WEBVTT 00:00.940 --> 00:05.260 Hello and welcome back to this session about user defined functions. 00:05.260 --> 00:11.710 So whenever we want to reuse or repeat the same or very similar code more than once it may be worth 00:11.740 --> 00:14.170 writing a reusable function. 00:14.170 --> 00:19.480 And by doing so this can help us to make our code more readable. 00:19.480 --> 00:21.040 Functions are nothing new. 00:21.040 --> 00:25.180 And we use a lot of built in functions so far but. 00:25.270 --> 00:35.290 First of all let's create a list ATL containing four integers 3 2 5 6 and we already know the function 00:35.320 --> 00:43.350 the built in function print so we can print our list and for example there's also the function sorted 00:43.380 --> 00:54.290 that we can start our list and then ascending minor so here we have 2 3 5 6 and saw it actually creates 00:54.410 --> 00:56.390 a sorted copy off. 00:56.480 --> 01:03.740 So if we want to really sort our list out we have to reassign l. 01:04.230 --> 01:05.640 So now let's print again. 01:05.650 --> 01:15.570 Oh and here we have these sorted lists L and we already know methods which are functions that are attached 01:15.570 --> 01:23.190 to a specific objects for instance we know that the list object has and the method append where we can 01:23.190 --> 01:25.220 append another element to the end of the list. 01:25.220 --> 01:33.270 So for example we can here append the element 7 to the end of the list and let's print out our list 01:33.270 --> 01:37.670 again and there we have 4 7 at the end of the list. 01:37.960 --> 01:44.880 And as a general rule functions work for different objects so you can print out a list or an integer 01:44.880 --> 01:47.850 on float with the function print. 01:47.850 --> 01:53.320 And in contrast to that methods are functions specifically designed for particular objects. 01:53.320 --> 01:59.010 So for example here append the append method is a particular function for. 01:59.010 --> 02:02.000 For the list object. 02:02.270 --> 02:02.750 All right. 02:02.750 --> 02:08.360 And we already know more functions so we know the lengths function which calculates them the amount 02:08.360 --> 02:17.480 of elements in our list so we have five elements and we already know the main function which calculates 02:17.480 --> 02:20.150 that the minimum element of our list. 02:21.710 --> 02:24.800 So it s 2 and we also know the max function 02:29.400 --> 02:35.470 and we've already seen also the SUM function which takes them the sum over all elements in our list. 02:38.480 --> 02:44.200 Now we have seen that the output and functions like minimal maximum sum and one could think that now 02:44.270 --> 02:49.200 there should be also a mean function which calculates them the mean of our list. 02:49.200 --> 02:57.650 And if we try to apply that mean function to our list L and then Python to assess them that then I mean 02:57.650 --> 02:58.400 is not defined. 02:58.400 --> 03:06.100 So there is no need function and now we can create our own user defined mean function that calculates 03:06.100 --> 03:07.380 them the mean of. 03:07.440 --> 03:07.680 Mm. 03:07.680 --> 03:08.150 Yeah. 03:08.160 --> 03:14.140 List and before we start creating the function I mean we should first of all think about how to calculate 03:14.140 --> 03:16.600 the mean of a sequence of numbers and the. 03:16.630 --> 03:23.940 Actually we just have to take them the sum of them all elements divided by the amount of elements itself. 03:24.580 --> 03:27.290 So let's check what we get here. 03:27.520 --> 03:33.750 So we have here the sum is 23 and the length is five. 03:33.750 --> 03:39.040 So twenty three divided by five gives us four point six. 03:39.130 --> 03:46.000 And now what we can do we can yeah define our new function mean and yeah we have to start with the keyword 03:46.040 --> 03:47.180 death. 03:47.730 --> 03:51.430 Here you see it in green it's a keyword and then we say mean. 03:51.720 --> 03:59.820 So our function so to have the name mean and then within the brackets we define a parameter x so x could 03:59.910 --> 04:08.300 stand for a list or another type of sequence like a tuple or so on and then we have to make a call on 04:08.310 --> 04:14.100 and then if we press and uh we see that the python automatically creates here and then. 04:14.460 --> 04:18.470 So this is called the function header that we defined the function. 04:18.990 --> 04:26.160 And now here is the function body where we define what are the functions of actually calculate. 04:26.160 --> 04:35.220 And here we have to make an indent so the function should calculate the sum over our parameter x divided 04:35.220 --> 04:45.960 by the length of our parameter X then we can delete here the empty line and then we have to define what 04:45.960 --> 04:48.310 our function so to actually return. 04:48.310 --> 04:54.030 So here in the function body we define what the functions calculate and then in the end we should define 04:54.420 --> 04:56.430 what our function should return. 04:56.490 --> 05:02.080 So we use them the keyword return and the function of such return us them they mean. 05:02.100 --> 05:03.510 So now we can run the cell 05:06.230 --> 05:07.490 and here once again. 05:07.490 --> 05:14.690 So this is some of the same quote here if you compare it and then what we can do we can call the function 05:14.720 --> 05:17.930 bypassing the argument added to the function. 05:17.930 --> 05:23.450 So Ellis our list and the function expects an argument for the parameter x. 05:23.490 --> 05:26.190 So our argument is then the list L.. 05:26.960 --> 05:29.710 And let's see what we get here. 05:29.720 --> 05:38.000 So here we get four point six and we can also do this more explicitly so we can call the function by 05:38.000 --> 05:43.150 saying okay our parameter X should be our list ello. 05:43.490 --> 05:48.250 So this is actually the same as above Yeah. 05:48.310 --> 05:54.020 And our function should not only work for lists but have also for other sequences like tuple so we can 05:54.020 --> 06:02.920 define that tuple t with the elements for five six seven and then we should also be able to calculate 06:02.920 --> 06:07.640 them the mean of our total because if you look at the function body here. 06:07.640 --> 06:14.080 So we calculate the sum of a sequence divided by the length of a sequence and some in length. 06:14.120 --> 06:20.930 Functions should also work for tuples so our function our user defined function means would also work 06:20.930 --> 06:21.610 for us. 06:21.620 --> 06:29.590 Let's see if it works and here we are and what I also want to show you. 06:29.590 --> 06:36.020 So there's no need to define yeah a variety of mean we can directly say okay. 06:36.090 --> 06:44.130 Please return me the sum of our sequence x divided by the length of our sequence X so this would also 06:44.130 --> 06:44.420 work 06:48.510 --> 06:54.650 and let's run it again and there can see it so it also works. 06:54.810 --> 07:00.540 All right so let's move to another example and we assign the integer seven to our arrival a 07:03.470 --> 07:05.950 and then we can calculate a squared. 07:07.520 --> 07:11.140 So it s forty nine yeah. 07:11.180 --> 07:12.680 Then if you want to change a. 07:12.710 --> 07:21.050 So let's say we want to change a two A's then we can rerun the cells and get sixty far and whenever 07:21.050 --> 07:27.860 we want to reuse a code we can also define a function so we can define the function square with them 07:27.890 --> 07:34.130 the parameter X and the function so to return us them the square of our parameter X 07:37.990 --> 07:43.120 so let's try for and call the function by passing the argument for 07:46.070 --> 07:49.900 and the function gives the sixteen and the same as above. 07:49.900 --> 07:57.060 We can also make it more explicitly by calling the function saying that the parameter X should be for 07:59.560 --> 08:06.100 so this is the same but actually what we have to do we have to pass the an argument to our function. 08:06.100 --> 08:11.500 So if we just call our function with no argument we get an error message. 08:12.670 --> 08:19.460 So you can see is our function squares missing 1 require positional argument x. 08:19.810 --> 08:26.820 However what we can do we can set a default value to our paramedics so we can define our functions create 08:26.830 --> 08:38.510 two by saying that the default value of our parameter X should be to and the function so return x cared. 08:38.520 --> 08:45.680 So now if you call the function without any argument the function automatically applies. 08:45.690 --> 08:47.320 Our default value too. 08:47.550 --> 08:56.580 So now we would expect here therefore as a return of the function and correct of course the function 08:56.580 --> 08:58.500 can also be called the new argument. 08:58.500 --> 09:08.010 So for example for gives us 16 and same as above we can make this more explicitly by saying our parameter 09:08.040 --> 09:09.080 X is for. 09:11.250 --> 09:13.380 And this also works. 09:13.380 --> 09:13.810 All right. 09:13.830 --> 09:18.510 These were the basics of youth that defined functions and the next two sat sessions we are going a bit 09:18.510 --> 09:20.850 more into detail so hope to see you there by.