1 00:00:04,569 --> 00:00:08,870 Alright, so let's now create the first Python file for this project. We're going 2 00:00:08,870 --> 00:00:20,720 to call it lists_intro.py. In Python, a list is enclosed in square brackets. 3 00:00:20,720 --> 00:00:23,960 Back in the previous section, we saw a slide with a list of computer 4 00:00:23,960 --> 00:00:28,020 parts, so let's type that in again. So computer_parts 5 00:00:28,020 --> 00:00:32,500 is equal to left and right square brackets, then in double quotes 6 00:00:32,500 --> 00:00:37,370 each item, so it can be computer in the first one in double quotes, comma monitor in 7 00:00:37,370 --> 00:00:43,700 double quotes comma keyboard in double quotes with a comma. 8 00:00:43,700 --> 00:00:49,900 Then mouse in double quotes with a comma, and finally mouse mat in double quotes and 9 00:00:49,910 --> 00:00:54,200 then we've got the right square bracket as well. Now you'll often see list literals, 10 00:00:54,200 --> 00:00:58,010 which these, are split up, so that each item is on a new line. I'll be talking 11 00:00:58,010 --> 00:01:00,980 about this later, when we've got a more complex list. 12 00:01:00,980 --> 00:01:05,560 For now what I'm going to do, is press ENTER after each of the commas, to split the list up, 13 00:01:12,100 --> 00:01:16,399 and also, the last right square bracket, I'll put onto a 14 00:01:16,400 --> 00:01:22,740 separate line as well. Alright, so that's a very simple list. So computer_parts 15 00:01:22,740 --> 00:01:25,070 is a list of strings here - the names of some common computer 16 00:01:25,070 --> 00:01:28,640 parts for a computer. I mentioned previously, that square brackets are used 17 00:01:28,640 --> 00:01:33,229 in three places in Python and this is the third use. We've seen them, previously, being 18 00:01:33,229 --> 00:01:38,270 used to index a sequence and for slices. They're also used to create lists, as we're 19 00:01:38,270 --> 00:01:42,220 doing here. So inside the square brackets, we've got the items in the list, 20 00:01:42,220 --> 00:01:46,670 separated by commas. Alright, so that's a very simple list, but because it is a 21 00:01:46,670 --> 00:01:50,539 sequence type, we can iterate over it, using a for loop. So let's go ahead and 22 00:01:50,539 --> 00:01:58,100 do that. So I'm going to type for part in computer_parts: and the next 23 00:01:58,100 --> 00:02:05,899 line indented, print parentheses part. Run the program, and you can see we've got 24 00:02:05,899 --> 00:02:12,830 each item of our list displayed in the output pane. So we can refer to 25 00:02:12,830 --> 00:02:17,120 individual items in the list, using their index position as well. It works exactly 26 00:02:17,120 --> 00:02:19,400 the same as we've seen with strings. 27 00:02:19,400 --> 00:02:24,080 The third item in the list is keyboard, so that's at index position 2, remembering 28 00:02:24,080 --> 00:02:29,000 that indexing starts at 0. So let's have a look at that: print left and right 29 00:02:29,000 --> 00:02:35,220 parentheses here, to make a space between lines, and then on the next line, print computer_parts 30 00:02:35,220 --> 00:02:38,540 left square bracket 2 right square bracket and right 31 00:02:38,540 --> 00:02:45,320 parenthesis. So if you run that, you can see we get the keyword, or the word 32 00:02:45,320 --> 00:02:49,700 keyboard, printed out. Now you can also slice a list, and that works 33 00:02:49,700 --> 00:02:53,180 exactly the same as we've seen for strings. What I'll do is print out a couple of 34 00:02:53,180 --> 00:02:56,650 examples to revise what we've already done. 35 00:02:59,460 --> 00:03:05,400 So print parenthesis computer_parts left square bracket 36 00:03:05,400 --> 00:03:10,840 0:3 right square bracket then right parenthesis, and on line 15, print 37 00:03:10,840 --> 00:03:17,180 parentheses computer_parts left parenthesis minus 1. We've got our 38 00:03:17,180 --> 00:03:25,540 right square bracket and right parenthesis. So if you run the program now, 39 00:03:25,540 --> 00:03:30,580 you see that the first slice - that's the code there on line 14 - 40 00:03:30,580 --> 00:03:34,720 that's produced the first 3 items from the list, in this case, computer, monitor and 41 00:03:34,730 --> 00:03:39,080 keyboard. Note that the output here is in square brackets, and that indicates that 42 00:03:39,080 --> 00:03:44,990 slicing a list produces another list. Now line 15 is a Python idiom, and returns 43 00:03:44,990 --> 00:03:49,430 the last item in the sequence - the last item in the list, in this case - which was 44 00:03:49,430 --> 00:03:56,240 mouse mat. And we can confirm that by looking at our setup on line 1. Alright, so 45 00:03:56,240 --> 00:04:00,200 that's all been revision so far. It's not surprising that indexing and slicing 46 00:04:00,200 --> 00:04:03,620 works the same with a list as it did with a string, and that's because they're both 47 00:04:03,620 --> 00:04:07,980 sequence types. But there's one big difference between strings and lists: 48 00:04:07,980 --> 00:04:12,500 strings are immutable, which means they can't be changed. Lists, on the other hand, 49 00:04:12,500 --> 00:04:16,100 are mutable. In other words, you can change the contents of a list. 50 00:04:16,100 --> 00:04:20,320 I'll explain those terms in more detail, in the next two videos, and then we'll look 51 00:04:20,329 --> 00:04:25,510 at ways to change a list's contents. See you in the next video.