Best way to store variable-length data in an R data.frame?

Question

I have some mixed-type data that I would like to store in an R data structure of some sort. Each data point has a set of fixed attributes which may be 1-d numeric, factors, or

Answer 1

Since the R data frame structure is based loosely on the SQL table, having each element of the data frame be anything other than an atomic data type is uncommon. However, it can be done, as you've shown, and this linked post describes such an application implemented on a larger scale.

An alternative is to store your data as a string and have a function to retrieve it, or create a separate function to which the data is attached and extract it using indices stored in your data frame.

> ## alternative 1
> tokens <- function(x,i=TRUE) Map(as.numeric,strsplit(x[i],","))
> d <- data.frame(id=c(1,2,3), token_lengths=c("5,5", "9", "4,2,2,4,6"))
> 
> tokens(d$token_lengths)
[[1]]
[1] 5 5

[[2]]
[1] 9

[[3]]
[1] 4 2 2 4 6

> tokens(d$token_lengths,2:3)
[[1]]
[1] 9

[[2]]
[1] 4 2 2 4 6

> 
> ## alternative 2
> retrieve <- local({
+   token_lengths <- list(c(5,5), 9, c(4,2,2,4,6))
+   function(i) token_lengths[i]
+ })
> 
> d <- data.frame(id=c(1,2,3), token_lengths=1:3)
> retrieve(d$token_lengths[2:3])
[[1]]
[1] 9

[[2]]
[1] 4 2 2 4 6