1 00:00:04,590 --> 00:00:09,870 Let's talk about boolean expressions, True or False. So a boolean value is a 2 00:00:09,870 --> 00:00:16,960 value that can either be True or False. There are only two values that a boolean can have. 3 00:00:16,960 --> 00:00:21,060 Python defines two constants, True and False. Note that they're written 4 00:00:21,060 --> 00:00:25,830 with a capital letter. All names in Python are case-sensitive, so True as a 5 00:00:25,830 --> 00:00:32,430 capital T and False has a capital F. A boolean expression is an expression that 6 00:00:32,430 --> 00:00:37,290 evaluates to either True or False. We've seen a few boolean expressions, so far in 7 00:00:37,290 --> 00:00:41,260 the course - things like age greater than equal to 16, for example. 8 00:00:41,260 --> 00:00:45,320 Boolean expressions aren't restricted to just testing numbers. We could also test a 9 00:00:45,330 --> 00:00:49,380 string value. Name is equal to Tim, so that's a boolean expression that will 10 00:00:49,380 --> 00:00:54,140 evaluate to True if the value of name equals the string Tim. 11 00:00:54,140 --> 00:00:58,640 It will evaluate to False if name has any other value. 12 00:00:58,640 --> 00:01:04,000 Boolean expressions can be very complex. For example, you might have a condition such as: if day is equal to 13 00:01:04,000 --> 00:01:08,680 Saturday, in double quotes, and temperature greater than 27 and not raining. 14 00:01:08,680 --> 00:01:11,780 So you can see in that example, we're checking to see whether day is 15 00:01:11,780 --> 00:01:15,800 equal to Saturday and whether temperature is greater than 27, and we're 16 00:01:15,800 --> 00:01:20,810 also using and not here. Now in this example, raining is a boolean variable. 17 00:01:20,810 --> 00:01:26,540 It has the value True or False. Not is used to reverse the value. 18 00:01:26,540 --> 00:01:33,780 So we can use not to reverse a boolean value: not True is False and not False is True. 19 00:01:33,780 --> 00:01:37,500 Alright, so back to the code now, so let's actually create a new Python file. 20 00:01:37,500 --> 00:01:46,200 I'm going to call this one truefalse. 21 00:01:46,200 --> 00:01:49,820 Alright, we're gonna start typing some code: day is equal to, 22 00:01:49,820 --> 00:01:57,680 in double quotes, Monday with a capital M. Line 2, temperature is equal to 30. 23 00:01:57,680 --> 00:02:03,860 Next line, raining is equal to True, remembering we need to use a capital T 24 00:02:03,860 --> 00:02:06,940 when using a boolean variable, as we've done there. Then we'll have a 25 00:02:06,950 --> 00:02:12,890 look at that condition, so if day is equal to, so it's two equal signs, Saturday 26 00:02:12,890 --> 00:02:18,780 in double quotes with a capital S, and temperature is greater than 27 27 00:02:18,780 --> 00:02:28,940 and not raining colon, print parentheses double quotes Go swimming. 28 00:02:28,940 --> 00:02:33,060 And the next line else : then on the line 8, indented, 29 00:02:33,060 --> 00:02:40,040 print parenthesis double quotes and within the double quotes, learn Python. 30 00:02:40,040 --> 00:02:44,959 Now didn't mention it when I created this file, truefalse.py, but be careful with 31 00:02:44,959 --> 00:02:48,230 the names you choose for your Python file names. You're starting to learn 32 00:02:48,230 --> 00:02:52,250 quite a few Python names and keywords now, so it's getting easier to avoid 33 00:02:52,250 --> 00:02:58,140 using them as the name of your file. Now if you'd called the file true.py with a capital T, 34 00:02:58,140 --> 00:03:00,680 then that wouldn't be a good idea. You might not get an 35 00:03:00,680 --> 00:03:04,340 error straight away but it could cause problems eventually. So just keep it in 36 00:03:04,340 --> 00:03:09,709 mind and try not to use Python names and keywords. And I do suggest experimenting 37 00:03:09,709 --> 00:03:13,040 with different boolean expressions to get used to how they work, as a good idea. 38 00:03:13,040 --> 00:03:17,100 We're obviously doing that here with the code that we saw on the slides. 39 00:03:17,100 --> 00:03:20,420 It's essential to attach the names to values before you use them in 40 00:03:20,420 --> 00:03:24,820 your boolean expressions. If you don't attach date or value, for example, 41 00:03:24,820 --> 00:03:29,700 then you'd get an error on line 5. The same for temperature and raining. 42 00:03:29,700 --> 00:03:34,540 Alright, so let's run the program, and we should see Learn Python, 43 00:03:34,540 --> 00:03:38,780 and you can see we're seeing that down the bottom there. To make sure this is working because we 44 00:03:38,780 --> 00:03:45,620 know that temperature is 30 and raining set to True, let's change the day to Saturday, 45 00:03:45,620 --> 00:03:48,920 remembering I need to type it exactly how I've typed Saturday on line 5, 46 00:03:48,920 --> 00:03:55,300 in this case, with a capital S. Run that, and in fact, we're still learning Python. Why? 47 00:03:55,300 --> 00:04:00,100 Well that's because it's raining. We set raining to True on line 3. 48 00:04:00,100 --> 00:04:05,800 All three parts of the condition, on line 5, must evaluate to True in order for the entire 49 00:04:05,800 --> 00:04:11,500 condition to be true. The last part of the condition on line 5 is: and not raining. 50 00:04:11,500 --> 00:04:16,459 So raining is true, as we can see on line 3. That means that not raining is false. 51 00:04:16,459 --> 00:04:22,060 So what we can do here is change the and, here in line 5, to an or, 52 00:04:22,060 --> 00:04:27,560 and once we do that and run the program again, this time we get the output, Go swimming. 53 00:04:27,560 --> 00:04:31,260 When you're experimenting, remember which strings have to match exactly. 54 00:04:31,260 --> 00:04:36,960 On line 1, we've got Saturday with a capital S, which matches Saturday in the condition on line 5. 55 00:04:36,960 --> 00:04:40,560 If you'd spelled one of them with a lowercase s, so I had another typo, 56 00:04:40,560 --> 00:04:44,700 they wouldn't compare equal and your program wouldn't work as expected. 57 00:04:44,700 --> 00:04:49,280 Before I finish the video, take a closer look at line 5 in our code. 58 00:04:49,280 --> 00:04:53,980 Can you tell by reading that line, what the true intention, or what I wanted that line to do? 59 00:04:53,980 --> 00:04:58,700 It's ambiguous to read, so we know how Python will interpret it because the 60 00:04:58,700 --> 00:05:01,790 precedents of the operators is well-documented, and you can find the 61 00:05:01,790 --> 00:05:06,000 table in the documentation in a link in the resources section of this video. 62 00:05:06,000 --> 00:05:11,320 But I can also bring it up on screen, which I'll do now. 63 00:05:11,320 --> 00:05:16,820 So there's the table. Now we discussed operator precedence for the arithmetic operators in the previous section, 64 00:05:16,820 --> 00:05:20,600 but boolean, or conditional operators also have precedence and you 65 00:05:20,600 --> 00:05:25,280 can see the or and not there, talking about boolean. So we can see here that and 66 00:05:25,280 --> 00:05:30,380 has a higher precedence than or. Now that's why we go swimming even though it's raining, 67 00:05:30,380 --> 00:05:35,600 looking back at the code again. Raining is True, as you can see on line 3. 68 00:05:35,600 --> 00:05:39,340 So is that a bug, or is that what I intended? The point of this is that 69 00:05:39,340 --> 00:05:43,300 that line of code doesn't really give you any indication of what I intended. 70 00:05:43,300 --> 00:05:48,140 Even if they aren't necessary, we should use parentheses to make it clear. 71 00:05:48,140 --> 00:05:52,280 So let's change the day to Friday to see what happens. 72 00:05:52,580 --> 00:06:00,820 Capital F there. We run the program. We can see Learn Python, outputted at the bottom of the screen. 73 00:06:00,820 --> 00:06:05,480 The condition day is equal to Saturday, evaluates to False. 74 00:06:05,480 --> 00:06:09,300 The condition temperature greater than 27 evaluates to True, 75 00:06:09,300 --> 00:06:15,780 but False and True is False, as we established in previous videos. Not raining is also False, 76 00:06:15,780 --> 00:06:19,870 which means the code in the else block is executed. So what would happen 77 00:06:19,870 --> 00:06:26,440 here if we changed raining to False. Let's do that. 78 00:06:26,440 --> 00:06:28,640 Run the code. 79 00:06:28,640 --> 00:06:33,760 This time we go swimming. As I said, you don't have any indication if that's what was intended 80 00:06:33,760 --> 00:06:37,640 Maybe I want to go swimming on hot Saturdays or on any other day 81 00:06:37,640 --> 00:06:41,960 that's not raining. So let's add some parentheses here, to make it clear 82 00:06:41,960 --> 00:06:45,880 how this code is being interpreted. So I'm going to put a parenthesis in front, 83 00:06:45,880 --> 00:06:51,180 a left parenthesis in front of day, and the other right parenthesis after 84 00:06:51,180 --> 00:06:57,320 temperature greater than 27, over here. Now this is how the code's interpreted at the moment, 85 00:06:57,320 --> 00:07:01,990 because and has a higher precedence than or. So even if that's 86 00:07:01,990 --> 00:07:05,280 what I meant, then I should still use the parentheses. 87 00:07:05,280 --> 00:07:09,180 They're not needed by Python but they make the code much easier to read. 88 00:07:09,180 --> 00:07:13,080 Looking at this code now, it's now clear that we'll go swimming on any day 89 00:07:13,080 --> 00:07:17,800 when it's not raining, as well as on hot Saturdays, whether it's raining or not. 90 00:07:17,800 --> 00:07:23,520 As a general rule, always use parentheses when you mix and and or in the same condition. 91 00:07:23,520 --> 00:07:26,900 Doing it makes it clear what you intended. More importantly, 92 00:07:26,900 --> 00:07:29,520 it makes it less likely that you'll introduce a bug. 93 00:07:29,520 --> 00:07:32,820 When you use parentheses, you don't have to worry about remembering 94 00:07:32,820 --> 00:07:35,980 whether and has a higher or lower precedence than or. 95 00:07:35,980 --> 00:07:40,200 Alright, so try different values for those three variables, and then have a go at creating your own 96 00:07:40,200 --> 00:07:44,760 boolean expressions. In the next video, we're gonna have a quick look at some truthy values. 97 00:07:44,760 --> 00:07:47,920 See you in the next video.