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       Treat-c,Treat-d,Treat-e

So it will be structured like this:

   CAT   COUNT1  COUNT2 COUNT3  TREAT1   TREAT2   TREAT3
    A    1       2      3       Treat-a  Treat-b  NA 
    B    4       5      NA      Treat-c  Treat-d  Treat-e 

Example code to generate the source dataframe:

df<-data.frame(CAT=c("A","B"))
df$COUNT <-list(1:3,4:5) 
df$TREAT <-list(paste("Treat-", letters[1:2],sep=""),paste("Treat-", letters[3:5],sep=""))

I believe I need a combination of rbind and unlist? Any help would be greatly appreciated. - Tim

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")