如何展平列表列表？

Question

The tm package extends c so that, if given a set of PlainTextDocument s it automatically creates a Corpus . Unfortunately, it appears that each PlainTextDocument must be specified separately.

eg if I had:

foolist <- list(a, b, c); # where a,b,c are PlainTextDocument objects

I'd do this to get a Corpus :

foocorpus <- c(foolist[[1]], foolist[[2]], foolist[[3]]);

I have a list of lists of 'PlainTextDocument s that looks like this:

> str(sectioned)
List of 154
 $ :List of 6
  ..$ :Classes 'PlainTextDocument', 'TextDocument', 'character'  atomic [1:1] Developing assessment models   Developing models
  .. .. ..- attr(*, "Author")= chr "John Smith"
  .. .. ..- attr(*, "DateTimeStamp")= POSIXlt[1:1], format: "2013-04-30 12:03:49"
  .. .. ..- attr(*, "Description")= chr(0) 
  .. .. ..- attr(*, "Heading")= chr "Research Focus"
  .. .. ..- attr(*, "ID")= chr(0) 
  .. .. ..- attr(*, "Language")= chr(0) 
  .. .. ..- attr(*, "LocalMetaData")=List of 4
  .. .. .. ..$ foo           : chr "bar"
  .. .. .. ..$ classification: chr "Technician"
  .. .. .. ..$ team          : chr ""
  .. .. .. ..$ supervisor    : chr "Bill Jones"
  .. .. ..- attr(*, "Origin")= chr "Smith-John_e.txt"

#etc., all sublists have 6 elements

So, to get all my PlainTextDocument s into a Corpus , this would work:

sectioned.Corpus <- c(sectioned[[1]][[1]], sectioned[[1]][[2]], ..., sectioned[[154]][[6]])

Can anyone suggest an easier way, please?

ETA: foo<-unlist(foolist, recursive=FALSE) produces a flat list of PlainTextDocuments, which still leaves me with the problem of feeding a list element by element to c

Answer 1

I expect that unlist(foolist) will help you. It has an option recursive which is TRUE by default.

So unlist(foolist, recursive = FALSE) will return the list of the documents, and then you can combine them by:

do.call(c, unlist(foolist, recursive=FALSE))

do.call just applies the function c to the elements of the obtained list

Answer 2

Here's a more general solution for when lists are nested multiple times and the amount of nesting differs between elements of the lists:

 flattenlist <- function(x){  
  morelists <- sapply(x, function(xprime) class(xprime)[1]=="list")
  out <- c(x[!morelists], unlist(x[morelists], recursive=FALSE))
  if(sum(morelists)){ 
    Recall(out)
  }else{
    return(out)
  }
}

Answer 3

Here's another method that worked for my list of lists.

df <- as.data.frame(do.call(rbind, lapply(foolist, as.data.frame)))

Or take a look at new functions in tidyr which work well.

rectangle a nested list into a tidy tibble

rectangling

    lst <-  list(
      list(
        age = 23,
        gender = "Male",
        city = "Sydney"
      ),
      list(
        age = 21,
        gender = "Female",
        city = "Cairns"
      )
    )
      
    tib <- tibble(lst)  %>% 
      unnest_wider(lst)

df <- as.data.frame(tib)