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An older post on this blog talked about several alternative base apply functions. This post will talk about how to apply a function across multiple vectors or lists with **Map** and **mapply** in R. These functions are generalizations of **sapply** and **lapply**, which allow you to more easily loop over multiple vectors or lists simultaneously.

**Map**

Suppose we have two lists of vectors and we want to divide the n^{th} vector in one list by the n^{th} vector in the second list. **Map** makes this straightforward to accomplish, while keeping the code clean to read. **Map** returns a list by default, similar to **lapply**.

Below, we create two sample lists of vectors.

values1 <- list(a = c(1, 2, 3), b = c(4, 5, 6), c = c(7, 8, 9)) values2 <- list(a = c(10, 11, 12), b = c(13, 14, 15), c = c(16, 17, 18))

Now, let’s do the operation we described above using **Map**. Here, we’ll input the function as the first parameter. In this case, the function takes two numeric values as input and divides the first value by the second. The remaining inputs to **Map** are the names of the lists we are looping over.

Map(function(num1, num2) num1 / num2, values1, values2)

*num1* refers to each individual element in the iteration over *values1*, while *num2* refers to each individual element in the iteration over *values2*. Each element in each list is a vector.

Below is another example. Here, we loop over our two lists of vectors, and get the pairwise union of the vectors across the lists.

Map(function(num1, num2) union(num1, num2), values1, values2)

**mapply**

**mapply**, similar to **sapply**, tries to return a vector result when possible. Like **Map**, one difference between **mapply** and **sapply** or **lapply** is that the function to be applied is input as the first parameter.

Let’s suppose we again have our two lists of vectors, but this time we want to get the maximum value across two pairwise vectors for each pair of vectors in the lists.

mapply(function(num1, num2) max(c(num1, num2)), values1, values2)

Here, **mapply** loops over each of the lists simultaneously. For the n^{th} vector in each list, **mapply** combines the two vectors and finds the maximum value.

**Map** is actually a wrapper around **mapply**, with the parameter SIMPLIFY set to FALSE. Setting this parameter to TRUE (which is default) means (as mentioned above) **mapply** will try to simplify the result to a vector if possible. Each of these functions can also be useful in iterating over lists of data frames.

That’s it for this post. Please click here to follow my blog on Twitter!

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