Improving performance of split() function in R?

Question

I have a data frame in a very simple form:

    X Y
    ---
    A 1
    A 2
    B 3
    C 1
    C 3

My end result should be a list like this

Answer 1

I found an elegant solution using similar code from dplyr and/or data.table. I looked for concatenate groups in R and I found this post:

Efficiently concate character content within one column, by group in R

And actually, it works quite nicely with

dt = data.table(content = sample(letters, 26e6, T), groups = LETTERS)
df = as.data.frame(dt)

system.time(dt[, paste(content, collapse = " "), by = groups])
#   user  system elapsed 
#   5.37    0.06    5.65 

system.time(df %>% group_by(groups) %>% summarise(paste(content, collapse = " ")))
#   user  system elapsed 
#   7.10    0.13    7.67 

Thanks for all your help