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in R Programming by (50.2k points)

I want to loop through a collection of files in R and just want to access the information from each one. I don't know why the loop is very slow. Is there a way I could vectorize this?

library("rjson")

all_files=list.files(path="~/p/a/t/h", recursive = TRUE)

for(i in seq_along(all_files)) {

    temp = fromJSON(file = all_files[i])

    if (length(temp$tags) != 0){

        songTags <- c(songTags, temp$tags)

        songTrack_id <- c(songTrack_id, temp$track_id)

    }

}

1 Answer

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by (108k points)

See in R programming, if you want to grow the objects in a loop, then usually the process is very slow. You can use lapply/sapply instead.

all_data <- do.call(rbind, lapply(all_files, function(x) {

                temp = jsonlite::fromJSON(file = x)

                if(length(temp$tags)) 

                   list(tags = temp$tags, track_id = temp$track_id)

            }))

If you want a more convenient one then you can sue purrr's map_df:

all_data <- map_df(all_files, ~{

               temp = jsonlite::fromJSON(file = .x)

               if(length(temp$tags)) 

                  list(tags = temp$tags, track_id = temp$track_id)

               })

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