WEBVTT 0 00:00.110 --> 00:05.750 Now, when you're working with lists, one of the most common errors you'll come across is something 1 00:05.750 --> 00:08.000 called the index out of range error. 2 00:08.030 --> 00:11.630 In fact, by this point, you might have already seen it. 3 00:11.810 --> 00:13.940 Now, what does it mean though? 4 00:14.030 --> 00:16.640 Well, let's take our states_of_america, 5 00:16.640 --> 00:20.330 and you might remember that there are 50 states in America, 6 00:20.330 --> 00:26.000 but if you don't, because you're a programmer, it's as easy as writing, len(), 7 00:26.000 --> 00:31.070 and then passing over the states_of_america, which will print 50. 8 00:31.100 --> 00:38.630 So now that we know that there are a total of 50 items in this list, and remember, because we start 9 00:38.630 --> 00:44.030 counting from 0, Hawaii is actually at index 49. 10 00:44.030 --> 00:51.500 So if we print states_of_america and then we try to get the item at 49, we hit print, you'll see that 11 00:51.500 --> 00:58.010 we get Hawaii printed. Now what if we went one beyond that? 12 00:58.010 --> 01:02.090 What if we tried to get the one at index 50? 13 01:02.090 --> 01:03.710 What do we get instead? 14 01:03.800 --> 01:05.870 Well, we get an error. 15 01:05.960 --> 01:07.850 It's called an IndexError. 16 01:07.850 --> 01:14.930 And this is because it's beyond Hawaii and there's nothing there, as we can see with our own eyes. 17 01:14.930 --> 01:21.000 But when you're working with large lists and you're not always looking at the data, then these errors 18 01:21.000 --> 01:23.460 can be a little bit more confusing. 19 01:23.520 --> 01:28.950 Very frequently when you're working with lists, you'll end up with an off-by-one error. 20 01:28.950 --> 01:36.630 So it's unusual that you'll try to get something at index number 90, because that's just way beyond 21 01:36.630 --> 01:37.740 your list size, 22 01:37.740 --> 01:44.280 but very frequently you might end up in a situation where you have some sort of value, say 23 01:44.280 --> 01:46.590 number_of_states = len(), 24 01:46.590 --> 01:50.850 and then we pass over the states_of_america. 25 01:50.850 --> 01:53.310 So this is going to be equal to 50. 26 01:53.310 --> 01:59.730 And then we pass that inside here as the index num_of_states, 27 01:59.730 --> 02:01.290 and then we hit Run, 28 02:01.290 --> 02:03.510 and we get the same error, right? 29 02:03.510 --> 02:06.870 In this list. It's again list index out of range. 30 02:06.870 --> 02:14.280 And this is an off-by-one error because all we need to do is just simply -1, 31 02:14.280 --> 02:19.380 so that 1 becomes 0, and 50 becomes 49. 32 02:19.380 --> 02:21.510 And then we get rid of that error. 33 02:22.860 --> 02:27.060 Now it might be easier if we work with something a little bit simpler. 34 02:27.330 --> 02:35.190 Recently I was reading online, and I came across the so-called "Dirty Dozen," where the Environmental 35 02:35.190 --> 02:41.130 Working Group, a bunch of people crunched through a whole lot of data, probably using Python, and 36 02:41.130 --> 02:47.820 they released their Dirty Dozen, a list of the fruits and vegetables that have the most pesticides. 37 02:47.820 --> 02:54.900 And it's kind of crazy that they actually washed and peeled all of these foods and then tested them 38 02:54.900 --> 03:01.860 for pesticides. And the list looks something like this, where strawberries are apparently one of the 39 03:01.860 --> 03:03.870 worst offenders for pesticides. 40 03:03.870 --> 03:07.380 So let's create a list of the dirty_dozen. 41 03:07.380 --> 03:14.640 But you'll notice that some of these are fruits like strawberries, apples, and other ones are vegetables. 42 03:14.640 --> 03:21.510 So how can we use our lists to still keep them inside the same sort of container? 43 03:21.540 --> 03:26.790 The dirty_dozen, but somehow separate them out into fruits and vegetables? 44 03:27.720 --> 03:33.210 Well, we could just simply create two lists fruits and vegetables. 45 03:33.210 --> 03:36.870 But these two lists kind of have a relationship, right? 46 03:36.870 --> 03:42.270 They're kind of related because they're all on the list of high pesticide foods. 47 03:42.270 --> 03:46.200 So how can we have lists within a list? 48 03:46.230 --> 03:49.200 Well, that's what's called a nested list. 49 03:49.200 --> 03:57.060 Instead of our original dirty_dozen, we could create a new list called dirty_dozen, and we set it 50 03:57.060 --> 04:00.870 equal to a list that contains two lists. 51 04:00.870 --> 04:04.890 It contains fruits and it contains vegetables. 52 04:05.220 --> 04:13.480 So now what effectively has happened is we've inserted this list inside here, and then we've inserted 53 04:13.480 --> 04:16.060 this list inside here. 54 04:16.120 --> 04:20.710 So we now have a list that contains two lists. 55 04:21.070 --> 04:26.260 And if I go ahead and print out this list you'll be able to see its structure. 56 04:27.820 --> 04:29.530 And it looks like this. 57 04:29.560 --> 04:34.660 You'll notice that there's two brackets at the beginning and at the end. 58 04:34.660 --> 04:38.680 And the reason is because this is one list, 59 04:38.920 --> 04:42.490 this is another list, 60 04:42.490 --> 04:45.040 and this is also a list. 61 04:45.040 --> 04:52.780 So this is yet another way of using lists and just showing you the flexibility of this particular data 62 04:52.780 --> 04:53.620 structure. 63 04:53.830 --> 04:58.660 It's something that you're going to use a lot when you're writing Python code.