问题
I am trying to create a function that reads in all sheets in an excel workbook using readxl::read_excel and binds them into a single data frame, and allows me to pass through additional arguments to read_excel. I can do the first part fine, but not the second part.
library(magrittr)
# example excel workbook with multiple sheets
path <- readxl::readxl_example("datasets.xlsx")
# function with simple forwarding
read_all <- function(path, ...) {
path %>%
readxl::excel_sheets() %>%
rlang::set_names() %>%
purrr::map_df(~ readxl::read_excel(path = path, sheet = .x, ...))
}
# errors with and without additional arguments
read_all(path)
read_all(path, skip = 5)
I should get back a single file, instead I get an error:
Error: Can't guess format of this cell reference: iris
In addition: Warning message: Cell reference follows neither the A1 nor R1C1 format. Example: iris NAs generated.
Without argument passing the function works fine:
# Function works without passing extra params
read_all_0 <- function(path) {
path %>%
readxl::excel_sheets() %>%
rlang::set_names() %>%
purrr::map_df(~ readxl::read_excel(path = path, sheet = .x))
}
read_all_0(path)
Argument passing works fine in a simple function without purrr::map_df
read_test <- function(path, ...) {
path %>% readxl::read_excel(...)
}
read_test(path, skip = 10)
回答1:
A possible solution is to create a named function which only takes one argument and pass it to map so that the only argument is the vector/list you are looping over.
Applied to your problem a solution would look like this:
# function with forwarding
read_all <- function(path, ...) {
# function within function that sets the arguments path and ellipsis as given and only leaves sheet to be determined
read_xl <- function(sheet) {
readxl::read_excel(path = path, sheet, ...)
}
path %>%
readxl::excel_sheets() %>%
rlang::set_names() %>%
purrr::map_df(read_xl)
}
# this allows you to pass along arguments in the ellipsis correctly
read_all(path)
read_all(path, col_names = FALSE)
It seems this problem is stemming from an improper environment handling of the purrr::as_mapper function. To circumvent this, I suggested using an anonymous function in the comments. Apparently, the approach below works as well.
read_all <- function(path, ...) {
path %>%
readxl::excel_sheets() %>%
rlang::set_names() %>%
purrr::map_df(function(x) {
readxl::read_excel(path = path, sheet = x, ...)
})
}
To verify that it is really the as_mapper function that is causing the problem, we can rewrite the named function-in-function from above using as_mapper. This again yields errors with and without additional arguments in the ellipsis.
# function with forwarding
read_all <- function(path, ...) {
# named mapper function
read_xl <- purrr::as_mapper(~ readxl::read_excel(path = path, sheet = .x, ...))
path %>%
readxl::excel_sheets() %>%
rlang::set_names() %>%
purrr::map_df(read_xl)
}
Update
Knowing that as_mapper is causing the issue allows us to dig deeper into the problem. Now we can inspect in the RStudio debugger what is happening under the hood when running a simple mapper version of read_excel:
read_xl <- purrr::as_mapper(~ readxl::read_excel(path = .x, sheet = .y, ...))
debugonce(read_xl)
read_xl(path, 1)
It seems that when the ellipsis is included in the mapper function, as_mapper maps the first argument not only to .x but also automatically to the ellipsis .... We can verify this by creating a simple mapper function paster taking two arguments .x and ....
paster <- purrr::as_mapper(~ paste0(.x, ...))
paster(1)
> [1] "11"
paster(2)
> [1] "22"
The question now is: is there another way we are supposed to use ellipsis in mapper functions or is this a bug.
回答2:
I would have thought the following would work:
read_all <- function(path, ...) {
path %>%
readxl::excel_sheets() %>%
purrr::set_names() %>%
map_df(~readxl::read_excel(path=path, sheet=.x), ...)
}
because the map family has a ... argument for passing additional arguments to the mapped function. However, the following code ignores the n_max argument and still returns all rows of the various data frames instead of a data frame with 8 rows (2 rows from each of the four sheets):
p <- readxl_example("datasets.xlsx")
read_all(p, n_max=2)
However, this works:
read_all <- function(path, ...) {
path %>%
excel_sheets() %>%
set_names() %>%
map_df(read_excel, path=path, ...)
}
p <- readxl_example("datasets.xlsx")
read_all(path=p, n_max=2)
In the above, path and any additional arguments in ... are passed to read_excel and (apparently) the sheet name (which would be the .x if we used it explicitly) is implicitly passed to the sheet argument, I guess because the path argument, which is first, has already been provided. I don't really get this and it doesn't seem like a particularly transparent approach, but I thought I'd put it out there in case someone else can explain what's going on and provide better code.
来源：https://stackoverflow.com/questions/58243710/forwarding-arguments-in-a-function-with-purrrmap-df