Using `lapply`, `sapply` and `vapply` in r

Example 1: a list contains one vector

x1 <- list(c(1, 2, 3))
lapply_x1 <- lapply(x1, median)
sapply_x1 <- sapply(x1, median)
vapply_x1 <- vapply(x1, median, numeric(1))

lapply_x1 is 2 in list class
sapply_x1 is 2 in numeric class
vapply_x1 is 2 in numeric class

Example 2: a list contains two vectors with same length

x2 <- list(c(1, 2, 3), c(4, 5, 6))
lapply_x2 <- lapply(x2, median)
sapply_x2 <- sapply(x2, median)
vapply_x2 <- vapply(x2, median, numeric(1))

lapply_x2 in list class
[[1]]
[1] 2

[[2]]
[1] 5

sapply_x2 in numeric class
[1] 2 5

vapply_x2 in numeric class
[1] 2 5

Example 3: a list contains two vectors with different and same length

x3 <- list(c(1, 2, 3), c(4, 5, 6, 7))
square_function <- function(x) x^2
lapply_x3 <- lapply(x3, square_function)
sapply_x3 <- sapply(x3, square_function)

lapply_x3 in list class
[[1]]
[1] 1 4 9

[[2]]
[1] 16 25 36 49

sapply_x3 in list class
[[1]]
[1] 1 4 9

[[2]]
[1] 16 25 36 49

vapply(x3, square_function, numeric(3))

Error in vapply(x3, square_function, numeric(3)): values must be length 3,
 but FUN(X[[2]]) result is length 4

x3_same_length <- list(c(1, 2, 3), c(4, 5, 6))
vapply_x3_same_length <- vapply(x3_same_length, square_function, numeric(3))

class(vapply_x3_same_length)
[1] "matrix" "array" 
vapply_x3_same_length
     [,1] [,2]
[1,]    1   16
[2,]    4   25
[3,]    9   36

Summary for lapply, sapply and vapply

	lapply	sapply	vapply
Return	list	vector / matrix / array / list	pre-specified type

References:

RDocumentation lapply: Apply a Function over a List or Vector