1 00:00:04,220 --> 00:00:08,420 Alright so how did you get on with the challenge from the last video. 2 00:00:08,420 --> 00:00:13,380 The first part of the challenge was to slice the string to produce the letters q, p and o. 3 00:00:13,389 --> 00:00:17,520 So let's have a go at doing that. So I'm going to type, on line 7, print 4 00:00:17,520 --> 00:00:25,760 parentheses letters square brackets 16:13:-1 5 00:00:25,760 --> 00:00:27,400 So we're standing at position 16. 6 00:00:27,400 --> 00:00:32,320 We're extending backwards, up to, but not including the character at position 13. 7 00:00:32,320 --> 00:00:39,220 So let's run that, and there's the letters q, p and o. So next we want a 8 00:00:39,220 --> 00:00:44,379 slice to produce the letters edbca. The easiest way to include the beginning 9 00:00:49,420 --> 00:00:58,900 is to type print parentheses letters[4::-1]. So if we 10 00:00:58,900 --> 00:01:02,840 run that, 11 00:01:02,840 --> 00:01:04,760 and there's the letters edcba. 12 00:01:04,760 --> 00:01:07,000 Finally, we wanted the last eight 13 00:01:07,000 --> 00:01:11,920 characters in reverse order. So we'll be using a negative step value, which means 14 00:01:11,920 --> 00:01:17,109 the start will default to the end of the string and can therefore be omitted. 15 00:01:17,109 --> 00:01:26,780 So to type that, I'm going to do a print parentheses letters square brackets 16 00:01:26,780 --> 00:01:33,020 :-9:-1 17 00:01:33,020 --> 00:01:41,859 Alright so running that, and there's the last 8 letters, zyxwvuts. Now there are other solutions you could have 18 00:01:41,859 --> 00:01:45,340 used, especially when returning items from the beginning and end of the 19 00:01:45,340 --> 00:01:50,049 sequence. Using default values for the start and stop is recommended, when 20 00:01:50,049 --> 00:01:53,409 trying to return a certain number of characters from the beginning or end of 21 00:01:53,409 --> 00:01:58,780 a sequence. If you managed to produce the correct strings with your slices, well done. 22 00:01:58,780 --> 00:02:02,580 Alright, so I'll finish talking about slices for now, by looking at some 23 00:02:02,580 --> 00:02:06,860 common idioms that Python programmers use and recognize. 24 00:02:06,860 --> 00:02:13,299 In the last video, I mentioned that a slice of ::-1 is a Python idiom. When you see 25 00:02:13,299 --> 00:02:17,110 that, you'll recognize it as reversing the sequence. 26 00:02:17,110 --> 00:02:21,640 Let's see some more idioms. The first is to return the last n items in a 27 00:02:21,640 --> 00:02:26,920 sequence. So let's say we wanted the last four characters from letters. So I'm 28 00:02:26,920 --> 00:02:33,510 going to come down, and on line 15 I'm going to type print parentheses letters 29 00:02:33,510 --> 00:02:40,510 [-4:] If I run that you should get the last 30 00:02:40,510 --> 00:02:47,830 four letters, wxyz. Whenever you see a slice with a negative start value and 31 00:02:47,830 --> 00:02:52,480 defaults for the stop and step, that just returns the end of the sequence. 32 00:02:52,480 --> 00:02:57,720 Our common use of that is to get the last item by specifying -1 for the start 33 00:02:57,730 --> 00:03:04,209 value. So let's have a go doing that. So I'm going to type print parentheses then 34 00:03:04,209 --> 00:03:15,640 letters[-1:] Run that, we get the last item, in this case, the z. 35 00:03:15,640 --> 00:03:21,070 Now you can also use a slice to get the first character, very similar. We just 36 00:03:21,070 --> 00:03:28,460 type print(letters[1:] 37 00:03:28,460 --> 00:03:33,310 Run that, and I actually did that the wrong way, because that obviously gave us 38 00:03:33,310 --> 00:03:38,120 everything but the first, starting at sequence position 1. It should have been [:1], 39 00:03:38,120 --> 00:03:43,480 which is what I meant to type. Running that, we now get the letter a, which is 40 00:03:43,480 --> 00:03:48,040 the first item in the sequence, the first character. Now that last one is useful 41 00:03:48,040 --> 00:03:52,239 because it doesn't give an error if the sequence is empty. It may though, be more 42 00:03:52,240 --> 00:03:56,880 natural to get the item at index position 0, and that works with a string. 43 00:03:56,880 --> 00:04:05,880 So we could also do it as print(letters[0]), 44 00:04:07,320 --> 00:04:11,950 and that's the same, gives us the same result. That works, but what would happen 45 00:04:11,950 --> 00:04:16,149 if our string was empty? So I'm going to go up and delete the letters, or the 46 00:04:16,149 --> 00:04:21,959 characters rather, on line 1. So I'll make that an empty string. 47 00:04:21,959 --> 00:04:27,300 Run the program, and we now get an error, and the error is: string index out of range. 48 00:04:27,310 --> 00:04:30,940 You can't access the first item of a sequence that doesn't 49 00:04:30,940 --> 00:04:37,210 any items. So looking at the code, the slice on line 17 produces an empty 50 00:04:37,210 --> 00:04:43,280 string, instead of crashing with an index error, and just to confirm, if I run it again, 51 00:04:43,280 --> 00:04:49,720 the error's actually talking, or is showing on line 18, as you can see there. 52 00:04:49,720 --> 00:04:55,990 So I had no problems with executing line 17. So the idiom on line 17 is 53 00:04:55,990 --> 00:05:00,970 useful for getting the first item in a sequence, without your code crashing. 54 00:05:00,970 --> 00:05:05,440 If the sequence is empty, you'll get an empty sequence back, and that's often 55 00:05:05,440 --> 00:05:09,080 what you'd want to happen. Alright so that's the end of slices, for now. 56 00:05:09,080 --> 00:05:14,940 In the next video, we'll look at string operators. See you in the next video.