1 00:00:05,360 --> 00:00:09,040 Alright, so let's have a look at the solution to the dictionary menu challenge 2 00:00:09,040 --> 00:00:13,920 from the last video. So how did you get on with the challenge? So as I mentioned in the iterating over 3 00:00:13,920 --> 00:00:19,440 a dictionary lecture, you can use the dot items method to get the keys and values at the same time. 4 00:00:19,440 --> 00:00:24,720 That's more efficient than iterating over the keys, and indexing into the dictionary again 5 00:00:24,720 --> 00:00:28,960 to get the values. So let's have a look at how to do that. What I'm going to do is start by adding an 6 00:00:28,960 --> 00:00:33,793 else block and then print a short message. So I'm going to add the else block, here on line 14, 7 00:00:33,793 --> 00:00:43,840 else colon. Then on line 15, print parentheses double quotes Please add options from the list. 8 00:00:46,960 --> 00:00:50,720 Alright, so far so good. Next what we're going to do is iterate over the items and 9 00:00:50,720 --> 00:00:57,680 print the keys and values. So I'm going to type, on line 16, for key comma value 10 00:00:57,680 --> 00:01:04,400 in available_parts.items with parentheses there, colon, 11 00:01:06,000 --> 00:01:15,040 and print parentheses f double quotes left and right curly braces key colon and a space, 12 00:01:15,040 --> 00:01:22,080 left and right curly braces again, for value - so printing out both key and value in an f-string. Then outside 13 00:01:22,080 --> 00:01:30,653 of the for loop, indented back one level, I'm going to print zero colon finish, or to finish. 14 00:01:32,400 --> 00:01:37,920 Alright and I'll push down that other output down to line 20. So now if we enter an invalid 15 00:01:37,920 --> 00:01:44,394 option, the menu will be printed. When we enter the loop for the first time, current_choice 16 00:01:44,394 --> 00:01:48,160 isn't a valid option - you can see it's set to None on line 9. So that means the program 17 00:01:48,160 --> 00:01:53,840 should start by printing the menu. So let's run the program and confirm if that is the case, 18 00:01:56,000 --> 00:01:58,400 and you see, good news that it is working. And what I'm going to 19 00:01:58,400 --> 00:02:03,243 do is move the run pane over to the right, using the cog icon over here, 20 00:02:06,960 --> 00:02:12,400 like so, and just move that over a little bit. Okay. Alright so that's looking good - we've got our 21 00:02:12,400 --> 00:02:17,840 seven items printing as a menu and zero to quit, showing on the screen as well. We're not adding the 22 00:02:17,840 --> 00:02:22,640 items to a shopping cart yet, but we can check that the correct items are being chosen. So I'm going to 23 00:02:22,640 --> 00:02:31,280 come over here to the run pane, and select 2 for monitor, add a dvd drive, which is option 6, 24 00:02:31,280 --> 00:02:36,659 and anything that isn't a valid choice will cause the menu to be re-printed. We'll put a 9 there - 25 00:02:36,659 --> 00:02:42,071 we get our menu showing again. Alright, we'll terminate the program by selecting zero 26 00:02:42,071 --> 00:02:45,840 Alright so that's working. The next step here now, 27 00:02:45,840 --> 00:02:50,381 is to add the items to a list or a dictionary. Because this section is about dictionaries, 28 00:02:50,381 --> 00:02:55,680 that's what I'll use. But you could take the existing code from my original buy_computer program, 29 00:02:55,680 --> 00:03:00,800 and add the items to a list, if you want to try that. If you remember, choosing an item again removes it 30 00:03:00,800 --> 00:03:05,280 from the list. But we have a challenge at the end of the video, to do just that. But we need a 31 00:03:05,280 --> 00:03:10,575 copy of this file - buy_computer_dict, because we're about to make some changes to it. 32 00:03:10,575 --> 00:03:15,280 So I'm going to open the project pane and then right click buy_computer_dict 33 00:03:15,280 --> 00:03:22,480 and choose Copy, and Copy again. Then I'm going to right click over here on the project and choose Paste. 34 00:03:22,480 --> 00:03:28,800 I'm going to call this one dict_list.py. You'll use that for the 35 00:03:28,800 --> 00:03:32,960 challenge at the end of this video. Until then, we'll close that video in the editor. 36 00:03:33,920 --> 00:03:39,840 So we're back at buy_computer_dict.py. So I'll create a dictionary to 37 00:03:39,840 --> 00:03:44,480 store the parts that the user chooses, in the next video. Before that, I'll finish this video 38 00:03:44,480 --> 00:03:51,520 by discussing lines 11 and 12. So on line 11, we're checking that the chosen part is in the dictionary. 39 00:03:51,520 --> 00:03:58,560 That means we don't have to use get on line 12. We can obtain the part name by indexing into our 40 00:03:58,560 --> 00:04:03,840 available_parts dictionary, using current_choice as the key. We know the 41 00:04:03,840 --> 00:04:09,200 code won't crash, because we've already checked that the key does exist. We did that on line 11. 42 00:04:09,200 --> 00:04:12,720 Another thing worth mentioning, is that this code is a bit shorter 43 00:04:12,720 --> 00:04:19,725 than the version that used a list for the menu. So if we go back to our buy_computer.py. 44 00:04:19,725 --> 00:04:25,000 So looking at the code there, we had to create a list of the valid choices - that's the code on 45 00:04:25,000 --> 00:04:33,040 lines 9 through 12. We need that, to check if the user's choice is a valid index position into the list. 46 00:04:33,040 --> 00:04:39,658 But in this new version, buy_computer_dict, the choices are the keys in the dictionary. 47 00:04:39,658 --> 00:04:44,566 So there's no need to create another list, because we can check if something is in the dictionary. 48 00:04:44,566 --> 00:04:49,760 Now we want to map the user's choice to an available part. If you remember, I mentioned 49 00:04:49,760 --> 00:04:56,960 that dictionaries are sometimes called mappings. So in our dictionary, on lines 1 through 7, the character 1 50 00:04:56,960 --> 00:05:03,120 maps to a computer, 2 is mapped to a monitor, and so on. A dictionary is a good choice here, 51 00:05:03,120 --> 00:05:08,560 because it automatically maps the choices to the items. So you might be wondering why I bothered 52 00:05:08,560 --> 00:05:13,680 showing you how to use a list, back in the Lists and Tuples section. Why do I show you one way, when I 53 00:05:13,680 --> 00:05:19,654 know there's a quote unquote better way available? Well the answer is that you may need to use both. 54 00:05:20,560 --> 00:05:24,640 So if you were writing a shopping cart for an online store, you wouldn't have the items 55 00:05:24,640 --> 00:05:29,920 typed into your code. The items available for sale might come from a database, or a file on 56 00:05:29,920 --> 00:05:35,280 disk. If the items appear in this format, you can see on screen there, the number a comma then the 57 00:05:35,280 --> 00:05:40,000 description, then it would be quite easy to create a dictionary by reading in that data. 58 00:05:42,240 --> 00:05:45,280 But what if the items were presented to you without the numbers? 59 00:05:45,280 --> 00:05:49,040 So you might read in a series of items, like you can see on screen; just the descriptions themselves. 60 00:05:49,840 --> 00:05:53,840 So in that case you'd have to generate the numerical characters that the user would type. 61 00:05:54,720 --> 00:05:59,760 If you've got to include that code anyway, then it might be easier to leave the items in a list. 62 00:05:59,760 --> 00:06:05,760 There's another reason why you might prefer to use a list; Python can sort lists easily. Right so I'm 63 00:06:05,760 --> 00:06:10,160 going to finish this video by demonstrating that, with the original buy_computer program. 64 00:06:11,920 --> 00:06:20,160 Alright, so I'm clicking on buy_computer.py here. Now when I run this, 65 00:06:20,160 --> 00:06:24,320 the items are appearing in an unsorted way, to the right hand side of the screen there. 66 00:06:24,320 --> 00:06:30,000 But humans can find things faster if items are sorted. So as you can see, the dvd drive appears last, 67 00:06:30,000 --> 00:06:35,200 when it should really appear after computer. So that's easy to fix. We can sort the list 68 00:06:35,200 --> 00:06:39,669 before using it. So we're going to enter 0 to terminate the program, 69 00:06:42,160 --> 00:06:48,346 and let's make a change. So what I'm going to do is come down here, before the while loop. 70 00:06:48,346 --> 00:06:55,520 Specifically on line 16, I'm going to type available_parts.sort, leaving the parentheses in, 71 00:06:55,520 --> 00:07:03,030 and I'll just put a line break between that and the while loop. So if we run the program again now, 72 00:07:03,030 --> 00:07:07,760 you can see the parts appear in alphabetical order. So you can iterate over a dictionary, 73 00:07:07,760 --> 00:07:13,040 in sorted order of the keys, but it's not easy or efficient to do that with the values. 74 00:07:13,040 --> 00:07:18,845 More importantly, if we did rearrange the dictionary, option 6 would appear after option 1, 75 00:07:18,845 --> 00:07:23,896 in second place in the menu. Now obviously, with our code, we could have typed 1 to 6 76 00:07:23,896 --> 00:07:29,840 in alphabetical order, but if the data came from an external source, we wouldn't be able to do that. 77 00:07:30,800 --> 00:07:35,760 So we've seen how to use a list and a dictionary to present our menu. So which one is quote unquote 78 00:07:35,760 --> 00:07:41,600 better depends on the nature of your original data, and exactly what you want to do with that data. 79 00:07:41,600 --> 00:07:47,120 We've seen some examples of why a dictionary might be more suitable here - shorter code for instance. 80 00:07:47,120 --> 00:07:51,200 But we've also seen that a list may be more appropriate, if you have to sort the values. 81 00:07:51,920 --> 00:07:56,400 So in the next video we'll return to our new program, and add items that the user chooses. 82 00:07:56,960 --> 00:08:03,840 Before that though, try this challenge. It's very common to use both lists and dictionaries in the 83 00:08:03,840 --> 00:08:09,440 same program. Have a go at adding and removing items from a computer_parts list 84 00:08:09,440 --> 00:08:15,200 and I'll add my code as a document after this video. So remember to use the dict_list.py 85 00:08:15,200 --> 00:08:20,480 file that we copied earlier. The new code that you'll add will be very similar to the code 86 00:08:20,480 --> 00:08:25,104 we've already got, in the original buy_computer.py program. 87 00:08:25,104 --> 00:08:29,207 Check out my solution using a list to do that, in the document that appears next, 88 00:08:29,207 --> 00:08:32,240 and then I'll see you in the following video.