1 1 00:00:00,670 --> 00:00:02,250 Hi guys, and welcome back. 2 2 00:00:02,250 --> 00:00:05,094 In this video, we're going to learn about lambda functions. 3 3 00:00:05,094 --> 00:00:08,680 A lambda function is a different type of function 4 4 00:00:08,680 --> 00:00:10,368 which doesn't have a name, 5 5 00:00:10,368 --> 00:00:14,038 and is only used to return values. 6 6 00:00:14,038 --> 00:00:16,490 Remember that we mentioned that functions can be used 7 7 00:00:16,490 --> 00:00:19,231 to perform actions such as printing something out, 8 8 00:00:19,231 --> 00:00:22,540 or operating with values and returning them, 9 9 00:00:22,540 --> 00:00:26,030 such as the sum function, or they can do both. 10 10 00:00:26,030 --> 00:00:28,600 But lambda functions are exclusively used 11 11 00:00:28,600 --> 00:00:31,150 to operate on inputs and return outputs. 12 12 00:00:31,150 --> 00:00:34,253 They are almost never used to perform actions. 13 13 00:00:35,340 --> 00:00:38,550 Let's start with a simple function that gets some input 14 14 00:00:38,550 --> 00:00:40,110 and returns some output. 15 15 00:00:40,110 --> 00:00:42,940 Let's go to our trusty add function 16 16 00:00:42,940 --> 00:00:45,764 that returns x + y. 17 17 00:00:45,764 --> 00:00:49,260 If you print add of five and seven, 18 18 00:00:49,260 --> 00:00:52,110 you've got that add five and seven 19 19 00:00:52,110 --> 00:00:53,690 is going to run this line here, 20 20 00:00:53,690 --> 00:00:56,240 add x and y together, which gives you 12, 21 21 00:00:56,240 --> 00:00:57,410 and then it's gonna return it, 22 22 00:00:57,410 --> 00:01:00,470 replacing essentially this entire function call by 23 23 00:01:00,470 --> 00:01:03,150 the value 12, and then we're going to print that out. 24 24 00:01:03,150 --> 00:01:06,043 So, if we press play on this, you get 12. 25 25 00:01:07,350 --> 00:01:10,860 You can rewrite this function as a lambda function, 26 26 00:01:10,860 --> 00:01:12,480 which would be like this. 27 27 00:01:12,480 --> 00:01:17,480 Lambda x, y: x + y. 28 28 00:01:17,550 --> 00:01:19,500 So what we've got here, 29 29 00:01:19,500 --> 00:01:21,050 and ignore this underline for a second, 30 30 00:01:21,050 --> 00:01:23,670 what we've got here is the lambda keyword, 31 31 00:01:23,670 --> 00:01:25,930 then we've got a space, and then we've got 32 32 00:01:25,930 --> 00:01:30,410 the list of parameters, which in this case are x and y, 33 33 00:01:30,410 --> 00:01:32,730 then we've got a colon, and then we've got 34 34 00:01:32,730 --> 00:01:34,610 the return value of the function, 35 35 00:01:34,610 --> 00:01:37,260 but we don't specify the return keyword. 36 36 00:01:37,260 --> 00:01:39,590 So lambda functions are used to return output, 37 37 00:01:39,590 --> 00:01:42,050 so you don't need to specify return because 38 38 00:01:42,050 --> 00:01:44,872 that's what they're for, so Python will infer that. 39 39 00:01:44,872 --> 00:01:47,780 So, this does exactly the same thing as the add function 40 40 00:01:47,780 --> 00:01:49,710 earlier, but it doesn't have a name. 41 41 00:01:49,710 --> 00:01:52,910 Notice that the add variable is never created. 42 42 00:01:52,910 --> 00:01:57,060 So to solve that problem, if you want your lambda functions 43 43 00:01:57,060 --> 00:01:58,820 to have a name, which sometimes you won't 44 44 00:01:58,820 --> 00:02:00,550 and I'll show you why not in a moment, 45 45 00:02:00,550 --> 00:02:03,300 you can just assign that to a variable. 46 46 00:02:03,300 --> 00:02:05,010 So here we've got the add variable, 47 47 00:02:05,010 --> 00:02:07,150 make it equal to our lambda function, 48 48 00:02:07,150 --> 00:02:09,630 and then you can call it in your programme using add. 49 49 00:02:09,630 --> 00:02:11,600 So again, there are four parts to a lambda function. 50 50 00:02:11,600 --> 00:02:13,130 You've got the lambda keyword, 51 51 00:02:13,130 --> 00:02:14,730 then you've got your parameter list, 52 52 00:02:14,730 --> 00:02:16,060 then you've got your colon, 53 53 00:02:16,060 --> 00:02:20,530 and finally, your return value without the return keyword. 54 54 00:02:20,530 --> 00:02:22,950 Lambdas are meant to be short functions, 55 55 00:02:22,950 --> 00:02:25,830 often used without giving them a name, 56 56 00:02:25,830 --> 00:02:27,210 for example in conjunction with the 57 57 00:02:27,210 --> 00:02:29,110 built in function called map. 58 58 00:02:29,110 --> 00:02:31,568 So we're going to learn about map in a moment. 59 59 00:02:31,568 --> 00:02:34,960 However, you can give them a name if you want. 60 60 00:02:34,960 --> 00:02:38,750 If you didn't want to give it a name, then you would have 61 61 00:02:38,750 --> 00:02:42,500 to use it in the same line where you define it. 62 62 00:02:42,500 --> 00:02:44,400 Otherwise as soon as you define this function, 63 63 00:02:44,400 --> 00:02:46,070 you have no way of actually using it, 64 64 00:02:46,070 --> 00:02:47,310 because it doesn't have a name, so it's 65 65 00:02:47,310 --> 00:02:48,920 kind of pointless to just define 66 66 00:02:48,920 --> 00:02:50,915 the lambda function without using it. 67 67 00:02:50,915 --> 00:02:53,868 If you wanted to use this lambda function, 68 68 00:02:53,868 --> 00:02:57,180 you need to surround it in brackets 69 69 00:02:57,180 --> 00:02:59,731 so that Python will treat this as single unit, 70 70 00:02:59,731 --> 00:03:02,810 and then call the function here, say with 71 71 00:03:02,810 --> 00:03:04,850 the values five and seven, 72 72 00:03:04,850 --> 00:03:07,429 and this here means that you are using 73 73 00:03:07,429 --> 00:03:11,060 the five and seven call on this thing, 74 74 00:03:11,060 --> 00:03:13,270 which is a lambda function, which essentially 75 75 00:03:13,270 --> 00:03:16,210 calls the function with the arguments five and seven. 76 76 00:03:16,210 --> 00:03:19,020 You can see why this is also not very often done, 77 77 00:03:19,020 --> 00:03:22,240 because it's extremely confusing, and this will still 78 78 00:03:22,240 --> 00:03:23,900 not do anything, 'cause you still need 79 79 00:03:23,900 --> 00:03:26,290 your print function around it as well. 80 80 00:03:26,290 --> 00:03:28,240 So lambda functions used in this way, 81 81 00:03:28,240 --> 00:03:30,030 not very common at all. 82 82 00:03:30,030 --> 00:03:32,330 Usually you will define a proper function 83 83 00:03:32,330 --> 00:03:34,860 and use it like we did earlier on instead. 84 84 00:03:34,860 --> 00:03:37,880 But let's get into some uses for lambda functions 85 85 00:03:37,880 --> 00:03:39,305 that are actually useful. 86 86 00:03:39,305 --> 00:03:42,260 Let's say you've got a function called double 87 87 00:03:42,260 --> 00:03:44,280 that doubles a number passed into it, 88 88 00:03:44,280 --> 00:03:46,700 and then you've got a sequence of numbers, 89 89 00:03:46,700 --> 00:03:49,040 such as one, three, five, and nine. 90 90 00:03:49,040 --> 00:03:51,870 And you want to double this sequence, or you want 91 91 00:03:51,870 --> 00:03:54,320 to double each number in this sequence. 92 92 00:03:54,320 --> 00:03:57,110 Something that you may see is something like doubled equal, 93 93 00:03:57,110 --> 00:04:02,110 and then use a list comprehension, x * 2 for x in sequence. 94 94 00:04:02,955 --> 00:04:07,910 So x * 2 is the same as this function returns 95 95 00:04:07,910 --> 00:04:11,160 for its given input, so instead we can do 96 96 00:04:11,160 --> 00:04:13,900 double x for x in sequence. 97 97 00:04:13,900 --> 00:04:17,500 The benefit of this is that using a function call 98 98 00:04:17,500 --> 00:04:20,210 allows you to potentially perform larger 99 99 00:04:20,210 --> 00:04:22,310 or more complicated mathematical expressions 100 100 00:04:22,310 --> 00:04:25,320 without making your list comprehension more complicated. 101 101 00:04:25,320 --> 00:04:29,150 This specific point in time it's not a big difference 102 102 00:04:29,150 --> 00:04:31,934 but it can be in other scenarios. 103 103 00:04:31,934 --> 00:04:34,980 So instead of doing this list comprehension, 104 104 00:04:34,980 --> 00:04:36,350 you can use the map function. 105 105 00:04:36,350 --> 00:04:41,350 And that is where lambda functions become extremely useful. 106 106 00:04:41,890 --> 00:04:43,730 So instead of using that, you can do 107 107 00:04:43,730 --> 00:04:45,330 doubled = map(double, sequence). 108 108 00:04:48,020 --> 00:04:51,140 And that does exactly the same as this list comprehension. 109 109 00:04:51,140 --> 00:04:54,960 What map does is it goes over each value in the sequence 110 110 00:04:54,960 --> 00:04:57,820 such as a list, a double, a set, and so on, 111 111 00:04:57,820 --> 00:05:01,350 and it applies the double function on each element, 112 112 00:05:01,350 --> 00:05:04,210 and then returns the final sequence out to you. 113 113 00:05:04,210 --> 00:05:05,360 So that's what map is for, 114 114 00:05:05,360 --> 00:05:07,770 it's very useful, so it is the counterpart 115 115 00:05:07,770 --> 00:05:10,240 for this list comprehension here. 116 116 00:05:10,240 --> 00:05:13,443 In Python, you would usually use list comprehensions, 117 117 00:05:13,443 --> 00:05:16,780 but many other programming languages don't have 118 118 00:05:16,780 --> 00:05:19,956 list comprehensions, so they have to use things like map. 119 119 00:05:19,956 --> 00:05:23,980 So, if you are working with developers familiar 120 120 00:05:23,980 --> 00:05:26,150 with other programming languages, and you want 121 121 00:05:26,150 --> 00:05:30,060 to use a programming style that they will be familiar with, 122 122 00:05:30,060 --> 00:05:31,570 you can use something like map. 123 123 00:05:31,570 --> 00:05:33,890 In addition, if you're working in a large project 124 124 00:05:33,890 --> 00:05:36,530 and you are using multiple different programming languages, 125 125 00:05:36,530 --> 00:05:39,368 such as JavaScript as well as Python, 126 126 00:05:39,368 --> 00:05:42,730 you may want to use map so that your Python code 127 127 00:05:42,730 --> 00:05:46,090 and your JavaScript code both have similar style. 128 128 00:05:46,090 --> 00:05:49,020 With that said though, the list comprehension 129 129 00:05:49,020 --> 00:05:53,300 is usually slightly faster, so there is 130 130 00:05:53,300 --> 00:05:56,190 no reason to use things like map 131 131 00:05:56,190 --> 00:06:00,373 over list comprehensions most of the time in Python. 132 132 00:06:00,373 --> 00:06:03,897 Let's re-write this example using a lambda function. 133 133 00:06:03,897 --> 00:06:08,440 Your doubled list comprehension here could define 134 134 00:06:08,440 --> 00:06:12,244 a lambda function here, lambda x: x * 2, 135 135 00:06:12,244 --> 00:06:16,636 and then use x as the argument to that function, 136 136 00:06:16,636 --> 00:06:19,300 and then you no longer need this doubled there, 137 137 00:06:19,300 --> 00:06:22,020 but this is getting quite confusing, 138 138 00:06:22,020 --> 00:06:24,330 because as I mentioned earlier on, you have your 139 139 00:06:24,330 --> 00:06:26,880 lambda function, which must be surrounded by brackets, 140 140 00:06:26,880 --> 00:06:29,070 and then you have another pair of brackets 141 141 00:06:29,070 --> 00:06:32,260 to pass in the argument, so this is a little bit unreadable. 142 142 00:06:32,260 --> 00:06:34,320 So if you want to use lambda functions 143 143 00:06:34,320 --> 00:06:38,320 you should use map instead, just because 144 144 00:06:38,320 --> 00:06:41,360 it makes it a little bit clearer without so many brackets 145 145 00:06:41,360 --> 00:06:42,445 what's going on. 146 146 00:06:42,445 --> 00:06:45,020 By the way, if you want to run this code 147 147 00:06:45,020 --> 00:06:47,350 and check the value returned by map, 148 148 00:06:47,350 --> 00:06:50,260 you need to turn it into a list in order to be able 149 149 00:06:50,260 --> 00:06:52,460 to print it out, just because the map function 150 150 00:06:52,460 --> 00:06:54,760 doesn't return a list of numbers, 151 151 00:06:54,760 --> 00:06:57,520 it will return something called a map object. 152 152 00:06:57,520 --> 00:06:59,430 We're going to learn more about what those things are 153 153 00:06:59,430 --> 00:07:01,020 as we move through these videos, 154 154 00:07:01,020 --> 00:07:02,630 but for now just turn it into a list 155 155 00:07:02,630 --> 00:07:05,470 if you want to see the values in your console. 156 156 00:07:05,470 --> 00:07:09,240 So just a recap, lambdas are functions without a name. 157 157 00:07:09,240 --> 00:07:11,498 You can give them a name by assigning them to a variable, 158 158 00:07:11,498 --> 00:07:15,010 but they don't have a name in the function definition. 159 159 00:07:15,010 --> 00:07:17,530 They are used to return a value calculated 160 160 00:07:17,530 --> 00:07:19,280 from its parameters, and they're almost 161 161 00:07:19,280 --> 00:07:21,366 never used to perform actions. 162 162 00:07:21,366 --> 00:07:24,810 They're almost always single line, so you don't want 163 163 00:07:24,810 --> 00:07:27,120 to be doing anything really complicated in them, 164 164 00:07:27,120 --> 00:07:29,260 and one of the big reasons why they're single line 165 165 00:07:29,260 --> 00:07:32,570 is because their syntax gets really complicated 166 166 00:07:32,570 --> 00:07:34,859 to read as they get longer. 167 167 00:07:34,859 --> 00:07:38,570 Very often it's better to define a normal function 168 168 00:07:38,570 --> 00:07:41,130 than a lambda function, in most situations. 169 169 00:07:41,130 --> 00:07:43,870 Lambda functions, I personally don't use them very much, 170 170 00:07:43,870 --> 00:07:46,100 because I find them that they are a bit difficult to read 171 171 00:07:46,100 --> 00:07:48,926 and keep track of, especially as your programmes grow. 172 172 00:07:48,926 --> 00:07:51,460 And that's it for this video, thanks for joining me, 173 173 00:07:51,460 --> 00:07:53,053 and I'll see you in the next one.