1 00:00:04,670 --> 00:00:08,840 Alright, so it's time to write the code that lets our user select songs from the 2 00:00:08,840 --> 00:00:13,129 albums lists. You'll have noticed, in the demonstration, that the program keeps 3 00:00:13,129 --> 00:00:17,380 displaying the menus until an invalid choice is made, now that's a good use for 4 00:00:17,380 --> 00:00:21,880 a while loop. We used a while look at the end of the previous section, when we 5 00:00:21,880 --> 00:00:26,779 created a simple menu program. We then extended that, to offer our menu of parts 6 00:00:26,779 --> 00:00:30,109 for a computer. You might want to review those earlier lectures, before continuing 7 00:00:30,109 --> 00:00:34,430 with this one. You'll notice a lot of similarity between the earlier menu 8 00:00:34,430 --> 00:00:39,079 program, and the code we're about to write. Alright, so I'm going to delete the 9 00:00:39,080 --> 00:00:46,240 line that printed the albums. Instead what we're going to do, is type in while 10 00:00:46,240 --> 00:00:50,940 True, with a capital T there and colon. Next we'll print some brief 11 00:00:50,940 --> 00:00:55,360 instructions and the titles of the albums. So I'm going to print, on line 4, 12 00:00:55,370 --> 00:01:03,949 parentheses double quotes Please choose your album, and in parentheses, Invalid 13 00:01:03,949 --> 00:01:09,890 choice exits, right parenthesis colon. Then we've got our double quote and 14 00:01:09,890 --> 00:01:15,120 right parenthesis there. Let's put the titles for the albums now, so it's for 15 00:01:15,120 --> 00:01:22,560 index comma. Then in parentheses I'm going to type title comma artist comma 16 00:01:22,560 --> 00:01:29,300 year and comma songs, then we've got our right parentheses there, in enumerate, 17 00:01:29,300 --> 00:01:35,659 then in parentheses, albums, then right parenthesis and then the colon. Then on the 18 00:01:35,660 --> 00:01:38,960 next line, we're going to print out the title, artist, year and song, so it's print 19 00:01:38,960 --> 00:01:44,420 parentheses double quotes, left and right curly braces for each, so basically 20 00:01:44,420 --> 00:01:53,780 duplicating that four times, one, two, three four- so a total of five curly braces there - 21 00:01:53,780 --> 00:01:57,280 then closing double quote. Then on the next line, just for formatting reasons, 22 00:01:57,289 --> 00:02:00,979 I'm going to type .format - you can do that on one line if you prefer. So it's dot 23 00:02:00,980 --> 00:02:10,460 format parentheses, index + 1 comma, then we want our title comma artist comma 24 00:02:10,460 --> 00:02:17,140 year comma and songs comma. Then we've got our two right parentheses. 25 00:02:17,140 --> 00:02:21,299 So we're using a for loop to iterate over the albums. We also want the index 26 00:02:21,299 --> 00:02:24,420 numbers and we've seen, previously, how to get them by using the enumerate 27 00:02:24,420 --> 00:02:29,790 function. I've split line 6 and 7 into two lines, just to show it all on 28 00:02:29,790 --> 00:02:34,110 video. Again, you can put that on one line if you prefer. So this code is very 29 00:02:34,110 --> 00:02:38,540 similar to the code we've seen before. Where it's different, though, is the parentheses 30 00:02:38,540 --> 00:02:44,740 around title, artist, year and songs on line 5. I introduced nested data 31 00:02:44,740 --> 00:02:48,120 structures by mentioning those parentheses. That was at the end of the 32 00:02:48,120 --> 00:02:52,110 More Unpacking video, earlier in this section. Once we've see why they're 33 00:02:52,110 --> 00:02:55,680 necessary, it makes perfect sense, and when you've seen the error you get when 34 00:02:55,680 --> 00:02:58,460 you forget them, you'll know that you need to use them. 35 00:02:58,460 --> 00:03:01,620 So let's start with the error. Our while loop's gonna go round and round forever, 36 00:03:01,620 --> 00:03:05,790 which isn't great, so I'm going to add a break to prevent that. So I'm going to 37 00:03:05,790 --> 00:03:11,420 put the break down here, on line 9, indented to the same level as the for loop. 38 00:03:11,420 --> 00:03:18,060 Alright, so let's just run this to confirm that it does work. Run the code. 39 00:03:18,060 --> 00:03:21,920 Alright, so that's working okay, as you can see. What happens if I remove those 40 00:03:21,930 --> 00:03:26,220 parentheses? Let's go ahead and do that. So I'm going to change this up here, to remove 41 00:03:26,220 --> 00:03:34,680 the parentheses, and you can see there's no errors showing, but if I run this now 42 00:03:34,680 --> 00:03:38,160 we actually get an error: not enough values to unpack, expected 43 00:03:38,160 --> 00:03:43,200 5, got 2. So we asked Python to unpack five values but enumerate only 44 00:03:43,200 --> 00:03:47,340 returns two. When you get an error like that, it's worth checking what 45 00:03:47,340 --> 00:03:51,630 enumerate is returning. So comment out this loop, which I'll do now, the loop 46 00:03:51,630 --> 00:03:58,120 that we've been using. We'll actually comment out those three lines. 47 00:03:58,120 --> 00:04:02,160 Instead, what I'm going to do, is come down here onto line 9 and type 48 00:04:02,160 --> 00:04:07,760 for space index comma value in enumerate and in 49 00:04:07,769 --> 00:04:13,139 parentheses, albums, right parentheses and : We're going to print, in parentheses, 50 00:04:13,140 --> 00:04:21,959 index comma value. Then I'll leave line 12 as it was. Run the program now, 51 00:04:21,959 --> 00:04:26,480 and we've got four tuples printed out. Their index positions are printed first, 52 00:04:26,480 --> 00:04:30,440 and then each tuple. If enumerate only returns two items, 53 00:04:30,440 --> 00:04:34,900 then we can only unpack two items in the for loop. So examine this output 54 00:04:34,900 --> 00:04:40,150 carefully, to check what those two items represent. The first is an int, the index 55 00:04:40,150 --> 00:04:44,480 position of each item, but the second item, as you can see there, is a tuple, 56 00:04:44,480 --> 00:04:50,220 and it actually contains four items; the title, the artist, the year, as well as the 57 00:04:50,220 --> 00:04:55,800 list of songs. If it's a tuple, we should also be able to unpack it, and we can, 58 00:04:55,800 --> 00:05:00,820 and once again, I'll take this slow. Let's start by unpacking value. So I'm gonna close 59 00:05:00,820 --> 00:05:05,470 down the run window. We'll go back to our for loop here. On line 10, I'm going to 60 00:05:05,470 --> 00:05:14,740 insert title comma artist comma year comma songs equals value. Instead of 61 00:05:14,740 --> 00:05:18,080 value, we're actually going to print out those four values. So we'll leave index in, 62 00:05:18,080 --> 00:05:27,360 and we'll print index, title, artist, year and songs. Run the program, and let's check out the output. 63 00:05:27,360 --> 00:05:32,620 You can see there, that line 10 is unpacking the value tuple into 64 00:05:32,620 --> 00:05:37,200 our four variables, and in the output now, we get those four variables printed out, 65 00:05:37,200 --> 00:05:44,320 each time around the loop. You can see that over to the right-hand side. Now that's not 66 00:05:44,320 --> 00:05:48,220 surprising - we've done things like that a few times now. Where it gets interesting, 67 00:05:48,220 --> 00:05:52,990 and why those parentheses were necessary, is when we combine lines 9 68 00:05:52,990 --> 00:05:58,630 and 10. So compare those two lines - close down the run window. So compare 69 00:05:58,630 --> 00:06:03,500 lines 9 and 10, to line 5, which is currently commented out. On line 5, 70 00:06:03,500 --> 00:06:08,780 we want to unpack a single tuple into four variables, just like we're doing on line 10. 71 00:06:08,780 --> 00:06:13,680 But because that tuple's one of the pair of values, we need to enclose title, 72 00:06:13,690 --> 00:06:23,950 artist, year end songs in parentheses. So let's do that. We're now doing all the 73 00:06:23,950 --> 00:06:28,840 unpacking on one line, instead of doing it in two steps on lines 9 and 10. 74 00:06:28,840 --> 00:06:38,140 I'm gonna un-comment that block now, and let's rerun the program. 75 00:06:38,140 --> 00:06:42,840 You can see that that works and we get a very similar output now, from both loops. The index numbers are 76 00:06:42,850 --> 00:06:47,500 different - that's because we added one to the index on line 7 - but we're 77 00:06:47,500 --> 00:06:51,880 getting the same values otherwise. And we'll do something similar again, when we 78 00:06:51,880 --> 00:06:55,780 come to display the list of songs for an album. Now that'll be in the video 79 00:06:55,780 --> 00:06:59,460 after the next one, because there's something else I want to show you - 80 00:06:59,460 --> 00:07:05,300 constants in Python. We'll look at those in the next video.