[R] Data.frames can not hold objects...What can be done in the following scenario?

[R. Michael Weylandt]

It is possible to chain together uses of `[[` -- e.g.,

x <- list(1:5, list(letters[1:5], list(LETTERS[1:5])))

x[[c(1,2)]] # 2L

x[[c(2,1,3)]] # "c"

x[[c(2,2,1,3)]] # "C"

which is sometimes useful.

Best,
Michael

On Mon, Jun 11, 2012 at 4:35 PM, Onur Uncu <onuruncu at gmail.com> wrote:
> Rui and the R-help team,
>
> In Rui's helpful answer below, the function returns a list as output.
> When we apply() the function to the data.frame, dataframe$newcolumn
> has 2 layers of list before we can access each vector elements. For
> instance, dataframe$newcolumn[[1]][[1]] is a vector whereas
> dataframe$newcolumn and dataframe$newcolumn[[1]] are lists. Is there a
> solution that involves less layers of lists? I am just trying to
> understand the R language better.
>
> Thank you.
>
>
> On Sun, Jun 10, 2012 at 3:18 PM, Rui Barradas <ruipbarradas at sapo.pt> wrote:
>> Hello,
>>
>> What you need is to have your function return a list, not a vector. Like
>> this
>>
>> testfun <- function (x, y) list(seq(x, y, 1))
>>
>> testframe<-data.frame(xvalues=c(2,3),yvalues=c(4,5))
>>
>> testframe$newcolumn <- apply(testframe, 1, function(x) testfun(x[1], x[2]))
>> class(testframe$newcolumn)  # [1] "list"
>>
>> Then you access lists and list elements.
>>
>> testframe$newcolumn[[1]]  # a list with just one element
>> testframe$newcolumn[[1]][[1]]  # that element, a vector
>> testframe$newcolumn[[1]][[1]][2]  # the vector's 2nd element
>>
>>
>> Since you want the function to return vectors in order to do further
>> computations, you'll access those vectors by varying the list index,
>>
>>
>> testframe$newcolumn[[1]][[1]]  # first list, it's only vector
>> testframe$newcolumn[[2]][[1]]  # second list, it's only vector
>>
>>
>> Etc.
>>
>> Hope this helps,
>>
>> Rui Barradas
>>
>> Em 10-06-2012 12:29, Onur Uncu escreveu:
>>>
>>> Thank you Duncan. A follow-up question is, how can I achieve the
>>> desired result in the earlier email? (i.e. Add the resulting vectors
>>> as a new column to the existing data.frame?)   I tried the following:
>>>
>>> testframe$newcolumn<-apply(testframe,1,function(x)testfun(x[1],x[2]))
>>>
>>> but I am getting the following error:
>>>
>>> Error in `$<-.data.frame`(`*tmp*`, "vecss", value = c(2, 3, 4, 3, 4, 5
>>> : replacement has 3 rows, data has 2
>>>
>>> Thanks for the help.
>>>
>>>
>>> On Sun, Jun 10, 2012 at 12:02 PM, Duncan Murdoch
>>> <murdoch.duncan at gmail.com> wrote:
>>>>
>>>> On 12-06-10 6:41 AM, Onur Uncu wrote:
>>>>>
>>>>>
>>>>> R-Help community,
>>>>>
>>>>> I understand that data.frames can hold elements of type double, string
>>>>> etc but NOT objects (such as a matrix etc).
>>>>
>>>>
>>>>
>>>> That is incorrect.  Dataframes can hold list vectors.  For example:
>>>>
>>>> A <- data.frame(x = 1:3)
>>>> A$y <- list(matrix(1, 2,2), matrix(2, 3,3), matrix(3,4,4))
>>>>
>>>> A[1,2] will now extract the 2x2 matrix, A[2,2] will extract the 3x3, etc.
>>>>
>>>> Duncan Murdoch
>>>>
>>>> This is not convenient for
>>>>>
>>>>>
>>>>> me in the following situation. I have a function that takes 2 inputs
>>>>> and returns a vector:
>>>>>
>>>>> testfun<- function (x,y) seq(x,y,1)
>>>>>
>>>>> I have a data.frame defined as follows:
>>>>>
>>>>> testframe<-data.frame(xvalues=c(2,3),yvalues=c(4,5))
>>>>>
>>>>> I would like to apply testfun to every row of testframe and then
>>>>> create a new column in the data.frame which holds the returned vectors
>>>>> as objects. Why do I want this? Because the returned vectors are an
>>>>> intermediate step towards further calculations. It would be great to
>>>>> keep adding new columns to the data.frame with the intermediate
>>>>> objects. But this is not possible since data.frames can not hold
>>>>> objects as elements. What do you suggest as an elegant solution in
>>>>> this scenario? Thank you for any help!
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>> I would love to hear if forum
>>>>>
>>>>> ______________________________________________
>>>>> R-help at r-project.org mailing list
>>>>> https://stat.ethz.ch/mailman/listinfo/r-help
>>>>> PLEASE do read the posting guide
>>>>> http://www.R-project.org/posting-guide.html
>>>>> and provide commented, minimal, self-contained, reproducible code.
>>>>
>>>>
>>>>
>>>
>>> ______________________________________________
>>> R-help at r-project.org mailing list
>>> https://stat.ethz.ch/mailman/listinfo/r-help
>>> PLEASE do read the posting guide
>>> http://www.R-project.org/posting-guide.html
>>> and provide commented, minimal, self-contained, reproducible code.
>>>
>>
>
> ______________________________________________
> R-help at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.