1 00:00:04,360 --> 00:00:09,520 Let's discuss truthy values. So I want to actually discuss something that you'll 2 00:00:09,520 --> 00:00:12,840 see being done when you're looking at code on the internet, for example, 3 00:00:12,840 --> 00:00:16,780 and I'm going to keep this short and I'm including it here for completeness. 4 00:00:16,780 --> 00:00:20,320 Don't worry if you don't understand this. It'll be there in the back of your mind and 5 00:00:20,320 --> 00:00:27,040 it'll make sense when we come to use it. So I'll start by referring to the Python documentation. 6 00:00:27,040 --> 00:00:28,980 I'm linking the resources section in this video. We're going to 7 00:00:28,989 --> 00:00:33,340 paste it into here. So it starts by saying that an object can be tested for 8 00:00:33,340 --> 00:00:38,109 truth value for use in an if or while condition. We'll see an example of that 9 00:00:38,109 --> 00:00:42,300 in a moment. Now just below that, you can see there's two bullets there, 10 00:00:42,300 --> 00:00:47,739 indicating values that will evaluate to False. So it mentions two constants - none 11 00:00:47,740 --> 00:00:53,300 and False. We haven't seen none yet but obviously, False is False. Zero is also 12 00:00:53,300 --> 00:00:57,460 considered False, whether that's an int zero or a floating point zero, it's 13 00:00:57,460 --> 00:01:03,660 still False. And finally, empty sequences and collections are also considered False. 14 00:01:03,660 --> 00:01:07,760 So what does all this mean? We're going to go back to the code 15 00:01:07,760 --> 00:01:11,580 and we're going to stay with our truefalse.py file from the previous video. We're going to 16 00:01:11,580 --> 00:01:14,500 delete all the code now. 17 00:01:14,500 --> 00:01:16,990 If you've been experimenting, and I hope you have, you 18 00:01:16,990 --> 00:01:19,930 may want to keep your code. In that case, just create a new python file for 19 00:01:19,930 --> 00:01:22,360 these examples. Alright so let's have a look at things 20 00:01:22,360 --> 00:01:27,370 that evaluate to False when used in a boolean expression. The documentation 21 00:01:27,370 --> 00:01:33,130 said that zero was treated as False, so let's type some code: if 0 colon, 22 00:01:33,130 --> 00:01:39,280 on the next line, print parentheses and True in double quotes. We'll add an else there, else 23 00:01:39,280 --> 00:01:47,360 colon print parentheses double quotes False. 24 00:01:47,360 --> 00:01:52,780 Now we don't actually have to execute this code because my IDE's telling me, if I hover over line 2 there, 25 00:01:52,780 --> 00:01:59,770 this code is unreachable. 0 won't evaluate to True, so line 2 can't be executed. 26 00:01:59,770 --> 00:02:06,880 We'll run it anyway and it prints False. Alright so zero is evaluated to False 27 00:02:06,880 --> 00:02:13,040 when it appears in a boolean expression. Empty sequences also evaluate to False. 28 00:02:13,040 --> 00:02:16,760 We've only seen one sequence type so far, the string type. 29 00:02:16,760 --> 00:02:19,920 So if an empty string evaluates to False, we can do something like the following 30 00:02:19,920 --> 00:02:26,920 I'm going to type in. So on line six, name equals input parentheses double quotes 31 00:02:26,920 --> 00:02:33,700 please enter your name, and we'll put a colon there and we want a space, and closing double quote and right 32 00:02:33,700 --> 00:02:39,280 parenthesis. Then we'll put if name colon print on the next line parentheses 33 00:02:39,280 --> 00:02:46,160 double quotes. We're going to type Hello Hello comma then left and right curly braces, 34 00:02:46,160 --> 00:02:51,139 then closing double quote dot format left parenthesis name, then we've got the two 35 00:02:51,140 --> 00:03:03,140 right parentheses there, else colon print Are you the man with no name? 36 00:03:03,140 --> 00:03:09,640 Alright, so both of those if statements on line 1 and line 7 look a bit strange. 37 00:03:09,640 --> 00:03:13,489 Python expects a boolean expression after if, which is why it tries to 38 00:03:13,489 --> 00:03:18,590 evaluate 0 and name as boolean values. We've already seen that 0 evaluates to 39 00:03:18,590 --> 00:03:24,380 False. Let's run the program and see what happens on line 7. 40 00:03:24,380 --> 00:03:31,040 Alright, so I'm going to start by entering my name Tim. Because Tim isn't an empty string, it evaluates as 41 00:03:31,040 --> 00:03:36,320 True on line 7 and we get the message Hello Tim. Run the program again. 42 00:03:36,320 --> 00:03:40,120 This time I'm just going to press enter here without typing anything. So that then 43 00:03:40,129 --> 00:03:44,329 results in name being an empty string which evaluates as False and that's why 44 00:03:44,329 --> 00:03:48,340 we get the message Are you the man with no name? So that's actually quite useful. 45 00:03:48,340 --> 00:03:51,820 We've got a quick way now to check if the user types something when we ask 46 00:03:51,820 --> 00:03:55,940 them for input. You'll see things like that being done, but I suggest you avoid 47 00:03:55,940 --> 00:04:00,040 doing it until you're really comfortable with Python. So until then what I do, 48 00:04:00,049 --> 00:04:03,620 I suggest you do, is write line 7 a different way, so I want to comment that 49 00:04:03,620 --> 00:04:10,159 out so you've still got it there, and instead I'm going to type it as if name is not 50 00:04:10,159 --> 00:04:14,930 equal, and two double quotes with a colon. So now what we're doing on line 8 is 51 00:04:14,930 --> 00:04:19,009 explicitly comparing name to an empty string. It's more obvious what we're 52 00:04:19,009 --> 00:04:22,660 doing there and that's the way I suggest you write your conditions to begin with. 53 00:04:22,660 --> 00:04:25,240 In the next video we'll finish discussing boolean expressions by 54 00:04:25,240 --> 00:04:29,140 looking at two more operators we can use; in and not in. 55 00:04:29,140 --> 00:04:31,980 See you in the next video.