How to read excel workbooks with varying sheet names in parallel?

I am trying to use parLapplyLB to read excel files

file.list <- list.files(path="folder path", pattern="*.xlsx", full.names=TRUE)

test <- parLapplyLB(cl, file.list, function(x){
    readxl::read_excel(x, sheet=sheet_in_file)
    }

My problem is that each file in file.list has a different "sheet" name that I need to read. For e.g. in file1, I need to read sheet = "abc", in file2, need to read sheet = "xyz" etc. I tried few things but I have not been able to achieve this, is there a way to do it using parLapplyLB?

Solution

parLapplyLB as parallelized variant of lapply essentially loops over one list. What you want is a multivariate variant which allows you to loop over the 1st, 2nd, 3rd, etc. element in both of your two lists, where the single-threaded version would be Map (with a big M). The parallelized version mcMap lives in the parallel package you are using as well.

We may use parallel::mcMap.

library(parallel)
r <- mcMap(openxlsx::read.xlsx, file.list, sheet_in_file)
r
# r$`./foo/wb1.xlsx`
# X1 X2 X3
# 1  1  1  1
# 2  1  1  1
# 
# $`./foo/wb2.xlsx`
# X1 X2 X3
# 1  2  2  2
# 2  2  2  2
# 
# $`./foo/wb3.xlsx`
# X1 X2 X3
# 1  3  3  3
# 2  3  3  3
# 
# $`./foo/wb4.xlsx`
# X1 X2 X3
# 1  4  4  4
# 2  4  4  4

(Should work similarly with readxl::read_excel, don't have it installed on this machine.)

Also see @HenrikB's comment below for a solution using a different package.

Data:

dir.create('foo')  ## create dir
## create four lists of four dataframes, write to dir as .xlsx
lapply(1:4, \(j) openxlsx::write.xlsx(lapply(1:4, \(i) data.frame(matrix(i, 2, 3))) |> setNames(LETTERS[1:4]), 
                                      file=sprintf('./foo/wb%s.xlsx', j), overwrite=TRUE)) |>invisible()

file.list <- list.files(path = "./foo", pattern="*.xlsx", full.names=TRUE)

sheet_in_file <- c('A', 'B', 'C', 'D')