WEBVTT 1 00:00:00.670 --> 00:00:02.250 Hi guys, and welcome back. 2 00:00:02.250 --> 00:00:05.094 In this video, we're going to learn about lambda functions. 3 00:00:05.094 --> 00:00:08.680 A lambda function is a different type of function 4 00:00:08.680 --> 00:00:10.368 which doesn't have a name, 5 00:00:10.368 --> 00:00:14.038 and is only used to return values. 6 00:00:14.038 --> 00:00:16.490 Remember that we mentioned that functions can be used 7 00:00:16.490 --> 00:00:19.231 to perform actions such as printing something out, 8 00:00:19.231 --> 00:00:22.540 or operating with values and returning them, 9 00:00:22.540 --> 00:00:26.030 such as the sum function, or they can do both. 10 00:00:26.030 --> 00:00:28.600 But lambda functions are exclusively used 11 00:00:28.600 --> 00:00:31.150 to operate on inputs and return outputs. 12 00:00:31.150 --> 00:00:34.253 They are almost never used to perform actions. 13 00:00:35.340 --> 00:00:38.550 Let's start with a simple function that gets some input 14 00:00:38.550 --> 00:00:40.110 and returns some output. 15 00:00:40.110 --> 00:00:42.940 Let's go to our trusty add function 16 00:00:42.940 --> 00:00:45.764 that returns x + y. 17 00:00:45.764 --> 00:00:49.260 If you print add of five and seven, 18 00:00:49.260 --> 00:00:52.110 you've got that add five and seven 19 00:00:52.110 --> 00:00:53.690 is going to run this line here, 20 00:00:53.690 --> 00:00:56.240 add x and y together, which gives you 12, 21 00:00:56.240 --> 00:00:57.410 and then it's gonna return it, 22 00:00:57.410 --> 00:01:00.470 replacing essentially this entire function call by 23 00:01:00.470 --> 00:01:03.150 the value 12, and then we're going to print that out. 24 00:01:03.150 --> 00:01:06.043 So, if we press play on this, you get 12. 25 00:01:07.350 --> 00:01:10.860 You can rewrite this function as a lambda function, 26 00:01:10.860 --> 00:01:12.480 which would be like this. 27 00:01:12.480 --> 00:01:17.480 Lambda x, y: x + y. 28 00:01:17.550 --> 00:01:19.500 So what we've got here, 29 00:01:19.500 --> 00:01:21.050 and ignore this underline for a second, 30 00:01:21.050 --> 00:01:23.670 what we've got here is the lambda keyword, 31 00:01:23.670 --> 00:01:25.930 then we've got a space, and then we've got 32 00:01:25.930 --> 00:01:30.410 the list of parameters, which in this case are x and y, 33 00:01:30.410 --> 00:01:32.730 then we've got a colon, and then we've got 34 00:01:32.730 --> 00:01:34.610 the return value of the function, 35 00:01:34.610 --> 00:01:37.260 but we don't specify the return keyword. 36 00:01:37.260 --> 00:01:39.590 So lambda functions are used to return output, 37 00:01:39.590 --> 00:01:42.050 so you don't need to specify return because 38 00:01:42.050 --> 00:01:44.872 that's what they're for, so Python will infer that. 39 00:01:44.872 --> 00:01:47.780 So, this does exactly the same thing as the add function 40 00:01:47.780 --> 00:01:49.710 earlier, but it doesn't have a name. 41 00:01:49.710 --> 00:01:52.910 Notice that the add variable is never created. 42 00:01:52.910 --> 00:01:57.060 So to solve that problem, if you want your lambda functions 43 00:01:57.060 --> 00:01:58.820 to have a name, which sometimes you won't 44 00:01:58.820 --> 00:02:00.550 and I'll show you why not in a moment, 45 00:02:00.550 --> 00:02:03.300 you can just assign that to a variable. 46 00:02:03.300 --> 00:02:05.010 So here we've got the add variable, 47 00:02:05.010 --> 00:02:07.150 make it equal to our lambda function, 48 00:02:07.150 --> 00:02:09.630 and then you can call it in your programme using add. 49 00:02:09.630 --> 00:02:11.600 So again, there are four parts to a lambda function. 50 00:02:11.600 --> 00:02:13.130 You've got the lambda keyword, 51 00:02:13.130 --> 00:02:14.730 then you've got your parameter list, 52 00:02:14.730 --> 00:02:16.060 then you've got your colon, 53 00:02:16.060 --> 00:02:20.530 and finally, your return value without the return keyword. 54 00:02:20.530 --> 00:02:22.950 Lambdas are meant to be short functions, 55 00:02:22.950 --> 00:02:25.830 often used without giving them a name, 56 00:02:25.830 --> 00:02:27.210 for example in conjunction with the 57 00:02:27.210 --> 00:02:29.110 built in function called map. 58 00:02:29.110 --> 00:02:31.568 So we're going to learn about map in a moment. 59 00:02:31.568 --> 00:02:34.960 However, you can give them a name if you want. 60 00:02:34.960 --> 00:02:38.750 If you didn't want to give it a name, then you would have 61 00:02:38.750 --> 00:02:42.500 to use it in the same line where you define it. 62 00:02:42.500 --> 00:02:44.400 Otherwise as soon as you define this function, 63 00:02:44.400 --> 00:02:46.070 you have no way of actually using it, 64 00:02:46.070 --> 00:02:47.310 because it doesn't have a name, so it's 65 00:02:47.310 --> 00:02:48.920 kind of pointless to just define 66 00:02:48.920 --> 00:02:50.915 the lambda function without using it. 67 00:02:50.915 --> 00:02:53.868 If you wanted to use this lambda function, 68 00:02:53.868 --> 00:02:57.180 you need to surround it in brackets 69 00:02:57.180 --> 00:02:59.731 so that Python will treat this as single unit, 70 00:02:59.731 --> 00:03:02.810 and then call the function here, say with 71 00:03:02.810 --> 00:03:04.850 the values five and seven, 72 00:03:04.850 --> 00:03:07.429 and this here means that you are using 73 00:03:07.429 --> 00:03:11.060 the five and seven call on this thing, 74 00:03:11.060 --> 00:03:13.270 which is a lambda function, which essentially 75 00:03:13.270 --> 00:03:16.210 calls the function with the arguments five and seven. 76 00:03:16.210 --> 00:03:19.020 You can see why this is also not very often done, 77 00:03:19.020 --> 00:03:22.240 because it's extremely confusing, and this will still 78 00:03:22.240 --> 00:03:23.900 not do anything, 'cause you still need 79 00:03:23.900 --> 00:03:26.290 your print function around it as well. 80 00:03:26.290 --> 00:03:28.240 So lambda functions used in this way, 81 00:03:28.240 --> 00:03:30.030 not very common at all. 82 00:03:30.030 --> 00:03:32.330 Usually you will define a proper function 83 00:03:32.330 --> 00:03:34.860 and use it like we did earlier on instead. 84 00:03:34.860 --> 00:03:37.880 But let's get into some uses for lambda functions 85 00:03:37.880 --> 00:03:39.305 that are actually useful. 86 00:03:39.305 --> 00:03:42.260 Let's say you've got a function called double 87 00:03:42.260 --> 00:03:44.280 that doubles a number passed into it, 88 00:03:44.280 --> 00:03:46.700 and then you've got a sequence of numbers, 89 00:03:46.700 --> 00:03:49.040 such as one, three, five, and nine. 90 00:03:49.040 --> 00:03:51.870 And you want to double this sequence, or you want 91 00:03:51.870 --> 00:03:54.320 to double each number in this sequence. 92 00:03:54.320 --> 00:03:57.110 Something that you may see is something like doubled equal, 93 00:03:57.110 --> 00:04:02.110 and then use a list comprehension, x * 2 for x in sequence. 94 00:04:02.955 --> 00:04:07.910 So x * 2 is the same as this function returns 95 00:04:07.910 --> 00:04:11.160 for its given input, so instead we can do 96 00:04:11.160 --> 00:04:13.900 double x for x in sequence. 97 00:04:13.900 --> 00:04:17.500 The benefit of this is that using a function call 98 00:04:17.500 --> 00:04:20.210 allows you to potentially perform larger 99 00:04:20.210 --> 00:04:22.310 or more complicated mathematical expressions 100 00:04:22.310 --> 00:04:25.320 without making your list comprehension more complicated. 101 00:04:25.320 --> 00:04:29.150 This specific point in time it's not a big difference 102 00:04:29.150 --> 00:04:31.934 but it can be in other scenarios. 103 00:04:31.934 --> 00:04:34.980 So instead of doing this list comprehension, 104 00:04:34.980 --> 00:04:36.350 you can use the map function. 105 00:04:36.350 --> 00:04:41.350 And that is where lambda functions become extremely useful. 106 00:04:41.890 --> 00:04:43.730 So instead of using that, you can do 107 00:04:43.730 --> 00:04:45.330 doubled = map(double, sequence). 108 00:04:48.020 --> 00:04:51.140 And that does exactly the same as this list comprehension. 109 00:04:51.140 --> 00:04:54.960 What map does is it goes over each value in the sequence 110 00:04:54.960 --> 00:04:57.820 such as a list, a double, a set, and so on, 111 00:04:57.820 --> 00:05:01.350 and it applies the double function on each element, 112 00:05:01.350 --> 00:05:04.210 and then returns the final sequence out to you. 113 00:05:04.210 --> 00:05:05.360 So that's what map is for, 114 00:05:05.360 --> 00:05:07.770 it's very useful, so it is the counterpart 115 00:05:07.770 --> 00:05:10.240 for this list comprehension here. 116 00:05:10.240 --> 00:05:13.443 In Python, you would usually use list comprehensions, 117 00:05:13.443 --> 00:05:16.780 but many other programming languages don't have 118 00:05:16.780 --> 00:05:19.956 list comprehensions, so they have to use things like map. 119 00:05:19.956 --> 00:05:23.980 So, if you are working with developers familiar 120 00:05:23.980 --> 00:05:26.150 with other programming languages, and you want 121 00:05:26.150 --> 00:05:30.060 to use a programming style that they will be familiar with, 122 00:05:30.060 --> 00:05:31.570 you can use something like map. 123 00:05:31.570 --> 00:05:33.890 In addition, if you're working in a large project 124 00:05:33.890 --> 00:05:36.530 and you are using multiple different programming languages, 125 00:05:36.530 --> 00:05:39.368 such as JavaScript as well as Python, 126 00:05:39.368 --> 00:05:42.730 you may want to use map so that your Python code 127 00:05:42.730 --> 00:05:46.090 and your JavaScript code both have similar style. 128 00:05:46.090 --> 00:05:49.020 With that said though, the list comprehension 129 00:05:49.020 --> 00:05:53.300 is usually slightly faster, so there is 130 00:05:53.300 --> 00:05:56.190 no reason to use things like map 131 00:05:56.190 --> 00:06:00.373 over list comprehensions most of the time in Python. 132 00:06:00.373 --> 00:06:03.897 Let's re-write this example using a lambda function. 133 00:06:03.897 --> 00:06:08.440 Your doubled list comprehension here could define 134 00:06:08.440 --> 00:06:12.244 a lambda function here, lambda x: x * 2, 135 00:06:12.244 --> 00:06:16.636 and then use x as the argument to that function, 136 00:06:16.636 --> 00:06:19.300 and then you no longer need this doubled there, 137 00:06:19.300 --> 00:06:22.020 but this is getting quite confusing, 138 00:06:22.020 --> 00:06:24.330 because as I mentioned earlier on, you have your 139 00:06:24.330 --> 00:06:26.880 lambda function, which must be surrounded by brackets, 140 00:06:26.880 --> 00:06:29.070 and then you have another pair of brackets 141 00:06:29.070 --> 00:06:32.260 to pass in the argument, so this is a little bit unreadable. 142 00:06:32.260 --> 00:06:34.320 So if you want to use lambda functions 143 00:06:34.320 --> 00:06:38.320 you should use map instead, just because 144 00:06:38.320 --> 00:06:41.360 it makes it a little bit clearer without so many brackets 145 00:06:41.360 --> 00:06:42.445 what's going on. 146 00:06:42.445 --> 00:06:45.020 By the way, if you want to run this code 147 00:06:45.020 --> 00:06:47.350 and check the value returned by map, 148 00:06:47.350 --> 00:06:50.260 you need to turn it into a list in order to be able 149 00:06:50.260 --> 00:06:52.460 to print it out, just because the map function 150 00:06:52.460 --> 00:06:54.760 doesn't return a list of numbers, 151 00:06:54.760 --> 00:06:57.520 it will return something called a map object. 152 00:06:57.520 --> 00:06:59.430 We're going to learn more about what those things are 153 00:06:59.430 --> 00:07:01.020 as we move through these videos, 154 00:07:01.020 --> 00:07:02.630 but for now just turn it into a list 155 00:07:02.630 --> 00:07:05.470 if you want to see the values in your console. 156 00:07:05.470 --> 00:07:09.240 So just a recap, lambdas are functions without a name. 157 00:07:09.240 --> 00:07:11.498 You can give them a name by assigning them to a variable, 158 00:07:11.498 --> 00:07:15.010 but they don't have a name in the function definition. 159 00:07:15.010 --> 00:07:17.530 They are used to return a value calculated 160 00:07:17.530 --> 00:07:19.280 from its parameters, and they're almost 161 00:07:19.280 --> 00:07:21.366 never used to perform actions. 162 00:07:21.366 --> 00:07:24.810 They're almost always single line, so you don't want 163 00:07:24.810 --> 00:07:27.120 to be doing anything really complicated in them, 164 00:07:27.120 --> 00:07:29.260 and one of the big reasons why they're single line 165 00:07:29.260 --> 00:07:32.570 is because their syntax gets really complicated 166 00:07:32.570 --> 00:07:34.859 to read as they get longer. 167 00:07:34.859 --> 00:07:38.570 Very often it's better to define a normal function 168 00:07:38.570 --> 00:07:41.130 than a lambda function, in most situations. 169 00:07:41.130 --> 00:07:43.870 Lambda functions, I personally don't use them very much, 170 00:07:43.870 --> 00:07:46.100 because I find them that they are a bit difficult to read 171 00:07:46.100 --> 00:07:48.926 and keep track of, especially as your programmes grow. 172 00:07:48.926 --> 00:07:51.460 And that's it for this video, thanks for joining me, 173 00:07:51.460 --> 00:07:53.053 and I'll see you in the next one.