WEBVTT 1 00:00:00.770 --> 00:00:02.510 Hi guys, and welcome back. 2 00:00:02.510 --> 00:00:05.220 In this video we're going to learn about if statements. 3 00:00:05.220 --> 00:00:07.979 An if statement allows us to use a Boolean 4 00:00:07.979 --> 00:00:10.340 to make a decision in our programme 5 00:00:10.340 --> 00:00:12.160 and potentially do different things, 6 00:00:12.160 --> 00:00:14.278 depending on the value of that Boolean. 7 00:00:14.278 --> 00:00:16.930 Of course, if statements are most useful 8 00:00:16.930 --> 00:00:19.020 when you have user input, 9 00:00:19.020 --> 00:00:22.220 such that your programme can potentially get different data, 10 00:00:22.220 --> 00:00:25.580 depending on who runs it and who's using your programme. 11 00:00:25.580 --> 00:00:27.830 So here we've got an input that asks the user 12 00:00:27.830 --> 00:00:29.430 for the day of the week, 13 00:00:29.430 --> 00:00:31.960 so what day of the week is it today? 14 00:00:31.960 --> 00:00:33.730 And then, we're going to check 15 00:00:33.730 --> 00:00:36.320 whether the current day of the week is Monday. 16 00:00:36.320 --> 00:00:37.440 So, how would you check 17 00:00:37.440 --> 00:00:39.260 whether the current day of the week is Monday? 18 00:00:39.260 --> 00:00:44.260 You would say day_of_week == the string "Monday". 19 00:00:45.050 --> 00:00:49.630 Let's print this out to see whether our Boolean works. 20 00:00:49.630 --> 00:00:52.870 So, all I've done is printed the Boolean out. 21 00:00:52.870 --> 00:00:54.813 And again, remember that this here 22 00:00:54.813 --> 00:00:58.053 will evaluate first, so Python will say, 23 00:00:58.053 --> 00:01:00.500 "Day of week, is it equal to Monday?" 24 00:01:00.500 --> 00:01:02.650 And if it is, then it will replace 25 00:01:02.650 --> 00:01:04.920 this entire thing by "true." 26 00:01:04.920 --> 00:01:07.597 Otherwise, it will be replaced by "false." 27 00:01:08.600 --> 00:01:09.990 And, if we run this, 28 00:01:09.990 --> 00:01:11.830 you see that we get our prompt, 29 00:01:11.830 --> 00:01:13.220 and you can type in "Monday," 30 00:01:13.220 --> 00:01:14.780 and then you get True. 31 00:01:14.780 --> 00:01:16.092 But, if you type "Tuesday," 32 00:01:16.092 --> 00:01:18.670 then you get False. 33 00:01:18.670 --> 00:01:20.750 So this is what we expected. 34 00:01:20.750 --> 00:01:22.930 Now, instead of printing this out, 35 00:01:22.930 --> 00:01:26.410 we can actually put it in an if statement. 36 00:01:26.410 --> 00:01:29.040 So the if statement is like this, 37 00:01:29.040 --> 00:01:33.450 you type in your if keyword, which is special in Python. 38 00:01:33.450 --> 00:01:36.040 Then you put a space, and then, 39 00:01:36.040 --> 00:01:37.772 without putting brackets or anything, 40 00:01:37.772 --> 00:01:41.980 you type your Boolean comparison. 41 00:01:41.980 --> 00:01:44.373 And then at the end you put your colon. 42 00:01:45.320 --> 00:01:49.140 This will evaluate as soon as the if statement runs, 43 00:01:49.140 --> 00:01:51.450 so as soon as Python reaches line 3, 44 00:01:51.450 --> 00:01:54.250 it's gonna calculate the result of this Boolean, 45 00:01:54.250 --> 00:01:56.630 which is gonna be true if the user entered "Monday," 46 00:01:56.630 --> 00:01:58.680 and false if they didn't. 47 00:01:58.680 --> 00:02:00.930 And if it is true, 48 00:02:00.930 --> 00:02:05.590 then we will run the code under the if statement. 49 00:02:05.590 --> 00:02:07.720 And here's one of the most important things 50 00:02:07.720 --> 00:02:09.330 in Python programming, 51 00:02:09.330 --> 00:02:11.350 which is that there are a few spaces 52 00:02:11.350 --> 00:02:13.050 in front of this line here. 53 00:02:13.050 --> 00:02:15.630 This is called indentation in Python, 54 00:02:15.630 --> 00:02:19.910 and after a colon that defines a block of code, 55 00:02:19.910 --> 00:02:22.740 such as an if statement or a function or a class, 56 00:02:22.740 --> 00:02:24.378 we're gonna learn more about those later on, 57 00:02:24.378 --> 00:02:27.580 you need to put indentation. 58 00:02:27.580 --> 00:02:30.315 That is how Python tells which code 59 00:02:30.315 --> 00:02:32.520 is inside the if statement, 60 00:02:32.520 --> 00:02:36.160 i.e. which code you want to run when this is true. 61 00:02:36.160 --> 00:02:38.607 So, for example, you can print 62 00:02:38.607 --> 00:02:41.207 "Have a great start to your week!" 63 00:02:42.470 --> 00:02:44.890 Now, this is the line of code 64 00:02:44.890 --> 00:02:47.140 that is inside the if statement. 65 00:02:47.140 --> 00:02:49.930 If this is true, this line of code will run, 66 00:02:49.930 --> 00:02:53.040 because it's got these four spaces in front. 67 00:02:53.040 --> 00:02:53.873 I'll say at this point that 68 00:02:53.873 --> 00:02:55.610 you don't have to use four space in Python, 69 00:02:55.610 --> 00:02:57.850 you can use two, you can use six, or eight, 70 00:02:57.850 --> 00:03:00.540 whatever you want, but be consistent. 71 00:03:00.540 --> 00:03:02.400 So, always use the same number of spaces 72 00:03:02.400 --> 00:03:03.938 that's gonna make your code work, 73 00:03:03.938 --> 00:03:05.690 otherwise it may not work, 74 00:03:05.690 --> 00:03:07.880 and also it's gonna make your code look much better. 75 00:03:07.880 --> 00:03:09.360 If you've got different indentations 76 00:03:09.360 --> 00:03:10.720 in different parts of your programme, 77 00:03:10.720 --> 00:03:11.920 people are gonna be really confused 78 00:03:11.920 --> 00:03:13.400 when they look at your code. 79 00:03:13.400 --> 00:03:16.423 I recommend using four spaces for Python code. 80 00:03:17.320 --> 00:03:21.200 Now, if you print something and you remove the indentation, 81 00:03:21.200 --> 00:03:22.393 the spaces in front, 82 00:03:26.370 --> 00:03:28.320 for example, like this, what you're gonna get 83 00:03:28.320 --> 00:03:31.200 is that this code is always going to run, 84 00:03:31.200 --> 00:03:33.720 if this is true, or if it is false. 85 00:03:33.720 --> 00:03:35.530 Because it's not in the if statement, 86 00:03:35.530 --> 00:03:38.200 it's completely unrelated to the if statement. 87 00:03:38.200 --> 00:03:40.640 So, Python will run this code if it's true, 88 00:03:40.640 --> 00:03:41.800 it will run this code. 89 00:03:41.800 --> 00:03:43.520 If it's false, it will not run this code, 90 00:03:43.520 --> 00:03:45.300 and then it will jump to the next line 91 00:03:45.300 --> 00:03:47.290 and it will continue running from here. 92 00:03:47.290 --> 00:03:49.180 So, I'm gonna run this down here again, 93 00:03:49.180 --> 00:03:50.445 and I'm gonna type "Monday," 94 00:03:50.445 --> 00:03:51.887 and then you see that we get, 95 00:03:51.887 --> 00:03:53.390 "Have a great start to your week!" 96 00:03:53.390 --> 00:03:55.250 and, "This always runs." 97 00:03:55.250 --> 00:03:56.771 But, if we type in "Tuesday," 98 00:03:56.771 --> 00:03:59.130 then you just see "This always runs." 99 00:03:59.130 --> 00:04:01.847 and you don't see the "great start to your week". 100 00:04:02.890 --> 00:04:04.295 If you want to print something 101 00:04:04.295 --> 00:04:07.429 if the day of the week is not Monday, 102 00:04:07.429 --> 00:04:09.870 then you can do something like this: 103 00:04:09.870 --> 00:04:14.870 If day_of_week is not Monday, then print 104 00:04:15.597 --> 00:04:17.230 "Full speed ahead!" 105 00:04:17.230 --> 00:04:19.980 Let's run that, and then I'll type "Monday," 106 00:04:19.980 --> 00:04:22.200 and you get "Have a great start to your week!" 107 00:04:22.200 --> 00:04:24.490 But, if we type "Tuesday," 108 00:04:24.490 --> 00:04:27.240 then you see "Full speed ahead!" 109 00:04:27.240 --> 00:04:29.150 Now this is actually not great Python code, 110 00:04:29.150 --> 00:04:31.368 because you have two checks 111 00:04:31.368 --> 00:04:34.740 that will always be evaluated by Python. 112 00:04:34.740 --> 00:04:37.400 Here you've got day_of_week == "Monday" 113 00:04:37.400 --> 00:04:39.500 and this will always be checked, 114 00:04:39.500 --> 00:04:41.760 whether it is true or false, by Python. 115 00:04:41.760 --> 00:04:45.080 And then you have day_of_week is not equal to "Monday" 116 00:04:45.080 --> 00:04:47.180 and this will always be checked by Python, 117 00:04:48.240 --> 00:04:51.060 regardless of whether this one was true. 118 00:04:51.060 --> 00:04:53.170 Evidently, if this one is true, 119 00:04:53.170 --> 00:04:56.380 this one cannot be, and vice versa. 120 00:04:56.380 --> 00:04:59.130 So, Python provides you a very nice piece of syntax 121 00:04:59.130 --> 00:05:01.017 called the else syntax. 122 00:05:01.017 --> 00:05:02.590 And what this does is, 123 00:05:02.590 --> 00:05:05.403 if this is true, then we run this code. 124 00:05:06.340 --> 00:05:08.540 Otherwise, run this code. 125 00:05:08.540 --> 00:05:11.713 So this is much nicer and also is much easier to read. 126 00:05:12.570 --> 00:05:14.100 Let me say at this stage, 127 00:05:14.100 --> 00:05:17.350 that only one of these two will run. 128 00:05:17.350 --> 00:05:19.640 You cannot possibly run both, 129 00:05:19.640 --> 00:05:21.670 because you can't have this be true 130 00:05:21.670 --> 00:05:23.523 and also false at the same time. 131 00:05:24.500 --> 00:05:26.908 In addition, if you wanted to print something else, 132 00:05:26.908 --> 00:05:29.370 if the day of the week is Tuesday, 133 00:05:29.370 --> 00:05:34.253 you can do elif day_of_week == "Tuesday" 134 00:05:35.810 --> 00:05:39.580 And what this does is it creates a chained if statement, 135 00:05:39.580 --> 00:05:42.637 an if statement with three different branches. 136 00:05:42.637 --> 00:05:44.290 First, Python will check 137 00:05:44.290 --> 00:05:46.372 whether the day of the week is Monday, 138 00:05:46.372 --> 00:05:48.760 and it will print this if it is true. 139 00:05:48.760 --> 00:05:49.920 But if it's not true, 140 00:05:49.920 --> 00:05:50.970 then Python will check 141 00:05:50.970 --> 00:05:52.710 whether the day of the week is Tuesday, 142 00:05:52.710 --> 00:05:55.450 and it will print this if it is true. 143 00:05:55.450 --> 00:05:57.650 But if neither of those are true, 144 00:05:57.650 --> 00:05:59.543 then it will go to the else. 145 00:06:00.730 --> 00:06:04.130 An important part of this syntax here is that 146 00:06:04.130 --> 00:06:06.193 if the day of the week is Monday, 147 00:06:07.040 --> 00:06:09.380 then Python will print this stuff, 148 00:06:09.380 --> 00:06:11.070 and it will ignore the rest. 149 00:06:11.070 --> 00:06:14.420 So it will never calculate this comparison. 150 00:06:14.420 --> 00:06:16.750 So this is actually good for performance, 151 00:06:16.750 --> 00:06:19.800 and also it lets you have a comparison here 152 00:06:19.800 --> 00:06:22.460 that when it evaluates, or when it's true, 153 00:06:22.460 --> 00:06:24.130 it ignores the rest. 154 00:06:24.130 --> 00:06:25.900 So there's actually a lot of stuff you can do 155 00:06:25.900 --> 00:06:27.030 with this knowledge, 156 00:06:27.030 --> 00:06:28.430 and you'll encounter that plenty 157 00:06:28.430 --> 00:06:30.803 while you're doing Python programming. 158 00:06:32.190 --> 00:06:35.180 A common problem when you are dealing with user input 159 00:06:35.180 --> 00:06:38.510 is that they may not enter exactly what you expect. 160 00:06:38.510 --> 00:06:40.100 For example, it is quite common 161 00:06:40.100 --> 00:06:43.603 for a user to enter all lowercase letters 162 00:06:43.603 --> 00:06:46.365 when you ask them for something, 163 00:06:46.365 --> 00:06:48.270 just because they can't be bothered 164 00:06:48.270 --> 00:06:49.970 typing the uppercase letters. 165 00:06:49.970 --> 00:06:51.740 So here, the user has typed "monday" 166 00:06:51.740 --> 00:06:54.397 all in lowercase, and then we have printed out 167 00:06:54.397 --> 00:06:56.860 "Full speed ahead!" because the day of the week 168 00:06:56.860 --> 00:06:59.740 is not exactly equal to the string "Monday" 169 00:06:59.740 --> 00:07:01.400 with a capital M. 170 00:07:01.400 --> 00:07:04.360 So, you have to do something about it 171 00:07:04.360 --> 00:07:06.610 if you want your programme to be able to detect 172 00:07:06.610 --> 00:07:08.460 whether the day of the week is Monday 173 00:07:08.460 --> 00:07:11.470 in uppercase and in lowercase. 174 00:07:11.470 --> 00:07:14.360 What I'd recommend is that you get the user's input 175 00:07:15.270 --> 00:07:19.200 and just after you type .lower 176 00:07:19.200 --> 00:07:23.490 And then, this is going to turn this entire string 177 00:07:24.610 --> 00:07:27.163 into it's lowercase variant. 178 00:07:28.310 --> 00:07:31.931 Now, if they enter Monday with the capital M, 179 00:07:31.931 --> 00:07:34.830 then it is also gonna be turned into monday 180 00:07:34.830 --> 00:07:36.402 with a lowercase m, 181 00:07:36.402 --> 00:07:38.810 which means that monday =="Monday" 182 00:07:38.810 --> 00:07:41.460 with an uppercase M will not match. 183 00:07:41.460 --> 00:07:42.970 So, instead what you should do 184 00:07:42.970 --> 00:07:45.500 is turn everything to lowercase. 185 00:07:45.500 --> 00:07:48.160 By turning the user's input into lowercase, 186 00:07:48.160 --> 00:07:50.740 and your checks into lowercase, 187 00:07:50.740 --> 00:07:52.680 you're always going to have the same case, 188 00:07:52.680 --> 00:07:54.690 because they're both lowercase. 189 00:07:54.690 --> 00:07:56.690 So if we save that and run it, 190 00:07:56.690 --> 00:07:59.190 you'll see that now I can type "monday," 191 00:07:59.190 --> 00:08:01.970 but I can also type "MONDAY" all in uppercase, 192 00:08:01.970 --> 00:08:04.850 and I can also type "Monday" as usual. 193 00:08:04.850 --> 00:08:06.970 So this is a good way of insuring that your programme 194 00:08:06.970 --> 00:08:10.973 will work, no matter what type of case the users type. 195 00:08:11.830 --> 00:08:12.890 Let's leave it at this, 196 00:08:12.890 --> 00:08:14.310 thank you for joining me in this video, 197 00:08:14.310 --> 00:08:15.440 I hope it's been useful, 198 00:08:15.440 --> 00:08:17.090 and I'll see you in the next one.