R lapply
lapply() runs a function on every item in a list or vector. It always gives the results back as a list.
lapply over a list
mean runs on each item of the list. Each result prints under its list name.
scores <- list(a = c(10, 20), b = c(30, 40, 50))
print(lapply(scores, mean))lapply over a vector
The input here is a plain vector. The result still prints as a list with numbered items.
print(lapply(c(1, 2, 3), function(x) x * 10))lapply with a written function
add_ten is a function written for this job. lapply runs it on each item of nums.
add_ten <- function(x) {
x + 10
}
nums <- list(first = c(1, 2), second = c(3, 4))
print(lapply(nums, add_ten))Single results stay in a list
length gives one number for each item. The two numbers still print inside a list.
scores <- list(a = c(10, 20), b = c(30, 40, 50))
print(lapply(scores, length))Example
teams <- list(red = c(5, 10), blue = c(20, 30))
totals <- lapply(teams, sum)
print(totals)
print(totals$red)Key points
- The first value in an lapply() call is the data.
- The second value is the function to run on each item.
- The result holds one item for each input item.
- Names on the input list carry over to the result list.
- The result is always a list, even when the input is a vector.
- A function written for the job can be passed by its name.
In this example lapply adds up each team vector and returns the totals as a list. The last line picks the red total out of the list with $.
Try it Yourself
Edit the code, then press Run.
Running is simulated · rdrr.io R Snippets
Exercise
What does lapply() always return?