 08:59.210 --> 09:01.790 one else if you want to add an else. 09:01.790 --> 09:03.410 But everything is optional. 09:03.440 --> 09:10.340 Okay, you can just the minimum code for a if structure for condition is basically this okay if condition 09:10.340 --> 09:16.040 colon and then a block of code with one line or as many lines as you want, and then you just need to 09:16.070 --> 09:23.000 go out of the indentation here to go back to the main execution of the code that will be line by line 09:23.000 --> 09:23.780 once again.