1 00:00:04,280 --> 00:00:09,720 Alright, so let's get back to the documentation about mutable sequence types. 2 00:00:09,720 --> 00:00:13,120 We'll place the link into the browser. 3 00:00:13,120 --> 00:00:17,900 So looking at this table again, it might look quite confusing. The first five 4 00:00:17,900 --> 00:00:21,960 operations are using indexing and slicing to change items in the sequence. 5 00:00:21,960 --> 00:00:25,230 Make sure that you're happy using indexes and slices. Review the videos in 6 00:00:25,230 --> 00:00:30,180 section five if necessary, because we'll be using slices in this video. So the 7 00:00:30,180 --> 00:00:35,780 first operation in this table uses indexing to replace an item in the list. 8 00:00:35,780 --> 00:00:39,620 So we're going to continue with lists _intro.py for this example. 9 00:00:39,620 --> 00:00:46,160 So I'm going to go back and open that up: lists_intro.py. 10 00:00:46,160 --> 00:00:51,320 Now we're going to delete everything here except the list on lines one through six. 11 00:00:52,190 --> 00:00:57,930 On line seven we're going to print out our lists our print parentheses computer 12 00:00:57,930 --> 00:01:04,220 underscore parts. Let's now replace the mouse in our list with a trackball. 13 00:01:04,220 --> 00:01:09,260 The mouse is at index position three, so let's modify it and then print it out again. 14 00:01:09,260 --> 00:01:12,580 So on line nine I'm going to type computer_ parts, 15 00:01:12,580 --> 00:01:18,000 left square bracket, 3, right square bracket, equals, then what we want to set it to, 16 00:01:18,000 --> 00:01:21,440 in this case, to trackball. So trackball, surrounded in double quotes 17 00:01:21,440 --> 00:01:24,899 because it's a string. Let's now print the list again, so print computer 18 00:01:24,899 --> 00:01:32,219 underscore parts. Let's run the program. You can see mouse is in the first list, 19 00:01:32,220 --> 00:01:36,240 printed out on line seven. And now that we've made the change on line nine, 20 00:01:36,240 --> 00:01:40,580 when we print out the second time, we see trackball replacing mouse. When we're 21 00:01:40,580 --> 00:01:44,520 dealing with strings, we could get the character at a particular index position. 22 00:01:44,520 --> 00:01:49,140 Strings are immutable, so we can't replace one character with another. When 23 00:01:49,140 --> 00:01:52,439 working with a mutable sequence type such as a list, though, we can also 24 00:01:52,439 --> 00:01:56,670 replace items. So let's go back to the browser, and that's what this first 25 00:01:56,670 --> 00:02:01,740 operation in that table's saying. If you've got a mutable sequence called s, we can 26 00:02:01,740 --> 00:02:06,659 assign a new value to the item at index position i. Now the second operation is 27 00:02:06,659 --> 00:02:11,310 similar, but this time it refers to a slice. Notice that it's referring to an 28 00:02:11,310 --> 00:02:15,140 iterable this time. X on the line above is an object. 29 00:02:15,140 --> 00:02:18,560 We used a string in our example, but any suitable object would also have 30 00:02:18,560 --> 00:02:22,640 worked. The second operation is talking about an iterable. 31 00:02:22,640 --> 00:02:25,360 Also notice the subtle difference in the two descriptions for those two 32 00:02:25,360 --> 00:02:29,380 operations. The description for the first one, s, 33 00:02:29,380 --> 00:02:35,300 i in square brackets equals x, says that item i is replaced by x. For the second 34 00:02:35,300 --> 00:02:39,350 operation, the description says that the slice will be replaced by the contents 35 00:02:39,350 --> 00:02:43,610 of t. So it's easy to miss subtle things like that, when you're just starting to 36 00:02:43,610 --> 00:02:47,030 read the documentation. But don't worry because as you get more experience with 37 00:02:47,030 --> 00:02:51,170 the documentation, it soon starts to make sense, and as you progress through this 38 00:02:51,170 --> 00:02:55,700 course, you'll start paying attention to the way things are phrased. When you get 39 00:02:55,700 --> 00:02:59,380 strange results, as we're about to do, it's easy to get frustrated. 40 00:02:59,380 --> 00:03:03,640 In fact, it's perfectly normal to get frustrated. Frustration is a natural 41 00:03:03,650 --> 00:03:07,520 state for a programmer. There are very few things that are as frustrating as 42 00:03:07,520 --> 00:03:11,150 learning to program, and that's especially true when you're trying to 43 00:03:11,150 --> 00:03:15,710 make sense of documentation like this, without an example. For an example, going 44 00:03:15,710 --> 00:03:19,550 back to our code, note that we don't really need a mouse mat if we're 45 00:03:19,550 --> 00:03:24,480 using a trackball. We can replace both items at once by specifying a slice 46 00:03:24,480 --> 00:03:28,400 instead of an index. So what I'm going to do is start out by printing out the 47 00:03:28,400 --> 00:03:32,150 slice, so that we can see what we're changing. I'm also going to comment out 48 00:03:32,150 --> 00:03:36,290 the trackball on line 9, and we're going to change line 10. Instead of me just 49 00:03:36,290 --> 00:03:40,070 printing out the list of computer parts, we're going to add a square bracket to 50 00:03:40,070 --> 00:03:45,300 the end of computer parts, 3: in those square brackets. 51 00:03:45,300 --> 00:03:48,500 So basically, we're printing out the items starting at position 3, extending 52 00:03:48,500 --> 00:03:52,560 up to the end of the list, and that should be the mouse and the mouse mat. 53 00:03:52,560 --> 00:03:58,300 So let's run the program and confirm that is the case. We can see that the slice, 54 00:03:58,300 --> 00:04:02,660 3: in square brackets, is the mouse and the mat. So we've got the correct slice here. 55 00:04:02,660 --> 00:04:06,980 At this point we can now replace the slice with a trackball, so let's try 56 00:04:06,980 --> 00:04:12,380 to do that. So our first attempt might be something along the lines of 57 00:04:12,380 --> 00:04:17,360 computer_parts square brackets 3: 58 00:04:17,360 --> 00:04:22,620 In those square brackets equals trackball in double quotes, 59 00:04:22,620 --> 00:04:28,040 print computer parts on the next line. Now that looks reasonable. Let's see what 60 00:04:28,040 --> 00:04:33,760 happens when I run that. Of course you knew that something strange was going to happen. 61 00:04:33,760 --> 00:04:36,600 I gave that away when we were looking at the documentation. 62 00:04:36,600 --> 00:04:40,540 What's happened, looking at the final line of the output, is that each of the 63 00:04:40,540 --> 00:04:44,620 characters in the string trackball, have been added to the list individually. 64 00:04:44,620 --> 00:04:48,200 And that's what the documentation means when it says that the slice is replaced 65 00:04:48,200 --> 00:04:52,760 by the contents of the iterable. To fix the problem, we want the iterable 66 00:04:52,760 --> 00:04:56,990 containing a single item - the string trackball. Now we're working with an 67 00:04:56,990 --> 00:05:01,280 iterable type at the moment - lists are iterables. In fact, all sequence types are 68 00:05:01,280 --> 00:05:07,040 iterables. To get the result we want, we can put the trackball string into a list. 69 00:05:07,040 --> 00:05:11,460 So I'm gonna make the right-hand side of the assignment on line 12 slightly different. 70 00:05:11,460 --> 00:05:15,620 So let's make that, well let's surround that, in square brackets, 71 00:05:15,620 --> 00:05:21,320 effectively making it a list. So this time when I run the program, we get the 72 00:05:21,320 --> 00:05:27,180 result we wanted. The two items - mouse and mouse mat - have now been replaced with trackball. 73 00:05:27,180 --> 00:05:29,720 The main thing I want you to learn from this video isn't about using 74 00:05:29,720 --> 00:05:33,050 a slice in a list. The really important part is about the 75 00:05:33,050 --> 00:05:36,560 documentation. Be aware of subtle differences in the way things are 76 00:05:36,560 --> 00:05:41,479 phrased. If two descriptions are different, like the first two back here, 77 00:05:41,479 --> 00:05:45,410 in the list of operations on mutable sequences, then you'll know that their 78 00:05:45,410 --> 00:05:49,260 behavior will be different. Alright, so let's end the video here and in the next one, 79 00:05:49,260 --> 00:05:55,760 we'll look at an example of deleting items from a list. See you in the next video.