1 00:00:04,400 --> 00:00:11,930 Alright, so let's now look at improving the code in our buy_computer program. So you can see I've got buy_computer.py 2 00:00:11,930 --> 00:00:16,460 on-screen, and we want to see how we can use a list to tidy up those conditions - 3 00:00:16,460 --> 00:00:21,040 these are the conditions on lines 15 through 26, inclusive. Now at the moment, 4 00:00:21,050 --> 00:00:25,489 our program isn't working because we haven't included 7 in the string, on 5 00:00:25,489 --> 00:00:32,000 line 13. Rather than hard coding the values 1, 2, 3, 4, 5, 6, what we can do is get 6 00:00:32,000 --> 00:00:37,370 Python to generate it from the number of items in the list. Ideally, we'd use a 7 00:00:37,370 --> 00:00:41,929 list comprehension for that. Now we'll be covering comprehensions and generators 8 00:00:41,929 --> 00:00:46,339 later in the course, but I'm going to put the code in now. I'm doing that because I 9 00:00:46,339 --> 00:00:50,539 want to show you good practices, when programming in Python. So our code is 10 00:00:50,539 --> 00:00:53,600 going to use a for loop, but I want you to be aware that there are more 11 00:00:53,600 --> 00:00:58,080 efficient ways of doing some things. So let's see this comprehension. 12 00:00:58,080 --> 00:01:02,820 So directly below line 8, I'm going to type valid_choices 13 00:01:02,820 --> 00:01:11,040 equals left square bracket str parentheses, with i in those parentheses. 14 00:01:11,040 --> 00:01:19,070 Then for i in range left parenthesis 1 comma len, and in parentheses, available 15 00:01:19,070 --> 00:01:24,020 underscore parts. Then we've got our right parenthesis plus 1, then our 16 00:01:24,020 --> 00:01:28,760 closing right parenthesis and then our closing right square bracket. Then we're 17 00:01:28,760 --> 00:01:34,490 going to print valid choices. So I'm printing out the valid choices list on 18 00:01:34,490 --> 00:01:42,960 line 10, just so you can see what's being produced. So let's run the program, 19 00:01:42,960 --> 00:01:48,500 and we can see that valid_choices is now a list containing the strings 1 to 7 - 20 00:01:48,500 --> 00:01:52,060 you can see that right at the top of the output pane. Alright, so looking at 21 00:01:52,070 --> 00:01:56,990 that code on line 9, it looks horrible. When you have worked through the 22 00:01:56,990 --> 00:02:00,100 lectures on comprehensions, though, it will make perfect sense. 23 00:02:00,100 --> 00:02:04,910 Until then, be aware that there are more efficient ways of doing things, but that 24 00:02:04,910 --> 00:02:08,508 you can't learn everything at once. Alright, so what I'm going to do is 25 00:02:08,508 --> 00:02:14,260 comment out that line and replace it with some code that'll make sense. 26 00:02:21,240 --> 00:02:27,380 So I'm going to type valid_choices is equal to an empty 27 00:02:27,380 --> 00:02:32,840 list and left and right square brackets. Then on the next line I'm going to type for i 28 00:02:32,840 --> 00:02:40,220 in range parentheses 1 comma len parentheses, available_parts 29 00:02:40,230 --> 00:02:44,760 in that parentheses, closing right parenthesis plus 1, then our closing 30 00:02:44,760 --> 00:02:52,680 right parenthesis and a colon. On the next line, valid_choices.append, 31 00:02:52,680 --> 00:02:58,240 and in parentheses, str, parentheses i and the two closing right parentheses. 32 00:02:58,240 --> 00:03:03,180 And we'll leave the print valid choices as it is on line 13. Now be 33 00:03:03,180 --> 00:03:07,830 careful with the parentheses on line 11. We add 1 to the length of available 34 00:03:07,830 --> 00:03:12,480 underscore parts, because the final value in range isn't included. The 1 35 00:03:12,480 --> 00:03:16,230 isn't inside the len function's arguments - we're adding 1 to the length and 36 00:03:16,230 --> 00:03:21,630 that's obviously outside of the parentheses there. You can see we're 37 00:03:21,630 --> 00:03:24,120 missing white space around operator there, so I need to put a space there, 38 00:03:24,120 --> 00:03:32,730 just to keep IntelliJ happy. Alright, so we run the program again now, stop and 39 00:03:32,730 --> 00:03:41,780 re-run there, and you can see we've still got the same output, 1, 2, 3, 4, 5, 6, 7 as a list. 40 00:03:41,780 --> 00:03:46,560 So we're using this str function, on line 12, because the customer is entering 41 00:03:46,560 --> 00:03:51,330 strings, and remember that input will return a string. If their input is going to 42 00:03:51,330 --> 00:03:54,989 match an item in this list, then this list should also contain strings. Ok now 43 00:03:54,989 --> 00:03:58,769 that we have a list that automatically contains the options allowed, we can use 44 00:03:58,769 --> 00:04:04,140 that list on line 18. Let's go ahead and do that, so instead of putting if current 45 00:04:04,140 --> 00:04:09,900 choice is in 1, 2, 3, 4, 5, 6, we'll just need to replace that, to be valid 46 00:04:09,900 --> 00:04:14,880 underscore choices. Alright, so far so good. The last step now, is to replace all 47 00:04:14,880 --> 00:04:19,589 those if elif statements with simpler code. We have a list of the available 48 00:04:19,589 --> 00:04:23,520 parts, and we know which part the user chose, so it's quite easy to get the 49 00:04:23,520 --> 00:04:27,780 part name from the list. We can index into available_parts to get 50 00:04:27,780 --> 00:04:31,940 the required item. There are two slight complications: 51 00:04:31,940 --> 00:04:36,140 First, the choice is one greater than the items index. That's easy to fix - we just 52 00:04:36,150 --> 00:04:39,690 subtract one from it. The second complication is that current 53 00:04:39,690 --> 00:04:44,850 underscore choice is a string, and indexes have to be ints. We saw how to handle 54 00:04:44,850 --> 00:04:49,050 that when we used the int function earlier. So let's have a look at changing 55 00:04:49,050 --> 00:04:54,330 this code. So directly below the print, I'm going to delete the other if 56 00:04:54,330 --> 00:05:01,800 code shortly. We're going to put index equals int parentheses current choice - 57 00:05:01,800 --> 00:05:08,300 again we're using minus one there - chosen_parts is equal to 58 00:05:08,300 --> 00:05:14,920 available_parts, and in square brackets, index. Then the third line, 59 00:05:14,920 --> 00:05:21,200 computer_parts.append, and in parentheses, chosen_part. Let's delete the 60 00:05:21,200 --> 00:05:28,280 rest of this if and elif block, or statements, just to tidy things up. 61 00:05:28,280 --> 00:05:32,280 I think you'd agree, that looks a lot tidier. We've effectively replaced twelve lines 62 00:05:32,280 --> 00:05:36,660 of code with three lines. We've also allowed the program to be updated, just 63 00:05:36,660 --> 00:05:40,500 by adding or removing items from the available_parts list, up there 64 00:05:40,500 --> 00:05:44,669 on lines 1 through 7. So let's check that the program works before 65 00:05:44,669 --> 00:05:48,800 changing the list. Go ahead and do that. 66 00:05:49,060 --> 00:05:57,680 So I'm gonna start by adding a computer, option 1, a dvd drive, option 7. 67 00:05:57,680 --> 00:06:01,620 I'm gonna try to add option 8 - that should display the menu again and it did do that, 68 00:06:01,620 --> 00:06:04,979 so that's working fine, and if you type anything else that's not a valid option, 69 00:06:04,980 --> 00:06:10,600 the menu will also be displayed again. Let's test that. I'll enter my name here, Tim, 70 00:06:10,600 --> 00:06:14,120 same deal - we get the menu back again. And that's a very strange thing for a 71 00:06:14,130 --> 00:06:18,060 customer to choose, but we've got no control over what people may type into 72 00:06:18,060 --> 00:06:21,060 our programs. You can see we've got the menu displayed again, so that's good - 73 00:06:21,060 --> 00:06:24,660 our code behaves sensibly and doesn't crash, it doesn't crash in that scenario. 74 00:06:24,660 --> 00:06:29,940 Alright, so let's finish by entering zero, and we get our final list which 75 00:06:29,940 --> 00:06:34,100 contains a computer and a dvd drive. So far so good. 76 00:06:34,100 --> 00:06:40,900 Let's go back up and what we'll do, is we'll delete the mouse mat from our list. 77 00:06:40,900 --> 00:06:47,230 Go ahead and do that. Run the program again, and you can see it still works but 78 00:06:47,230 --> 00:06:50,800 now it's minus the mouse mat, because we've deleted it from our available 79 00:06:50,800 --> 00:06:54,340 underscore parts list. Alright, you can see that things are still working. 80 00:06:54,340 --> 00:06:57,520 The menu now only contains six items and we can add them to the list of parts we 81 00:06:57,520 --> 00:07:04,570 want to buy. Let's try a few; computer then hdmi cable, option 7 - we now go 82 00:07:04,570 --> 00:07:08,410 back to the menu, whereas previously, of course, we had option 7 as being the 83 00:07:08,410 --> 00:07:14,160 dvd drive - zero to exit and the two items I chose were computer and hdmi cable. 84 00:07:14,160 --> 00:07:18,100 Everything's working as it should do. So we've used a few lists in this program. 85 00:07:18,100 --> 00:07:22,440 On lines 1 to 7, we've got a list of parts that are available to buy. On lines 86 00:07:22,440 --> 00:07:27,080 9 to 11, we create an empty list and then add the valid choices to it. 87 00:07:27,080 --> 00:07:31,620 Line 14 defines another empty list, computer_parts, which 88 00:07:31,620 --> 00:07:38,540 will contain the parts that the customer wants to buy, and we're appending their items - 89 00:07:38,540 --> 00:07:42,310 ones that they've chosen to buy - on line 21, so that ultimately 90 00:07:42,310 --> 00:07:46,479 contains the list of items that they want to purchase. So using lists for the 91 00:07:46,479 --> 00:07:51,310 menu items makes the code much shorter. More importantly, though, it's much easier 92 00:07:51,310 --> 00:07:55,320 to maintain. We can add and remove items from the available_parts list 93 00:07:55,320 --> 00:07:58,660 and we don't have to change any other code, as a result of doing that, 94 00:07:58,660 --> 00:08:02,349 as you saw. So test the program thoroughly. You might want to add things like a 95 00:08:02,349 --> 00:08:06,789 webcam, microphone and loudspeakers to the list. Also, make sure it copes with 96 00:08:06,789 --> 00:08:10,690 invalid choices. When testing a program like this, don't just focus on the 97 00:08:10,690 --> 00:08:15,380 correct input - make sure you try to think of other things that a user might type. 98 00:08:15,380 --> 00:08:18,220 Alright, so in the next video, moving ahead, we're gonna get the program to 99 00:08:18,220 --> 00:08:23,700 cope if the customer changes their mind. See you in the next video.