How to flatten R data frame that contains lists?

Question

I want to find the best \"R way\" to flatten a dataframe that looks like this:

  CAT    COUNT     TREAT
   A     1,2,3     Treat-a, Treat-b
   B     4,5              


        

Answer 1

Here is a solution using base R, accepting vectors of any length inside your list and no need to specify which columns of the dataframe you want to collapse. Part of the solution was generated using this answer.

df2 <- do.call(cbind,lapply(df,function(x){
  #check if it is a list, otherwise just return as is
  if(is.list(x)){
    return(data.frame(t(sapply(x,'[',seq(max(sapply(x,length)))))))
  } else{
  return(x)
  }
}))

As of R 3.2 there is lengths to replace sapply(x, length) as well,

df3 <- do.call(cbind.data.frame, lapply(df, function(x) {
  # check if it is a list, otherwise just return as is
  if (is.list(x)) {
    data.frame(t(sapply(x,'[', seq(max(lengths(x))))))
  } else {
   x
 }
}))

data used:

df <- structure(list(CAT = structure(1:2, .Label = c("A", "B"), class = "factor"), 
    COUNT = list(1:3, 4:5), TREAT = list(c("Treat-a", "Treat-b"
    ), c("Treat-c", "Treat-d", "Treat-e"))), .Names = c("CAT", 
"COUNT", "TREAT"), row.names = c(NA, -2L), class = "data.frame")