[R] data.frame with variable-length list

[William Dunlap]

Note that
  c(list(1,2,3), list(4, 5,6), list(7,8,9, 10))
is identical to
  list(1, 2, 3, 4, 5, 6, 7, 8, 9, 10)

You want either
   list(list(1,2,3), list(4,5,6), list(7,8,9,10)) # list of 3 lists of numeric scalars
or
   list(c(1,2,3), c(4,5,6), c(7,8,9,10)) # list of 3 numeric vectors

In any case, you need to wrap it in I() when passing it to data.frame so it doesn't
get converted to columns of a data.frame.  E.g.,

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

> df[[2, "rtn"]]
[1] 4 5 6
> df
  name type         rtn
1    a    1     1, 2, 3
2    b    2     4, 5, 6
3    c    3 7, 8, 9, 10

Bill Dunlap
Spotfire, TIBCO Software
wdunlap tibco.com


> -----Original Message-----
> From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-project.org] On Behalf
> Of Kevin Zembower
> Sent: Friday, March 08, 2013 4:49 PM
> To: r-help at r-project.org
> 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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