# 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').

##### Usage

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

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

##### Arguments

- .x
A list or data frame.

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

- ...
Additional arguments passed on to

`.f`

.- .p
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.- .at
A character vector of names or a numeric vector of positions. Only those elements corresponding to

`.at`

will be modified.

##### Details

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`

.

##### Value

If `.x`

is a list, a list. If `.x`

is a data
frame, a data frame.

##### See Also

##### Examples

```
# NOT RUN {
# 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))
}
out
}
# 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)
dplyr::as_data_frame(out)
}
# 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)
# }
```

*Documentation reproduced from package purrr, version 0.2.2.2, License: GPL-3 | file LICENSE*