1 1 00:00:00,770 --> 00:00:02,510 Hi guys, and welcome back. 2 2 00:00:02,510 --> 00:00:05,220 In this video we're going to learn about if statements. 3 3 00:00:05,220 --> 00:00:07,979 An if statement allows us to use a Boolean 4 4 00:00:07,979 --> 00:00:10,340 to make a decision in our programme 5 5 00:00:10,340 --> 00:00:12,160 and potentially do different things, 6 6 00:00:12,160 --> 00:00:14,278 depending on the value of that Boolean. 7 7 00:00:14,278 --> 00:00:16,930 Of course, if statements are most useful 8 8 00:00:16,930 --> 00:00:19,020 when you have user input, 9 9 00:00:19,020 --> 00:00:22,220 such that your programme can potentially get different data, 10 10 00:00:22,220 --> 00:00:25,580 depending on who runs it and who's using your programme. 11 11 00:00:25,580 --> 00:00:27,830 So here we've got an input that asks the user 12 12 00:00:27,830 --> 00:00:29,430 for the day of the week, 13 13 00:00:29,430 --> 00:00:31,960 so what day of the week is it today? 14 14 00:00:31,960 --> 00:00:33,730 And then, we're going to check 15 15 00:00:33,730 --> 00:00:36,320 whether the current day of the week is Monday. 16 16 00:00:36,320 --> 00:00:37,440 So, how would you check 17 17 00:00:37,440 --> 00:00:39,260 whether the current day of the week is Monday? 18 18 00:00:39,260 --> 00:00:44,260 You would say day_of_week == the string "Monday". 19 19 00:00:45,050 --> 00:00:49,630 Let's print this out to see whether our Boolean works. 20 20 00:00:49,630 --> 00:00:52,870 So, all I've done is printed the Boolean out. 21 21 00:00:52,870 --> 00:00:54,813 And again, remember that this here 22 22 00:00:54,813 --> 00:00:58,053 will evaluate first, so Python will say, 23 23 00:00:58,053 --> 00:01:00,500 "Day of week, is it equal to Monday?" 24 24 00:01:00,500 --> 00:01:02,650 And if it is, then it will replace 25 25 00:01:02,650 --> 00:01:04,920 this entire thing by "true." 26 26 00:01:04,920 --> 00:01:07,597 Otherwise, it will be replaced by "false." 27 27 00:01:08,600 --> 00:01:09,990 And, if we run this, 28 28 00:01:09,990 --> 00:01:11,830 you see that we get our prompt, 29 29 00:01:11,830 --> 00:01:13,220 and you can type in "Monday," 30 30 00:01:13,220 --> 00:01:14,780 and then you get True. 31 31 00:01:14,780 --> 00:01:16,092 But, if you type "Tuesday," 32 32 00:01:16,092 --> 00:01:18,670 then you get False. 33 33 00:01:18,670 --> 00:01:20,750 So this is what we expected. 34 34 00:01:20,750 --> 00:01:22,930 Now, instead of printing this out, 35 35 00:01:22,930 --> 00:01:26,410 we can actually put it in an if statement. 36 36 00:01:26,410 --> 00:01:29,040 So the if statement is like this, 37 37 00:01:29,040 --> 00:01:33,450 you type in your if keyword, which is special in Python. 38 38 00:01:33,450 --> 00:01:36,040 Then you put a space, and then, 39 39 00:01:36,040 --> 00:01:37,772 without putting brackets or anything, 40 40 00:01:37,772 --> 00:01:41,980 you type your Boolean comparison. 41 41 00:01:41,980 --> 00:01:44,373 And then at the end you put your colon. 42 42 00:01:45,320 --> 00:01:49,140 This will evaluate as soon as the if statement runs, 43 43 00:01:49,140 --> 00:01:51,450 so as soon as Python reaches line 3, 44 44 00:01:51,450 --> 00:01:54,250 it's gonna calculate the result of this Boolean, 45 45 00:01:54,250 --> 00:01:56,630 which is gonna be true if the user entered "Monday," 46 46 00:01:56,630 --> 00:01:58,680 and false if they didn't. 47 47 00:01:58,680 --> 00:02:00,930 And if it is true, 48 48 00:02:00,930 --> 00:02:05,590 then we will run the code under the if statement. 49 49 00:02:05,590 --> 00:02:07,720 And here's one of the most important things 50 50 00:02:07,720 --> 00:02:09,330 in Python programming, 51 51 00:02:09,330 --> 00:02:11,350 which is that there are a few spaces 52 52 00:02:11,350 --> 00:02:13,050 in front of this line here. 53 53 00:02:13,050 --> 00:02:15,630 This is called indentation in Python, 54 54 00:02:15,630 --> 00:02:19,910 and after a colon that defines a block of code, 55 55 00:02:19,910 --> 00:02:22,740 such as an if statement or a function or a class, 56 56 00:02:22,740 --> 00:02:24,378 we're gonna learn more about those later on, 57 57 00:02:24,378 --> 00:02:27,580 you need to put indentation. 58 58 00:02:27,580 --> 00:02:30,315 That is how Python tells which code 59 59 00:02:30,315 --> 00:02:32,520 is inside the if statement, 60 60 00:02:32,520 --> 00:02:36,160 i.e. which code you want to run when this is true. 61 61 00:02:36,160 --> 00:02:38,607 So, for example, you can print 62 62 00:02:38,607 --> 00:02:41,207 "Have a great start to your week!" 63 63 00:02:42,470 --> 00:02:44,890 Now, this is the line of code 64 64 00:02:44,890 --> 00:02:47,140 that is inside the if statement. 65 65 00:02:47,140 --> 00:02:49,930 If this is true, this line of code will run, 66 66 00:02:49,930 --> 00:02:53,040 because it's got these four spaces in front. 67 67 00:02:53,040 --> 00:02:53,873 I'll say at this point that 68 68 00:02:53,873 --> 00:02:55,610 you don't have to use four space in Python, 69 69 00:02:55,610 --> 00:02:57,850 you can use two, you can use six, or eight, 70 70 00:02:57,850 --> 00:03:00,540 whatever you want, but be consistent. 71 71 00:03:00,540 --> 00:03:02,400 So, always use the same number of spaces 72 72 00:03:02,400 --> 00:03:03,938 that's gonna make your code work, 73 73 00:03:03,938 --> 00:03:05,690 otherwise it may not work, 74 74 00:03:05,690 --> 00:03:07,880 and also it's gonna make your code look much better. 75 75 00:03:07,880 --> 00:03:09,360 If you've got different indentations 76 76 00:03:09,360 --> 00:03:10,720 in different parts of your programme, 77 77 00:03:10,720 --> 00:03:11,920 people are gonna be really confused 78 78 00:03:11,920 --> 00:03:13,400 when they look at your code. 79 79 00:03:13,400 --> 00:03:16,423 I recommend using four spaces for Python code. 80 80 00:03:17,320 --> 00:03:21,200 Now, if you print something and you remove the indentation, 81 81 00:03:21,200 --> 00:03:22,393 the spaces in front, 82 82 00:03:26,370 --> 00:03:28,320 for example, like this, what you're gonna get 83 83 00:03:28,320 --> 00:03:31,200 is that this code is always going to run, 84 84 00:03:31,200 --> 00:03:33,720 if this is true, or if it is false. 85 85 00:03:33,720 --> 00:03:35,530 Because it's not in the if statement, 86 86 00:03:35,530 --> 00:03:38,200 it's completely unrelated to the if statement. 87 87 00:03:38,200 --> 00:03:40,640 So, Python will run this code if it's true, 88 88 00:03:40,640 --> 00:03:41,800 it will run this code. 89 89 00:03:41,800 --> 00:03:43,520 If it's false, it will not run this code, 90 90 00:03:43,520 --> 00:03:45,300 and then it will jump to the next line 91 91 00:03:45,300 --> 00:03:47,290 and it will continue running from here. 92 92 00:03:47,290 --> 00:03:49,180 So, I'm gonna run this down here again, 93 93 00:03:49,180 --> 00:03:50,445 and I'm gonna type "Monday," 94 94 00:03:50,445 --> 00:03:51,887 and then you see that we get, 95 95 00:03:51,887 --> 00:03:53,390 "Have a great start to your week!" 96 96 00:03:53,390 --> 00:03:55,250 and, "This always runs." 97 97 00:03:55,250 --> 00:03:56,771 But, if we type in "Tuesday," 98 98 00:03:56,771 --> 00:03:59,130 then you just see "This always runs." 99 99 00:03:59,130 --> 00:04:01,847 and you don't see the "great start to your week". 100 100 00:04:02,890 --> 00:04:04,295 If you want to print something 101 101 00:04:04,295 --> 00:04:07,429 if the day of the week is not Monday, 102 102 00:04:07,429 --> 00:04:09,870 then you can do something like this: 103 103 00:04:09,870 --> 00:04:14,870 If day_of_week is not Monday, then print 104 104 00:04:15,597 --> 00:04:17,230 "Full speed ahead!" 105 105 00:04:17,230 --> 00:04:19,980 Let's run that, and then I'll type "Monday," 106 106 00:04:19,980 --> 00:04:22,200 and you get "Have a great start to your week!" 107 107 00:04:22,200 --> 00:04:24,490 But, if we type "Tuesday," 108 108 00:04:24,490 --> 00:04:27,240 then you see "Full speed ahead!" 109 109 00:04:27,240 --> 00:04:29,150 Now this is actually not great Python code, 110 110 00:04:29,150 --> 00:04:31,368 because you have two checks 111 111 00:04:31,368 --> 00:04:34,740 that will always be evaluated by Python. 112 112 00:04:34,740 --> 00:04:37,400 Here you've got day_of_week == "Monday" 113 113 00:04:37,400 --> 00:04:39,500 and this will always be checked, 114 114 00:04:39,500 --> 00:04:41,760 whether it is true or false, by Python. 115 115 00:04:41,760 --> 00:04:45,080 And then you have day_of_week is not equal to "Monday" 116 116 00:04:45,080 --> 00:04:47,180 and this will always be checked by Python, 117 117 00:04:48,240 --> 00:04:51,060 regardless of whether this one was true. 118 118 00:04:51,060 --> 00:04:53,170 Evidently, if this one is true, 119 119 00:04:53,170 --> 00:04:56,380 this one cannot be, and vice versa. 120 120 00:04:56,380 --> 00:04:59,130 So, Python provides you a very nice piece of syntax 121 121 00:04:59,130 --> 00:05:01,017 called the else syntax. 122 122 00:05:01,017 --> 00:05:02,590 And what this does is, 123 123 00:05:02,590 --> 00:05:05,403 if this is true, then we run this code. 124 124 00:05:06,340 --> 00:05:08,540 Otherwise, run this code. 125 125 00:05:08,540 --> 00:05:11,713 So this is much nicer and also is much easier to read. 126 126 00:05:12,570 --> 00:05:14,100 Let me say at this stage, 127 127 00:05:14,100 --> 00:05:17,350 that only one of these two will run. 128 128 00:05:17,350 --> 00:05:19,640 You cannot possibly run both, 129 129 00:05:19,640 --> 00:05:21,670 because you can't have this be true 130 130 00:05:21,670 --> 00:05:23,523 and also false at the same time. 131 131 00:05:24,500 --> 00:05:26,908 In addition, if you wanted to print something else, 132 132 00:05:26,908 --> 00:05:29,370 if the day of the week is Tuesday, 133 133 00:05:29,370 --> 00:05:34,253 you can do elif day_of_week == "Tuesday" 134 134 00:05:35,810 --> 00:05:39,580 And what this does is it creates a chained if statement, 135 135 00:05:39,580 --> 00:05:42,637 an if statement with three different branches. 136 136 00:05:42,637 --> 00:05:44,290 First, Python will check 137 137 00:05:44,290 --> 00:05:46,372 whether the day of the week is Monday, 138 138 00:05:46,372 --> 00:05:48,760 and it will print this if it is true. 139 139 00:05:48,760 --> 00:05:49,920 But if it's not true, 140 140 00:05:49,920 --> 00:05:50,970 then Python will check 141 141 00:05:50,970 --> 00:05:52,710 whether the day of the week is Tuesday, 142 142 00:05:52,710 --> 00:05:55,450 and it will print this if it is true. 143 143 00:05:55,450 --> 00:05:57,650 But if neither of those are true, 144 144 00:05:57,650 --> 00:05:59,543 then it will go to the else. 145 145 00:06:00,730 --> 00:06:04,130 An important part of this syntax here is that 146 146 00:06:04,130 --> 00:06:06,193 if the day of the week is Monday, 147 147 00:06:07,040 --> 00:06:09,380 then Python will print this stuff, 148 148 00:06:09,380 --> 00:06:11,070 and it will ignore the rest. 149 149 00:06:11,070 --> 00:06:14,420 So it will never calculate this comparison. 150 150 00:06:14,420 --> 00:06:16,750 So this is actually good for performance, 151 151 00:06:16,750 --> 00:06:19,800 and also it lets you have a comparison here 152 152 00:06:19,800 --> 00:06:22,460 that when it evaluates, or when it's true, 153 153 00:06:22,460 --> 00:06:24,130 it ignores the rest. 154 154 00:06:24,130 --> 00:06:25,900 So there's actually a lot of stuff you can do 155 155 00:06:25,900 --> 00:06:27,030 with this knowledge, 156 156 00:06:27,030 --> 00:06:28,430 and you'll encounter that plenty 157 157 00:06:28,430 --> 00:06:30,803 while you're doing Python programming. 158 158 00:06:32,190 --> 00:06:35,180 A common problem when you are dealing with user input 159 159 00:06:35,180 --> 00:06:38,510 is that they may not enter exactly what you expect. 160 160 00:06:38,510 --> 00:06:40,100 For example, it is quite common 161 161 00:06:40,100 --> 00:06:43,603 for a user to enter all lowercase letters 162 162 00:06:43,603 --> 00:06:46,365 when you ask them for something, 163 163 00:06:46,365 --> 00:06:48,270 just because they can't be bothered 164 164 00:06:48,270 --> 00:06:49,970 typing the uppercase letters. 165 165 00:06:49,970 --> 00:06:51,740 So here, the user has typed "monday" 166 166 00:06:51,740 --> 00:06:54,397 all in lowercase, and then we have printed out 167 167 00:06:54,397 --> 00:06:56,860 "Full speed ahead!" because the day of the week 168 168 00:06:56,860 --> 00:06:59,740 is not exactly equal to the string "Monday" 169 169 00:06:59,740 --> 00:07:01,400 with a capital M. 170 170 00:07:01,400 --> 00:07:04,360 So, you have to do something about it 171 171 00:07:04,360 --> 00:07:06,610 if you want your programme to be able to detect 172 172 00:07:06,610 --> 00:07:08,460 whether the day of the week is Monday 173 173 00:07:08,460 --> 00:07:11,470 in uppercase and in lowercase. 174 174 00:07:11,470 --> 00:07:14,360 What I'd recommend is that you get the user's input 175 175 00:07:15,270 --> 00:07:19,200 and just after you type .lower 176 176 00:07:19,200 --> 00:07:23,490 And then, this is going to turn this entire string 177 177 00:07:24,610 --> 00:07:27,163 into it's lowercase variant. 178 178 00:07:28,310 --> 00:07:31,931 Now, if they enter Monday with the capital M, 179 179 00:07:31,931 --> 00:07:34,830 then it is also gonna be turned into monday 180 180 00:07:34,830 --> 00:07:36,402 with a lowercase m, 181 181 00:07:36,402 --> 00:07:38,810 which means that monday =="Monday" 182 182 00:07:38,810 --> 00:07:41,460 with an uppercase M will not match. 183 183 00:07:41,460 --> 00:07:42,970 So, instead what you should do 184 184 00:07:42,970 --> 00:07:45,500 is turn everything to lowercase. 185 185 00:07:45,500 --> 00:07:48,160 By turning the user's input into lowercase, 186 186 00:07:48,160 --> 00:07:50,740 and your checks into lowercase, 187 187 00:07:50,740 --> 00:07:52,680 you're always going to have the same case, 188 188 00:07:52,680 --> 00:07:54,690 because they're both lowercase. 189 189 00:07:54,690 --> 00:07:56,690 So if we save that and run it, 190 190 00:07:56,690 --> 00:07:59,190 you'll see that now I can type "monday," 191 191 00:07:59,190 --> 00:08:01,970 but I can also type "MONDAY" all in uppercase, 192 192 00:08:01,970 --> 00:08:04,850 and I can also type "Monday" as usual. 193 193 00:08:04,850 --> 00:08:06,970 So this is a good way of insuring that your programme 194 194 00:08:06,970 --> 00:08:10,973 will work, no matter what type of case the users type. 195 195 00:08:11,830 --> 00:08:12,890 Let's leave it at this, 196 196 00:08:12,890 --> 00:08:14,310 thank you for joining me in this video, 197 197 00:08:14,310 --> 00:08:15,440 I hope it's been useful, 198 198 00:08:15,440 --> 00:08:17,090 and I'll see you in the next one.