purrr (version 0.2.4)

lmap: Apply a function to list-elements of a list


lmap(), lmap_at() and lmap_if() are similar to map(), map_at() and map_if(), with the difference that they operate exclusively on functions that take and return a list (or data frame). Thus, instead of mapping the elements of a list (as in .x[[i]]), they apply a function .f to each subset of size 1 of that list (as in .x[i]). We call those those elements list-elements).


lmap(.x, .f, ...)

lmap_if(.x, .p, .f, ...)

lmap_at(.x, .at, .f, ...)



A list or data frame.


A function that takes and returns a list or data frame.


Additional arguments passed on to .f.


A single predicate function, a formula describing such a predicate function, or a logical vector of the same length as .x. Alternatively, if the elements of .x are themselves lists of objects, a string indicating the name of a logical element in the inner lists. Only those elements where .p evaluates to TRUE will be modified.


A character vector of names or a numeric vector of positions. Only those elements corresponding to .at will be modified.


If .x is a list, a list. If .x is a data frame, a data frame.


Mapping the list-elements .x[i] has several advantages. It makes it possible to work with functions that exclusively take a list or data frame. It enables .f to access the attributes of the encapsulating list, like the name of the components it receives. It also enables .f to return a larger list than the list-element of size 1 it got as input. Conversely, .f can also return empty lists. In these cases, the output list is reshaped with a different size than the input list .x.

See Also

Other map variants: imap, invoke, map2, map, modify


Run this code
# Let's write a function that returns a larger list or an empty list
# depending on some condition. This function also uses the names
# metadata available in the attributes of the list-element
maybe_rep <- function(x) {
  n <- rpois(1, 2)
  out <- rep_len(x, n)
  if (length(out) > 0) {
    names(out) <- paste0(names(x), seq_len(n))

# The output size varies each time we map f()
x <- list(a = 1:4, b = letters[5:7], c = 8:9, d = letters[10])
x %>% lmap(maybe_rep)

# We can apply f() on a selected subset of x
x %>% lmap_at(c("a", "d"), maybe_rep)

# Or only where a condition is satisfied
x %>% lmap_if(is.character, maybe_rep)

# A more realistic example would be a function that takes discrete
# variables in a dataset and turns them into disjunctive tables, a
# form that is amenable to fitting some types of models.

# A disjunctive table contains only 0 and 1 but has as many columns
# as unique values in the original variable. Ideally, we want to
# combine the names of each level with the name of the discrete
# variable in order to identify them. Given these requirements, it
# makes sense to have a function that takes a data frame of size 1
# and returns a data frame of variable size.
disjoin <- function(x, sep = "_") {
  name <- names(x)
  x <- as.factor(x[[1]])

  out <- lapply(levels(x), function(level) {
    as.numeric(x == level)

  names(out) <- paste(name, levels(x), sep = sep)

# Now, we are ready to map disjoin() on each categorical variable of a
# data frame:
iris %>% lmap_if(is.factor, disjoin)
mtcars %>% lmap_at(c("cyl", "vs", "am"), disjoin)
# }

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