data.frame with variable-length list

HI,
Try this:
?df<-data.frame(name=c("a","b","c"),type=c(1,2,3),rtn=do.call(cbind,list(list(1:3,4:6,7:10))))
?str(df)
#'data.frame':??? 3 obs. of? 3 variables:
# $ name: Factor w/ 3 levels "a","b","c": 1 2 3
# $ type: num? 1 2 3
# $ rtn :List of 3
?# ..$ : int? 1 2 3
? #..$ : int? 4 5 6
? .#.$ : int? 7 8 9 10
A.K.




----- Original Message -----
From: Kevin Zembower <kzembower at verizon.net>
To: r-help at r-project.org
Cc: 
Sent: Friday, March 8, 2013 7:49 PM
Subject: [R] data.frame with variable-length list

Hello,

I'm trying to create a data frame with three columns, one of which is a
variable-length list. I tried:

df <- data.frame(name = c("a", "b", "c"),
? ? ? ? ? ? ? ?  type=c(1, 2, 3),
? ? ? ? ? ? ? ?  rtn = c(list(1,2,3), list(4, 5,6), list(7,8,9, 10)
? ? ? ? ? ? ? ? )
)

This would be useful, for example, if the 'rtn' is a variable number of
observations.

That gave me:

df

? name type rtn.1 rtn.2 rtn.3 rtn.4 rtn.5 rtn.6 rtn.7 rtn.8 rtn.9 rtn.10
1? ? a? ? 1? ?  1? ?  2? ?  3? ?  4? ?  5? ?  6? ?  7? ?  8? ?  9? ?  10
2? ? b? ? 2? ?  1? ?  2? ?  3? ?  4? ?  5? ?  6? ?  7? ?  8? ?  9? ?  10
3? ? c? ? 3? ?  1? ?  2? ?  3? ?  4? ?  5? ?  6? ?  7? ?  8? ?  9? ?  10

What I wanted is something like this, conceptually:

df

? name type? ?  rtn
1? ? a? ? 1? ?  list(1, 2, 3)
2? ? b? ? 2? ?  list(4, 5, 6)
3? ? c? ? 3? ?  list(7, 8, 9, 10)

I discovered this in the R Language Definition manual:

"A data frame can contain a list that is the same length as the other
components. The list can contain elements of differing lengths thereby
providing a data structure for ragged arrays. However, as of this
writing such arrays are not generally handled correctly. (2.3.2)"

Is this still the case? What does 'not handled correctly' mean? Can I do
something like:

sample(df$rtn, 1)

If this isn't the way to do this, can you suggest the correct way?

Thanks for your help and advice.

-Kevin

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