WEBVTT 00:00.990 --> 00:04.640 Welcome back to part two of our introduction to lists. 00:04.710 --> 00:10.650 So in the last session we learned about the general structure of lists and how to create lists. 00:10.680 --> 00:17.100 So now in this session we learn how to select single and multiple element elements from a list and how 00:17.100 --> 00:21.000 to manipulate and change elements of a list. 00:21.030 --> 00:27.480 And we also learn how to slice lists and make some selection of lists. 00:27.600 --> 00:28.170 So what now. 00:28.170 --> 00:31.470 Let's start with our lists which we already know 00:36.540 --> 00:43.630 in our first task is we want to select the very first element of this list and the general rule is that 00:43.660 --> 00:45.810 elements of a list can be assessed. 00:45.820 --> 00:54.100 This sped up records so we take our spot bracket and let's try to get them the very first element of 00:54.100 --> 00:54.670 the list. 00:54.760 --> 01:00.220 So you type in 1 and let's see what we get so we get two. 01:00.250 --> 01:02.150 So two is. 01:02.250 --> 01:02.520 Yeah. 01:02.530 --> 01:04.260 Actually the second element. 01:05.410 --> 01:08.590 So how do we get now our first element. 01:08.580 --> 01:14.260 And yeah the others actually that we have to do type in 0 as index. 01:14.260 --> 01:16.230 And let us see what we get. 01:16.240 --> 01:20.700 So if we're type in 0 to get the first element here it's 1. 01:21.840 --> 01:25.720 And this is neither a mistake or our special last change behavior. 01:25.740 --> 01:33.910 Python Yeah it's a common commonly seen pattern in programming or coding that you have the so-called 01:33.950 --> 01:35.620 zero based indexing. 01:36.070 --> 01:42.570 So the very first element of a sequence has the index Position ZERO. 01:42.760 --> 01:49.240 And then the second tests the index position one the third element has the index to show position 2 01:49.240 --> 01:50.240 and so on. 01:50.230 --> 01:55.120 Now this is a general rule in many programming languages. 01:55.120 --> 01:58.540 So once again it's called zero based indexing. 01:58.540 --> 01:58.870 All right. 01:58.870 --> 02:02.300 So what if now we want to get the fifth element of our list. 02:02.310 --> 02:05.130 So the fifth element is five. 02:05.140 --> 02:10.330 So consequently we have to uh input the index position far. 02:11.080 --> 02:13.840 And now it should return us five. 02:14.200 --> 02:14.980 And this is true. 02:15.850 --> 02:23.880 Alternatively we can also assess the very last element of a sequence by typing the index position minus 02:23.880 --> 02:29.600 one so minus one gets us the very last element of a sequence. 02:29.610 --> 02:38.180 So in this case it's five and the second very last element of our list can be assessed by the index 02:38.180 --> 02:41.220 position minus two. 02:41.450 --> 02:47.330 And consequently the first element of our list can also be indexed by minus five 02:51.320 --> 02:55.100 so that two ways to two index our lists. 02:55.220 --> 02:58.370 So that's the zero based indexing starting from zero. 02:58.370 --> 03:11.710 So our element 1 is has an exposition 0 in exposition 1 in exposition 2 index position 4 sorry in exposition 03:11.720 --> 03:20.900 3 and index position 4 and the alternative phase would be to have index position minus 1 index position 03:20.900 --> 03:30.780 minus 2 index position minus 3 index position minus 4 and index position minus 5. 03:30.830 --> 03:36.280 So and this holds true fire almost every kind of sequence. 03:36.280 --> 03:42.600 So not only false but also for two tuples which we see later off on umpire race and so on. 03:42.620 --> 03:47.170 So yeah this is a very important and basic basic rule here. 03:47.180 --> 03:54.070 How to to assess elements of a sequence and what you can also do you can do it the other way around. 03:54.220 --> 04:03.460 So you can foreign give given element let's say 5 percent can give you the index position so and um 04:03.760 --> 04:09.130 you need to the method index and let's type in 5. 04:09.130 --> 04:19.270 So we would expect B five is at index position far so we would expect here to return for and that's 04:19.270 --> 04:20.800 correct. 04:20.800 --> 04:22.570 Lists are generally mutable. 04:22.570 --> 04:24.020 So what does it mean mutable. 04:24.040 --> 04:26.850 So you can change elements of Alice. 04:27.250 --> 04:29.150 And how do we do this. 04:29.200 --> 04:35.800 So we first have to select our element here we want to select our very the very first element that index 04:35.800 --> 04:41.960 position zero and then we assign with the equals sign a new value here it's hundred. 04:42.430 --> 04:43.720 So let's see what we get. 04:45.460 --> 04:47.320 And here you can see now our lists. 04:47.380 --> 04:49.450 The first element is a hundred and not one. 04:49.450 --> 04:56.710 So we simply changed the first element at an exposure Position ZERO and we can also change the very 04:56.710 --> 04:59.620 last element in our sequence. 04:59.830 --> 05:04.400 So l at an exposition minus one equals hundred one. 05:04.680 --> 05:06.460 Let's see if it works. 05:07.360 --> 05:08.560 And yes it works. 05:08.560 --> 05:14.980 Now what if we want to select many elements of a list and we can do this with a so-called slicing by 05:14.980 --> 05:17.980 using a colon inside the spout brackets. 05:17.980 --> 05:24.130 So in this example here we have a slice from the second element that in exposition one. 05:24.150 --> 05:33.190 And by definition this element is including so from first position unto the fourth index position but 05:33.550 --> 05:38.080 here excluding so let's see what happens. 05:38.110 --> 05:40.350 So we have two three and four. 05:40.360 --> 05:46.990 So we are slicing the list from index position 1 including until index position 4 excluding. 05:47.200 --> 05:48.910 So let's check again here. 05:48.970 --> 05:49.870 Let's see at the list. 05:49.870 --> 06:00.430 So here is an exposition one including until an exposition for but excluding so we get to three and 06:00.430 --> 06:00.820 four 06:03.650 --> 06:07.040 and we can also slice from index position 1 to index position five. 06:07.050 --> 06:10.070 So here in our list there is no index position 5. 06:10.320 --> 06:17.230 So by default we get all elements from index position 1 until the very end. 06:17.430 --> 06:22.700 So let's check and we get to 3 4 and yeah here 101. 06:22.800 --> 06:30.530 And um the alternative way to slice until the very last element including the last element is to to 06:30.540 --> 06:33.990 have just one colon and then nothing 06:38.940 --> 06:44.730 so this means we are slicing from one until the very last element including the last element. 06:44.730 --> 06:48.390 And we can also slice from our very first element. 06:48.480 --> 06:56.850 So at an exposition 0 2 let's say in exposition 3 excluding let's see what we get. 06:56.870 --> 07:03.460 So one hundred two and three as we have including the very first element and the index position 3 so 07:03.470 --> 07:05.860 0 1 2 3. 07:05.880 --> 07:10.470 But excluding so one hundred two and three and the alternative way to do this. 07:10.480 --> 07:18.000 So slicing from the very first element to let's say the element that anyway in exposition 3 is to have 07:18.000 --> 07:20.570 just hear nothing. 07:20.590 --> 07:23.800 Colon 3. 07:23.980 --> 07:26.220 So this is the same actually. 07:26.260 --> 07:27.750 So on one more example. 07:27.760 --> 07:31.440 So let's slice from index position 1 including 2. 07:31.450 --> 07:43.060 Next position 3 excluding and we get 2 and 3 and we can also select all elements by just having a colon 07:43.150 --> 07:51.440 and on the left side of the colon on the right side of the colon nothing gives less than the whole list. 07:52.360 --> 07:55.850 All right so let's enlarge our list with some more elements. 07:55.850 --> 08:03.940 So from 1 to 10 and what we can also do we can choose every second element of our list and we can do 08:03.940 --> 08:06.950 this by having uh two colons. 08:07.010 --> 08:13.900 Um this gives us here every second element beginning from the first including so we have fear starting 08:13.900 --> 08:17.350 from the very first element every second element. 08:17.350 --> 08:28.800 So 1 3 5 7 and 9 and we can also select every third element beginning from the first element by having 08:28.860 --> 08:32.070 double colon and then every three 08:36.360 --> 08:41.820 so we are starting from the first element and then selecting every third element. 08:41.820 --> 08:53.370 So we have 1 4 7 and 10 and we can also start with. 08:53.410 --> 09:01.510 For example the second element in exposition 1 and then select every second element by defining here. 09:01.580 --> 09:09.730 Okay let's start from the second element add an exposition 1 but please only give me every second element. 09:11.110 --> 09:14.530 So we get 2 4 6 8 and 10. 09:14.530 --> 09:17.180 So we are starting from the second element. 09:18.380 --> 09:20.680 And then slicing every second element. 09:20.720 --> 09:29.200 So we have to for 6 8 and 10. 09:29.370 --> 09:31.040 So let's start again with ETL. 09:31.470 --> 09:32.280 That's our list. 09:32.370 --> 09:34.210 So from one to 10. 09:34.320 --> 09:38.160 And we can also assign many new value values to many elements. 09:38.160 --> 09:46.410 So for example if I want to assign to the element that takes position for 50 and to the element that 09:46.410 --> 09:54.570 an exposition 5 51 and to the element that an exposition 652 I can do this by slicing L and assigning 09:54.570 --> 09:57.570 the new values which are here in the list 10:00.730 --> 10:11.110 so as you can see here we have at an exposition 5 or so 0 1 2 3 4 we are 50 50 one that the next position 10:11.110 --> 10:14.060 5 and 50 two at an exposition 6. 10:14.080 --> 10:14.410 All right. 10:14.400 --> 10:17.670 So we cannot do as these so-called broadcasting. 10:17.920 --> 10:25.150 So if we want to have that an exposition for five and six if we want to have 50 then our code as we 10:25.150 --> 10:26.680 have it here is not working. 10:28.720 --> 10:30.040 So we get an error message. 10:30.070 --> 10:36.970 And alternatively how we have to assign two uh in exposition 450 550 and 650. 10:37.240 --> 10:38.680 So we have to do it manually 10:42.160 --> 10:49.540 and then we have 50 at an exposition for five and six alternatively we could have created a list of 10:49.750 --> 10:50.960 three times 50. 10:51.280 --> 10:57.910 So that's that's not out of fear and what we can also do we can delete an element at a certain index 10:57.950 --> 11:03.530 processional so let's say we want to delete them the very first element that in the next position zero. 11:03.560 --> 11:10.830 We can do this and as you can see here the first element so one is removed. 11:10.890 --> 11:15.070 Another thing I want to show you is a nested list or lists of lists. 11:15.320 --> 11:22.580 So the last session we learned that we can actually put any data type into a list and this holds also 11:22.580 --> 11:28.310 true for our list itself so you can just put one on many lists into a list. 11:28.310 --> 11:32.970 And for example we have here a list of let's say football players. 11:32.990 --> 11:40.340 So in each football player has a name and if for example as the height one point eighty six metres and 11:40.400 --> 11:46.550 has a rate so eighty two kilograms and we have four players on our list then for each player we have 11:46.910 --> 11:56.030 one list with the name length and the height so we can create the list and then we have the list with. 11:56.260 --> 12:03.190 So it's a nested list with the four elements and each element is the list itself and we can also print 12:04.170 --> 12:07.320 the type and the length of the list. 12:07.350 --> 12:18.900 Let's do this so we have of course this is a list and it has four elements and we can also slice or 12:18.900 --> 12:25.060 select elements off our list so we can select the very first element at an exposition zero. 12:25.080 --> 12:26.850 So I would expect to get them. 12:26.850 --> 12:27.150 Yeah. 12:27.190 --> 12:27.990 And here 12:31.710 --> 12:32.330 exactly. 12:32.340 --> 12:38.420 So we get we have high end video and use one point it's six meters tall and yes eighty two kilograms. 12:38.490 --> 12:46.600 You can also get the very last player at an exposition three or minus one as you like. 12:46.740 --> 12:52.870 So we get Mike so now and but we can also do we can select the height off from the very first player. 12:52.880 --> 12:56.610 So let's say Andy and we can do that by. 12:56.640 --> 12:59.760 First of all it's select Yeah. 13:00.260 --> 13:05.600 So now we have a sub selection we fear the subtlest and then we want to select the second element of 13:05.600 --> 13:11.210 the index position one we can do this. 13:11.390 --> 13:13.710 And so we have selected the height of India. 13:13.730 --> 13:15.860 So it's one point eighty six meters. 13:16.190 --> 13:22.790 And what we can also do we can create a list containing only all four names of our players. 13:22.850 --> 13:25.510 So we create a list called names. 13:25.610 --> 13:32.800 We open this up brackets and then we say OK the first element should be the very first player. 13:33.070 --> 13:34.210 And the first element. 13:34.210 --> 13:39.720 So at the next position zero is the name here it's Andy. 13:40.600 --> 13:46.630 And then give us the name of the second player give us the name of the third player. 13:47.110 --> 13:48.580 And the name of the fourth player 13:51.610 --> 13:56.400 so now we have a list containing only the names of our players. 13:56.680 --> 13:59.130 So we have a list of four strings. 13:59.230 --> 14:01.740 And that's another thing I want to show you here. 14:01.840 --> 14:05.080 Let's create a list X with four elements. 14:05.080 --> 14:08.290 And then we create another variable Y. 14:08.500 --> 14:10.130 And we assign x. 14:10.810 --> 14:13.590 So here y plus reference to the same list. 14:13.600 --> 14:19.900 And now if you change the very first element of X let's say x that in exposition 0 is 100 14:23.080 --> 14:26.680 so now the list of X is one hundred two three four. 14:26.680 --> 14:29.180 And now if you are checking again I y y. 14:29.340 --> 14:31.450 So why has also changed. 14:31.510 --> 14:33.220 So why is this the case. 14:33.310 --> 14:40.450 Well and what Tyson does internally here it creates a list object and references it makes a reference 14:40.450 --> 14:42.900 to rival x. 14:43.000 --> 14:51.630 And here we assigning y to X so we are creating a reference from X to the same list object. 14:51.660 --> 14:57.600 So we only have one list object and we have two references so one reference to X and one reference to 14:57.600 --> 14:58.560 Y. 14:58.600 --> 15:07.110 And now if you change our object by changing the first element of X Y references to the changed object 15:07.440 --> 15:10.070 so maybe it was our intention that also y changed. 15:10.080 --> 15:18.480 But what if it's not our intention that Y changes if exchanges and what we can do here is that we say 15:19.670 --> 15:27.480 so let's start again here with X equals one two three far and then we create a copy here of the list 15:27.510 --> 15:30.690 by saying okay please create me a copy of X 15:33.630 --> 15:40.010 and to sign it to y so now Y is referencing to a copy of x. 15:40.040 --> 15:42.200 So not to to here the. 15:42.230 --> 15:43.280 The object itself. 15:43.280 --> 15:50.090 So now we created two objects so we have one object the original one that is referencing two X or Xs 15:50.090 --> 15:57.380 referencing to the object and we have a second object which is actually the same elements but only Y 15:57.380 --> 16:01.410 is referencing to the new through the copy a new object. 16:01.700 --> 16:04.670 And if you change it here X now 16:08.410 --> 16:13.380 so why does not change and that's another way to do this. 16:13.380 --> 16:15.080 So let's start again. 16:15.170 --> 16:17.900 So we we are creating an object. 16:17.990 --> 16:19.970 One two three four and assigning X 16:26.110 --> 16:33.060 so we can also create a copy of X by saying okay please give me all elements of x. 16:33.160 --> 16:37.960 So by writing it here and in this manner we are also creating a copy. 16:38.710 --> 16:45.970 So why is it now referencing to our copy so we have two objects object one referencing two X and object 16:45.970 --> 16:49.470 two we are only at Y it's referencing to it. 16:49.610 --> 16:50.800 Now if you change X 16:54.850 --> 16:56.430 also there's no change with Y. 16:56.440 --> 16:59.530 So we have two alternative solutions where we prevent that. 16:59.530 --> 17:02.760 Also Y is changed if we we change x. 17:03.040 --> 17:03.420 All right. 17:03.430 --> 17:10.150 So at the beginning of the session yeah I said that the elements of sequences can be assessed with spec 17:10.150 --> 17:15.760 brackets and this not only holds far lists but also for another sequences or other data types which 17:15.760 --> 17:17.740 are sequences like strings. 17:17.740 --> 17:26.500 So if you create a string Doc dog is a sequence of three characters d o g. 17:27.170 --> 17:33.140 And we can also select here the first element at an exposition 0. 17:33.140 --> 17:38.870 So it's an d and we can also select the very last element G. 17:39.380 --> 17:44.940 And before we set that list mutable so we can change elements in our list. 17:45.410 --> 17:49.160 And that's the difference to strings so strings are immutable. 17:49.250 --> 17:58.100 If we want to assign a new value to let's say in exposition one let's say a stack instead of dark and 17:58.130 --> 18:00.350 then Python gives us an error message. 18:00.620 --> 18:02.220 So this is an example of that. 18:02.280 --> 18:06.510 String objects or as a sequence that is are immutable. 18:06.940 --> 18:07.340 All right. 18:07.340 --> 18:13.860 So now we have finished our session here and I hope you have a better understanding of lists and in 18:13.880 --> 18:21.980 our next session we are going to the next the data type which is lossless sequence tuples so happy to 18:21.980 --> 18:23.540 see you in the next session by.