Using the apply family of functions in R

September 12, 2015

(This article was first published on DataScience+, and kindly contributed to R-bloggers)

In this article, I will demonstrate how to use the apply family of functions in R. They are extremely helpful, as you will see.


apply can be used to apply a function to a matrix.
For example, let’s create a sample dataset:

data <- matrix(c(1:10, 21:30), nrow = 5, ncol = 4)

     [,1] [,2] [,3] [,4]
[1,]    1    6   21   26
[2,]    2    7   22   27
[3,]    3    8   23   28
[4,]    4    9   24   29
[5,]    5   10   25   30

Now we can use the apply function to find the mean of each row as follows:

apply(data, 1, mean)
13.5 14.5 15.5 16.5 17.5

The second parameter is the dimension. 1 signifies rows and 2 signifies columns. If you want both, you can use c(1, 2).


lapply is similar to apply, but it takes a list as an input, and returns a list as the output.
Let’s create a list:

data <- list(x = 1:5, y = 6:10, z = 11:15)

1 2 3 4 5
6  7  8  9 10
11 12 13 14 15

Now, we can use lapply to apply a function to each element in the list. For example:

lapply(data, FUN = median)

[1] 3
[1] 8
[1] 13


sapply is the same as lapply, but returns a vector instead of a list.

sapply(data, FUN = median)

x  y  z 
3  8 13


tapply splits the array based on specified data, usually factor levels and then applies the function to it.
For example, in the mtcars dataset:

tapply(mtcars$wt, mtcars$cyl, mean)

       4        6        8 
2.285727 3.117143 3.999214 

The tapply function first groups the cars together based on the number of cylinders they have, and then calculates the mean weight for each group.


mapply is a multivariate version of sapply. It will apply the specified function to the first element of each argument first, followed by the second element, and so on. For example:

x <- 1:5
b <- 6:10
mapply(sum, x, b)

7  9 11 13 15

It adds 1 with 6, 2 with 7, and so on.

Let me know if you have any questions by leaving a comment below or reaching out to me on Twitter.

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