1 00:00:04,710 --> 00:00:09,930 Alright, so let's talk more about conditional operators in Python, but before we 2 00:00:09,930 --> 00:00:13,820 look at other conditional operators, let's review the code from the last video. 3 00:00:13,820 --> 00:00:18,660 So on line 6, we test to see if the original guess is less than 5. If it is, we print 4 00:00:18,660 --> 00:00:23,960 another message and allow another guess. If the original guess is not less than 5, 5 00:00:23,960 --> 00:00:29,400 then line 13 checks if it's greater than 5 using elif. We allow another guess 6 00:00:29,400 --> 00:00:33,989 if it's greater than 5. If neither of those conditions are true, then the guess must 7 00:00:33,989 --> 00:00:38,980 have been equal to 5. This is picked up by the final else on line 20. 8 00:00:38,980 --> 00:00:42,640 There are better ways to write code like this, as we'll see shortly. 9 00:00:42,640 --> 00:00:48,540 We've duplicated exactly the same code in the if and elif blocks. The duplicate lines are lines 8 10 00:00:48,540 --> 00:00:54,690 to 12 and also 15 to 19. Whenever you see duplication like this happening, that's a 11 00:00:54,690 --> 00:00:56,900 good indication that it can be done better. 12 00:00:56,900 --> 00:01:03,020 For now, though, this example has illustrated a number of important concepts. 13 00:01:03,020 --> 00:01:07,700 Alright, so let's talk about if blocks. So a block can be quite complex, including further 14 00:01:07,700 --> 00:01:11,990 if and else blocks, and much more contained within it. When testing for 15 00:01:11,990 --> 00:01:16,520 equality, we can't use a single equal symbol. The single equal is used for 16 00:01:16,520 --> 00:01:20,600 assigning values to variables, so when testing for equality, we have to use two 17 00:01:20,600 --> 00:01:25,160 equal signs, the equals equals. An if statement can include many elif parts, 18 00:01:25,160 --> 00:01:29,680 but there can only be one else. elif, by the way, is short for else if. 19 00:01:29,680 --> 00:01:35,300 The else, if there is one, must come after all the elif blocks. Duplicating code is 20 00:01:35,300 --> 00:01:40,000 generally a bad idea. There's almost always a better way. 21 00:01:41,320 --> 00:01:45,360 When testing conditions, we can use any of the value comparison operators. 22 00:01:45,360 --> 00:01:51,360 Now there are other types of comparisons we can perform but we'll focus on these 6 for now; 23 00:01:51,360 --> 00:01:53,640 less than, less than or equal to, greater than, 24 00:01:53,640 --> 00:01:58,120 greater than or equal to, equal to and not equal to. 25 00:01:58,120 --> 00:02:03,600 Once again, remember that we test for equality using two equal symbols. That's something you'll 26 00:02:03,610 --> 00:02:07,810 forget at first, but you'll get a syntax error if you use a single equal, which 27 00:02:07,810 --> 00:02:12,000 should remind you. Alright, so now we know how to test for 28 00:02:12,000 --> 00:02:16,020 not equal we can simplify our program. What I'm going to do is change the first 29 00:02:16,020 --> 00:02:20,820 condition on line 6, to check if the guess is not equal to the answer. 30 00:02:20,820 --> 00:02:24,180 Now so that you can compare the new code with what we've got at the moment, 31 00:02:24,180 --> 00:02:30,180 I'm going to comment out lines 6 through 21, and put the new code before it. In the JetBrains 32 00:02:30,180 --> 00:02:34,230 IDEs you can comment out a block of code by using control / or command slash on 33 00:02:34,230 --> 00:02:41,580 a Mac. So I'm going to select lines 6 through 21, control slash in my case, 34 00:02:41,580 --> 00:02:45,930 because I'm on Windows. Now I'm not aware how to do, have a way to do that in IDLE. 35 00:02:45,930 --> 00:02:50,980 You may have to edit each line individually if you're using your IDLE. 36 00:02:51,620 --> 00:02:55,040 Now comments are ignored by the Python interpreter, and they're there to 37 00:02:55,040 --> 00:02:59,840 document your code. In this case, the comments prevent the code from being executed. 38 00:02:59,840 --> 00:03:02,680 Generally, you should use them to explain when you've learned you've 39 00:03:02,690 --> 00:03:06,280 done something in a certain way, or as a reminder of certain conditions or 40 00:03:06,280 --> 00:03:11,590 variable states that may not be obvious by looking at the code. 41 00:03:11,590 --> 00:03:15,200 Remember that you may come back to modify code, weeks or months, or even 42 00:03:15,200 --> 00:03:20,330 years after you wrote it. Documentation can be extremely helpful when you work 43 00:03:20,330 --> 00:03:26,400 on another programmers code for the first time, or come back to your own code after a gap. 44 00:03:26,430 --> 00:03:30,420 In Python, comments, as you can see, start with the hash symbol, and they actually 45 00:03:30,420 --> 00:03:34,230 can appear in a line on their own, or at the end of a line of code. You can't 46 00:03:34,230 --> 00:03:37,769 place them in the middle of a line, though. The hash inside the string is 47 00:03:37,769 --> 00:03:41,640 treated just like any other character, and doesn't include a comment. If you 48 00:03:41,640 --> 00:03:46,760 want a comment to span several lines, and start each line with a hash, as I've done here. 49 00:03:46,760 --> 00:03:49,590 Alright, so let's now start typing in some new code. So I'm going to come up 50 00:03:49,590 --> 00:03:56,880 to the top here, and specifically on line 6, I'm going to start typing, if guess 51 00:03:56,880 --> 00:04:02,160 is not equal, using the not equal comparison operator answer : 52 00:04:02,160 --> 00:04:09,040 and the next line, line 7, I'm going to do, if guess is less than answer : 53 00:04:09,040 --> 00:04:15,960 print parentheses double quotes and Please guess higher, in those double quotes. 54 00:04:15,960 --> 00:04:18,660 Otherwise I'm going to do an else, noting that I've gone, indented back 55 00:04:18,660 --> 00:04:23,920 at the same level as the if on line 7, else : and I'm going to put a comment here; 56 00:04:23,920 --> 00:04:33,980 hash guess must be greater than answer. I'm going to print, or type in print, 57 00:04:33,980 --> 00:04:40,600 parentheses double quotes Please guess lower. Then on the next line, 58 00:04:40,600 --> 00:04:45,000 I'm going to go back a level again, so at the same level as the else now, 59 00:04:45,000 --> 00:04:51,120 guess equals int left parenthesis input left and right parenthesis and closing right parenthesis 60 00:04:51,120 --> 00:04:55,920 for our guess. Then I'm going to type if guess is equal to 61 00:04:55,920 --> 00:05:02,159 answer, so two equal signs, colon print parentheses double quotes and 62 00:05:02,159 --> 00:05:05,720 Well done, you guessed it, in double quotes. 63 00:05:06,460 --> 00:05:13,100 else, again at the same level as the if, colon print parentheses double quotes, 64 00:05:13,100 --> 00:05:22,220 Sorry, you have not guessed correctly, correctly, and then I'm gonna go back, 65 00:05:22,220 --> 00:05:25,700 back again, so we're all the way to the left margin, again lined up with the if 66 00:05:25,700 --> 00:05:34,580 on line 6, else colon print parentheses double quotes, You got it first time. 67 00:05:34,580 --> 00:05:38,300 Okay, so there's our code. So once again this highlights the 68 00:05:38,310 --> 00:05:43,350 importance of indentation in Python. Our second guess on line 11 is at the 69 00:05:43,350 --> 00:05:49,020 same indentation level as the if and else statements on lines 7 and 9. 70 00:05:49,020 --> 00:05:53,520 It's not dependent on either condition, in other words. We get a different message if 71 00:05:53,520 --> 00:05:59,310 guess is greater than 5 or less than 5, but line 11 is executed, regardless of 72 00:05:59,310 --> 00:06:05,100 the outcome of the if-else clauses. I've also added a comment to the code on line 9. 73 00:06:05,100 --> 00:06:08,400 This makes it clear to any humans reading the code, what's happening. If you 74 00:06:08,400 --> 00:06:11,430 were to read the inner block in isolation, it may not be obvious that the 75 00:06:11,430 --> 00:06:16,700 guess must be greater than 5, rather than a greater than equal to, i.e. not less than 5. 76 00:06:16,700 --> 00:06:20,420 The comment draws attention to the fact that we would not have gotten to the 77 00:06:20,430 --> 00:06:24,600 inner block if guess as equal to 5. You can often do the same thing in many 78 00:06:24,600 --> 00:06:28,980 different ways, and this is an example of doing that. This new code does exactly 79 00:06:28,980 --> 00:06:32,360 the same thing as the previous code - that's the code now that has been 80 00:06:32,360 --> 00:06:38,540 commented out on lines 19 through 34, and I'm gonna take the opportunity to remove our breakpoint there. 81 00:06:38,540 --> 00:06:40,700 Now this can often confuse new programmers 82 00:06:40,700 --> 00:06:43,820 because there's usually more than one way to do the same thing. 83 00:06:43,820 --> 00:06:47,580 Both of these bits of code will produce exactly the same results when given the 84 00:06:47,580 --> 00:06:53,730 same inputs. Of course, the code on lines 19 through 34 won't do anything now because they 85 00:06:53,730 --> 00:06:57,120 start with a hash. In other words, it's been commented out. They're ignored when 86 00:06:57,120 --> 00:07:00,870 we run the program, but if they weren't commented out, they'd produce the same 87 00:07:00,870 --> 00:07:05,370 results as the code we've now got on lines 6 through 17. So let's check that 88 00:07:05,370 --> 00:07:09,660 by running the program to make sure that it still works. Alright, so I'm going to 89 00:07:09,660 --> 00:07:13,500 use the same guesses that I used in the previous video. The first guess was 9, 90 00:07:13,500 --> 00:07:16,600 so I'm going to do that again, and we get the message 91 00:07:16,600 --> 00:07:22,780 Please guess lower. When I enter 5 as the second guess, Well done, you guessed it. 92 00:07:22,780 --> 00:07:28,540 We're going to run the program again and use 1 as our first guess. 93 00:07:28,540 --> 00:07:33,300 So Please guess higher is outputted to the screen. 5 is my second guess, Well done you guessed it. 94 00:07:33,300 --> 00:07:38,900 So that's still working. Run it one more time, run the program again, 1. 95 00:07:38,900 --> 00:07:44,540 This time we're gonna enter 9 - Sorry you have not guessed correctly. So that's working fine 96 00:07:44,540 --> 00:07:49,300 and produces exactly the same results as the previous, now commented out, code. 97 00:07:49,300 --> 00:07:52,960 You may also want to set breakpoints and run the code in the debugger. 98 00:07:52,960 --> 00:07:56,700 Remember to think about which lines should be executed before stepping over 99 00:07:56,700 --> 00:08:00,100 the conditions to verify your results. Alright, so I'm going to finish this 100 00:08:00,100 --> 00:08:06,340 video now with a challenge and we'll go over the solution to the challenge in the next video. 101 00:08:06,340 --> 00:08:11,880 So the challenge is to change the condition on line 6 to, if guess is equal 102 00:08:11,880 --> 00:08:16,680 to answer, and you saw that previously it was not equal to answer. So change it so 103 00:08:16,680 --> 00:08:20,400 it says if guess is equal to answer, and then change the program in the 104 00:08:20,400 --> 00:08:24,480 appropriate places to give the correct results. Alright, so that's the challenge. 105 00:08:24,480 --> 00:08:28,080 See how you go with the challenge and we'll go over the solution to the challenge 106 00:08:28,080 --> 00:08:30,920 in the next video.