1 00:00:04,900 --> 00:00:08,940 Python sequence types let you create a slice. 2 00:00:08,940 --> 00:00:12,660 The only sequence type we've looked at so far is the string type, so let's 3 00:00:12,660 --> 00:00:16,440 continue with our strings2 program. What I'll do is delete all the code 4 00:00:16,440 --> 00:00:23,520 except for the comment and the parrot definition, so let's go ahead and do that. 5 00:00:23,520 --> 00:00:28,620 So we can produce a slice by providing three numbers separated by colons. 6 00:00:28,620 --> 00:00:34,700 Now these numbers are the start, stop and step values. I'll introduce the step value 7 00:00:34,710 --> 00:00:39,649 soon, but first let's look at slicing without a step. So we're going to type print 8 00:00:39,649 --> 00:00:46,379 parentheses parrot left square bracket 0 colon 6 and right square bracket, then 9 00:00:46,379 --> 00:00:52,710 our right parentheses. I'm going to add a comment as well, hash N o w, sorry 10 00:00:52,710 --> 00:00:59,159 N o r w e g. So here we're telling Python to start at position 0 and slice 11 00:00:59,159 --> 00:01:04,199 up to, but not including, position 6, and I've added a comment with a hash 12 00:01:04,199 --> 00:01:07,350 character there to indicate the result that we should get. So if we run the 13 00:01:07,350 --> 00:01:15,320 program now, we get the output in the output pane, Norweg. 14 00:01:15,320 --> 00:01:17,260 So when I said not including, 15 00:01:17,260 --> 00:01:20,940 that's important and something that new Python programmers often forget. 16 00:01:20,940 --> 00:01:26,970 The last character you specify isn't included in the slice. It turns out that 17 00:01:26,970 --> 00:01:31,740 not including that last character makes slices much easier to use, but it'll 18 00:01:31,740 --> 00:01:35,520 probably catch you out at first. In fact, not including that final value is 19 00:01:35,520 --> 00:01:39,500 something that happens in other places in Python, such as ranges. 20 00:01:39,500 --> 00:01:43,380 Now don't do this if you're at work, in an open-plan office, but if you're somewhere safe 21 00:01:43,380 --> 00:01:47,940 where you're not going to get funny looks, say out loud 10 times, "up to but not 22 00:01:47,940 --> 00:01:53,790 including", "up to but not including", "up to but not including". Alright, so getting 23 00:01:53,790 --> 00:02:00,090 back to our code, the slice on line 5 produces the characters from index 0 up 24 00:02:00,090 --> 00:02:06,120 to, but not including, index 6. So let's see some more examples to understand how 25 00:02:06,120 --> 00:02:11,370 slices work. So let's say we wanted the characters w e from the string, then we'd 26 00:02:11,370 --> 00:02:17,040 slice from position 3, the w, up to but not including position 5, 27 00:02:17,040 --> 00:02:21,659 the g. So let's type in the code for that. So on line 6 I'm going to type 28 00:02:21,659 --> 00:02:26,970 print left parenthesis parrot left square bracket, then we're going to type 3 29 00:02:26,970 --> 00:02:30,620 colon 5, as I mentioned, then our right square bracket, right parenthesis, 30 00:02:30,620 --> 00:02:36,510 and we'll run the program. We should see w e outputted, and there's w e in the 31 00:02:36,510 --> 00:02:41,099 output pane. So what else can we do? Well we can get the first word by 32 00:02:41,099 --> 00:02:46,019 getting the slice from position 0, up to but not including position 9. So let's try that. 33 00:02:46,019 --> 00:02:54,420 So print left parenthesis parrot left square bracket 0 colon 9, then our 34 00:02:54,420 --> 00:02:58,400 right square bracket and right parenthesis, and we'll run that, 35 00:02:58,400 --> 00:03:04,139 and there's the entire word, Norwegian. When creating a slice, the first value is the 36 00:03:04,139 --> 00:03:07,680 position to start at, as you've seen. That's separated from the position to 37 00:03:07,680 --> 00:03:11,879 stop at with a colon, which again, you've seen. But if you don't provide a start 38 00:03:11,879 --> 00:03:16,470 value, you still need the colon. So we could write that last slice, that we've 39 00:03:16,470 --> 00:03:22,440 got on line 7, differently, by typing print, on line 8, a left parenthesis 40 00:03:22,440 --> 00:03:27,480 parrot left square bracket, then just colon 9 this time, right square 41 00:03:27,480 --> 00:03:34,260 bracket right parenthesis, and we can run that. And as you see we get exactly the 42 00:03:34,260 --> 00:03:37,739 same result, and that's this time, the difference is, we didn't provide a start 43 00:03:37,739 --> 00:03:43,019 value. Basically, our lines 7 & 8 both do the same thing because the start value 44 00:03:43,019 --> 00:03:47,459 defaults to the start of the sequence, and obviously on line 7 we included the 45 00:03:47,460 --> 00:03:50,780 start value and line 8, we didn't include it. 46 00:03:50,780 --> 00:03:56,240 Alright so let's look at some slides to explain exactly what's going on here. 47 00:04:44,700 --> 00:04:46,650 So at this point you should be able to create a slice 48 00:04:46,650 --> 00:04:50,580 that returns the word blue, so pause the video and come back when you've had a 49 00:04:50,580 --> 00:04:54,600 go at that. 50 00:04:54,600 --> 00:04:58,620 Alright, so welcome back. So to slice the word blue from our string, we need to 51 00:04:58,620 --> 00:05:03,380 start at position 10, and extend up to position 14. So let's do that. 52 00:05:03,380 --> 00:05:09,940 So I'm going to type print left parenthesis parrot left square bracket, 10 colon 14 53 00:05:09,940 --> 00:05:16,160 right square bracket and right parenthesis, and if we run that, we should see the word Blue. 54 00:05:16,160 --> 00:05:18,820 And there's Blue on the screen. And again, 55 00:05:18,820 --> 00:05:22,840 there's no character at position 14, but remember that slices extend up to, but 56 00:05:22,840 --> 00:05:28,180 not including, the stop value. We saw that we can leave out the start value and it 57 00:05:28,180 --> 00:05:32,290 will default to the start of the sequence. If we leave out the stop value 58 00:05:32,290 --> 00:05:35,830 then it defaults to the end of the sequence. So that means that we can 59 00:05:35,830 --> 00:05:42,970 rewrite the last line, line 10, on line 11, as print left parenthesis parrot left 60 00:05:42,970 --> 00:05:47,770 square bracket 10 colon, and this time we're leaving off the other value. So 61 00:05:47,770 --> 00:05:53,500 we've got a right square bracket and right parenthesis and if I run that, we 62 00:05:53,500 --> 00:05:58,210 get the same output, in this case, the word Blue. It's important to include the 63 00:05:58,210 --> 00:06:02,860 colon that separates the start and stop values. We haven't provided a stop value, 64 00:06:02,860 --> 00:06:06,640 but we still need the colon, so that Python knows that we want a slice. 65 00:06:06,640 --> 00:06:11,740 Without the colon, that would be in index, and we'd get a single character at 66 00:06:11,740 --> 00:06:16,000 position 10. I mentioned that there are four different things that square 67 00:06:16,000 --> 00:06:20,650 brackets are used for, and slicing is the second thing. They're used for indexing 68 00:06:20,650 --> 00:06:25,240 and they used for slicing. If you use square brackets for slicing, you must 69 00:06:25,240 --> 00:06:30,380 have at least one colon, otherwise it's in fact, an index and not a slice. 70 00:06:30,380 --> 00:06:35,320 Alright so I've got a question for you. Before I ask it, we'll have a quick recap. 71 00:06:35,320 --> 00:06:39,130 If the first number in a slice is omitted, the slice will start from the 72 00:06:39,130 --> 00:06:43,450 beginning of the string, and if the second number is omitted, it'll run to 73 00:06:43,450 --> 00:06:47,790 the end of the string. So it's a recap, 74 00:06:48,030 --> 00:06:54,670 print parentheses parrot left square bracket colon 6 right square bracket 75 00:06:54,670 --> 00:07:00,280 right parenthesis, and print left parenthesis parrot left square bracket 76 00:07:00,280 --> 00:07:06,430 6 colon, then a right square bracket and right parenthesis. So again, the first number in 77 00:07:06,430 --> 00:07:09,850 a slice is omitted, the slice starts at the beginning of the string, and if 78 00:07:09,850 --> 00:07:13,980 the second number is omitted, it'll run to the end of the string. 79 00:07:13,980 --> 00:07:21,040 The slice indices are defined such as string square bracket colon n + string square 80 00:07:21,040 --> 00:07:26,050 bracket n colon, is the same as the original string. So to show you what I 81 00:07:26,050 --> 00:07:30,460 mean I'm going to type some code on line 16 this time, print left parenthesis 82 00:07:30,460 --> 00:07:35,820 parrot left square bracket colon 6. We've got a right square bracket, 83 00:07:35,820 --> 00:07:40,920 + this time, I'm going to add a space there, and a plus, another space then parrot 84 00:07:40,920 --> 00:07:45,430 left square bracket 6 colon, then we've got our right square bracket and our 85 00:07:45,430 --> 00:07:48,360 right parenthesis again. 86 00:07:48,360 --> 00:07:52,740 So if we run this, 87 00:07:52,740 --> 00:07:55,880 you can see, in fact, we've got the 88 00:07:55,880 --> 00:07:59,710 entire Norwegian Blue outputted to the screen. So at this point, my question to 89 00:07:59,710 --> 00:08:04,030 you is, what happens if we only have a colon in the slice? In other words, what 90 00:08:04,030 --> 00:08:06,980 will this produce? 91 00:08:06,980 --> 00:08:12,490 print left parenthesis parrot left square bracket colon and right 92 00:08:12,490 --> 00:08:16,330 square bracket parentheses. So have a think about that and see if you can 93 00:08:16,330 --> 00:08:19,840 figure it out before running the program. Pause the video now and come back when 94 00:08:19,840 --> 00:08:22,500 you've got an answer. 95 00:08:24,140 --> 00:08:26,800 Alright so welcome back. Did you work it out? 96 00:08:26,800 --> 00:08:29,060 Well if you don't provide a start value, 97 00:08:29,060 --> 00:08:32,219 the slice starts at the beginning of the sequence. So there's no start 98 00:08:32,219 --> 00:08:36,929 value on line 18, so we start at the beginning. Now if you don't provide a stop 99 00:08:36,929 --> 00:08:40,890 value, the slice extends up to the end of the sequence, and we haven't got a stop 100 00:08:40,890 --> 00:08:45,029 value on line 18 either. So that means the slice will start at the beginning 101 00:08:45,029 --> 00:08:51,020 and extend right up to the end. So if we run the program now, 102 00:08:51,020 --> 00:08:53,760 we get the complete string printed out. 103 00:08:53,760 --> 00:08:57,260 So you might be wondering at this point, why we'd don't want to do that. 104 00:08:57,260 --> 00:08:59,840 When dealing with strings you probably wouldn't, and I'll 105 00:08:59,840 --> 00:09:03,200 come back to it when we look at another sequence type, a list. 106 00:09:03,200 --> 00:09:07,740 When dealing with lists, a slice like this one, that produces a copy of the entire original, 107 00:09:07,740 --> 00:09:12,240 can be very useful. I've covered it now because we're looking at start and stop 108 00:09:12,240 --> 00:09:16,290 values in a slice, and we'll see some practical applications of it, later in 109 00:09:16,290 --> 00:09:19,040 the course. Alright I'm going to stop the video here. 110 00:09:19,040 --> 00:09:20,620 Before watching the next video, 111 00:09:20,620 --> 00:09:24,500 experiment with slices with different strings. So a good one that you 112 00:09:24,500 --> 00:09:29,660 might want to experiment with, would be this one which I'll add to the source code. 113 00:09:29,660 --> 00:09:41,260 So letters equals, and we're going to add the letters of the alphabet, 114 00:09:41,260 --> 00:09:42,240 like so. 115 00:09:42,240 --> 00:09:46,520 So that can be a good string to actually experiment with, but you can use any 116 00:09:46,520 --> 00:09:50,500 strings you want. Create some slices and make sure you get the results you wanted. 117 00:09:50,500 --> 00:09:54,680 Once you've done, that I'll see you in the next video.