1 00:00:04,400 --> 00:00:09,599 We've seen that you can use negative start and stop values in a slice. You can 2 00:00:09,599 --> 00:00:13,770 also use a negative step. There's only one thing to watch out for, and it's 3 00:00:13,770 --> 00:00:18,060 one of those things that's obvious, once it's been pointed out. If I asked you to 4 00:00:18,060 --> 00:00:23,100 count backwards from 99, and stop when you reach 100, how would you get on? 5 00:00:23,100 --> 00:00:27,380 You'd tell me it's impossible, of course, and you can't possibly get to 100 by 6 00:00:27,390 --> 00:00:32,940 counting backwards from 99. Well, neither can Python. Now that you know that, you 7 00:00:32,940 --> 00:00:37,020 won't make the most common error that people make when using negative steps in 8 00:00:37,020 --> 00:00:40,980 a slice. Alright, so let's see some examples. We want to create a new Python 9 00:00:40,980 --> 00:00:44,600 file called sliceback. 10 00:00:52,700 --> 00:01:06,360 So I'm going to start by typing letters equals and the alphabet abcdefg etc. 11 00:01:06,360 --> 00:01:13,020 Then on line 3 I'm going to type backwards equals letters square brackets 12 00:01:13,020 --> 00:01:19,990 25:0:-1 then the closing right square bracket, and we're 13 00:01:19,990 --> 00:01:24,759 going to print backwards. Alright so I've created a slice that 14 00:01:24,759 --> 00:01:30,970 starts at index 25, which is the z, and I set the stop value to 0 and use the 15 00:01:30,970 --> 00:01:35,770 step of -1. So what do you expect the output to be here? So pause the video 16 00:01:35,770 --> 00:01:41,250 and have a careful think, and come back when you think you've got the answer. 17 00:01:42,600 --> 00:01:48,800 Alright, so let's run the program now to see if you were right. 18 00:01:48,800 --> 00:01:54,300 So we get the letters of the alphabet in reverse order from z down to b. Notice that the letter 19 00:01:54,310 --> 00:02:00,430 a isn't included. We used a stop value of zero, and slices extend up to, but not 20 00:02:00,430 --> 00:02:05,020 including, the stop value. Before I ask you another question, let's review what 21 00:02:05,020 --> 00:02:10,389 we've got here, on line 3. Because we're using a negative step, the start 22 00:02:10,389 --> 00:02:14,620 value must be greater than the stop value. Our slice starts at index position 23 00:02:14,620 --> 00:02:20,350 25, and it extends down to index position 0, but doesn't include the character 24 00:02:20,350 --> 00:02:25,150 at that position. We're stepping by -1, which produces all the letters from 25 00:02:25,150 --> 00:02:30,190 z down to b. So my second question is, what stop value will let the slice 26 00:02:30,190 --> 00:02:34,660 include the letter a? There's two obvious answers to that but only one of them 27 00:02:34,660 --> 00:02:39,310 works. So have a think and run your code, to try your answer before starting the 28 00:02:39,310 --> 00:02:42,840 video again. So pause the video now. 29 00:02:43,520 --> 00:02:47,780 Alright so how did you get on? So one obvious answer; if a slice doesn't 30 00:02:47,780 --> 00:02:52,900 include the position at the stop value, is to use a stop value of -1. 31 00:02:52,900 --> 00:02:56,720 But unfortunately that's the one that doesn't work, so if we try that, and use a 32 00:02:56,720 --> 00:02:59,660 stop value of -1, 33 00:02:59,660 --> 00:03:04,520 when we run it, we don't get any output. So you may have 34 00:03:04,520 --> 00:03:08,750 worked out why. Remember that negative stop values count backwards from the 35 00:03:08,750 --> 00:03:14,030 end of the sequence. Minus 1 means the last character in the string, which means 36 00:03:14,030 --> 00:03:19,340 we've requested a slice that extends from the z up to, but not including, the z, 37 00:03:19,340 --> 00:03:23,810 and therefore we get nothing. The other obvious answer is to omit the 38 00:03:23,810 --> 00:03:28,190 stop value. Python will default to the start or end of the sequence, if we don't 39 00:03:28,190 --> 00:03:32,750 specify start or stop values. So it's clever enough to work out that it should 40 00:03:32,750 --> 00:03:37,280 reverse the defaults when we're using a negative step. So if I change that now, 41 00:03:37,280 --> 00:03:44,710 get rid of the -1 for our stop. Run this, and you can see that the code works. 42 00:03:44,710 --> 00:03:49,850 With a negative step, the start value will default to the end of the string, 43 00:03:49,850 --> 00:03:55,310 and the stop value defaults to the start of the string. That means we can ommit 44 00:03:55,310 --> 00:04:03,100 our start value as well. So we can go back and do that, and if we run it now, 45 00:04:03,100 --> 00:04:10,220 once again we get the string reversed. A slice of ::-1 is a Python idiom. 46 00:04:10,220 --> 00:04:15,080 When you see that, you'll recognize it as reversing the sequence. I'll cover a few 47 00:04:15,080 --> 00:04:18,769 more slicing idioms in the next video. There's not much else that 48 00:04:18,769 --> 00:04:23,150 I can say about slicing backwards. Just use a negative step and make sure the 49 00:04:23,150 --> 00:04:27,530 start value is greater than the stop value. Experiment with different values 50 00:04:27,530 --> 00:04:31,550 and make sure you're comfortable with what a negative step produces. I'll 51 00:04:31,550 --> 00:04:35,060 finish this video with a challenge, and go over the solution to that challenge 52 00:04:35,060 --> 00:04:37,200 in the next video. 53 00:04:56,790 --> 00:05:00,090 That's your challenge. Have a go at that, and as mentioned, I'll go through the 54 00:05:00,090 --> 00:05:03,770 solution to it in the next video.