mapply is a loop function that tries, is a multivariate version of the kind of lapply and sapply functions that you've seen previously. And the idea is that it applies a function in parallel over a set of different arguments. So one thing we have noticed about the previous functions, lapply, sapply, tapply, is that, they have only, they only apply a function over the elements of a single object. So, for example, if you think about lapply, the input to lapply was a list. A single list, that is. And then the function was applied over the elements of that list. So, what happens if you have two lists that you want to apply a function over? And so, and suppose you have two lists, and the elements of the first list go into one argument of the function, and the elements of the second list go into another argument of the function. So lapply and sapply can't really be used for that purpose. So one way to do that is just to write a for loop, where the for loop will index the elements of each of the different lists, and then you can pass a function to each of those elements of the list. Another way to do that though is with mapply, where mapply can take multiple list arguments and then apply a function to the, to the elements of those, of the multiple lists, in parallel. So the function arguments for mapply are a little bit different, just because it has to allow for the possibility of a variable number of arguments. So here, the first argument to mapply is the function that you want to apply. And the function that you're going to pass to mapply has to have, the number of arguments that the function takes has to be at least as many as the number of lists that you're going to pass to mapply. So the list that, the things that will be coerced to a list, will be passed through the dot dot dot argument. And so if you have three lists, you'll pass three objects and then your function has to take at least three arguments to it. So, the more args, argument is just if you have more arguments and you need to pass to your function. And a simplified argument is similar to the simplify arguments that you saw in sapply and also in in tapply. So, here, for example, I'm creating a list here, and the list has, I'm going to repeat 1 four times, the integer 1 four times, I'm going to repeat 2 three times, I'm going to repeat 3 two times, and repeat 4 just once. So it's a little bit tedious to have to type something to do, even though this is a fairly artificial example, but with mapply, it's actually quite simple. I can just do mapply rep, so, rep is the repeat function And then, repeat, it has two arguments, so, the first set of arguments is going to be 1 through 4, and the second set of arguments is going to be 4 through 1, and you can see that in list above here. The first argument was 1, 2, 3, and 4, and the second argument was 4, 3, 2, and 1. So, that's, so those are the two sets of arguments that I'm going to pass to mapply. And you can see that when I do that, I get my list of four 1s, three 2s, two 3s and one 4, just like the list that I have above here. So mapply is, can be used to apply a function to multiple sets of arguments. So, here's just another very simple function. It just generates some random normal noise. And these, and, see, the rnorm, the, the, I'm sorry, the function has three arguments the, the number of observations the value of the mean, and the value of the standard deviation. So, if I just apply noise to, with a single set of arguments, 5, 1 and 2. I get 5 random normal variables with the mean 1 and standard variation 2. However, this function doesn't work really correctly if I pass it a vector of arguments. So, now what's happening is, I get a vector of 5, here. When I pass it 1 through 5 and 1 through 5. But, really what I want to happen is to have one, one random normal with mean 1, two random normals with mean 2, three random normals with mean 3, etc, and then five random normals with mean 5. And, so that's what I get here, when I use the mapply function onto the and if I vectorize this noise function I give it, you know, three sets of arguments, so it's 1 through 5, 1 through 5, and then 2. So I, I'm always going to fix the standard deviation to be 2, but I'm going to be changing the n and I'm going to be changing the mean. So now I've got one random variable with a mean 1, I got two with mean 2, three with mean 3, four with mean 4 and then five with mean 5. So that's how I can instantly vectorize a function that doesn't allow for vector arguments. So this is the same as, as I were to manually type out a list with these five different function calls. So this way, it's, it's nice to be able to instantly, kind of, create a function that doesn't allow for vector inputs and to and to kind of instantly vectorize it.
