Output number of element of a list `l_dply` applies a function to

Question

When applying a function to each element of a list using l_ply, how can I generate an output showing me the number of the element of the list the function is momentarily applied to? It's similar to a progress bar (l_dply(list, function, .progress = progress_text(char = '*')), but what I'd like is an indicator of the position of the element in the list currently being processed by the function.

For example, if my list has 100 elements, I would like output from 1 to 100 for each iteration the function is applied.

I am using l_ply with a downloader function, so my ultimate goal is to be able to find out for which list element the download did not work.

Not sure it would be the best way to do it. You should look at http://r4ds.had.co.nz/iteration.html#dealing-with-failure. — F. Privé, Sep 06 '17 at 08:58
Something like this? https://stackoverflow.com/questions/9950144/access-lapply-index-names-inside-fun If I need anything like that, I usually resort to a for loop or `mapply` where one variable is a list of "iterators" and the second one is actually the things I'm trying to process. Oh, and here's the solution mentioned by Ana: https://stackoverflow.com/a/29833989/322912 — Roman Luštrik, Sep 06 '17 at 09:31

moodymudskipper · Accepted Answer · 2017-09-06T11:12:59.167

Maybe this would work for you:

l_ply2<- function(.data, .fun = NULL, ..., .progress = "none", .inform = FALSE,
                   .print = FALSE, .parallel = FALSE, .paropts = NULL){
  i <- 0
  fun2 <- function(x){
    i <<- i+1
    print(i)
    .fun(x)
  }
  plyr::l_ply(.data, fun2, ..., .progress = .progress, .inform = .inform,
         .print = .print, .parallel = .parallel, .paropts = .paropts)
}

l_ply2(list("a","b","c"),print)
# [1] 1
# [1] "a"
# [1] 2
# [1] "b"
# [1] 3
# [1] "c"

EDIT:

I fixed it to work with ellipsis (...), it's ugly though, the kind of ugly that usually gets me downvotes :), anyway here goes, and if you can, help me make it beautiful:

l_ply2<- function(.data, .fun = NULL, ..., .progress = "none", .inform = FALSE,
                   .print = FALSE, .parallel = FALSE, .paropts = NULL){
  i <- 0
  str <- paste0(deparse(args(.fun))[1],
                "{i<<-i+1;print(i);",
                as.character(substitute(.fun)),
                "(",paste(paste(formalArgs(.fun),"=",formalArgs(.fun)),collapse=","),")}")
  fun2 <- eval(parse(text = str))
  plyr::l_ply(.data, fun2, ..., .progress = .progress, .inform = .inform,
         .print = .print, .parallel = .parallel, .paropts = .paropts)
}

l_ply2(list("a","b","c",123.456),print,digits=4)
# [1] 1
# [1] "a"
# [1] 2
# [1] "b"
# [1] 3
# [1] "c"
# [1] 4
# [1] 123.5

Relevant: https://stackoverflow.com/questions/12982528/how-to-create-an-r-function-programmatically — moodymudskipper, Sep 06 '17 at 21:48

Output number of element of a list `l_dply` applies a function to

1 Answers1