Dataframe within dataframe?

Question

Consider this example:

df <- data.frame(id=1:10,var1=LETTERS[1:10],var2=LETTERS[6:15])

fun.split <- function(x) tolower(as.character(x))
df$new.letter         


        

Answer 1

In this case R doesn't behave like one would expect but maybe if we dig deeper we can solve it. What is a data frame? as Norman Matloff says in his book (chapter 5):

a data frame is a list, with the components of that list being equal-length vectors

The following code might be useful to understand.

class(df$new.letters)
[1] "matrix"


str(df)
'data.frame':   10 obs. of  4 variables:
 $ id         : int  1 2 3 4 5 6 7 8 9 10
 $ var1       : Factor w/ 10 levels "A","B","C","D",..: 1 2 3 4 5 6 7 8 9 10
 $ var2       : Factor w/ 10 levels "F","G","H","I",..: 1 2 3 4 5 6 7 8 9 10
 $ new.letters: chr [1:10, 1:2] "a" "b" "c" "d" ...
  ..- attr(*, "dimnames")=List of 2
  .. ..$ : NULL
  .. ..$ : chr  "var1" "var2"

Maybe the reason why it looks strange is in the print methods. Consider this:

colnames(df$new.letters)
[1] "var1" "var2"

maybe there must something in the print methods that combine the sub-names of objects and display them all.

For example here the vectors that constitute the df are:

names(df)
[1] "id"          "var1"        "var2"        "new.letters"

but in this case the vector new.letters also has a dim attributes (in fact it is a matrix) were dimensions have names var1 and var1 too. See this code:

attributes(df$new.letters)
$dim
[1] 10  2

$dimnames
$dimnames[[1]]
NULL

$dimnames[[2]]
[1] "var1" "var2"

but when we print we see all of them like they were separated vectors (and so columns of the data.frame!).

Edit: Print methods

Just for curiosity in order to improve this question I looked inside the methods of the print functions:

methods(print)

The previous code produces a very long list of methods for the generic function print but there is no one for data.frame. The one that looks for data frame (but I am sure there is a more technically way to find out that) is listof.

getS3method("print", "listof")
function (x, ...) 
{
    nn <- names(x)
    ll <- length(x)
    if (length(nn) != ll) 
        nn <- paste("Component", seq.int(ll))
    for (i in seq_len(ll)) {
        cat(nn[i], ":\n")
        print(x[[i]], ...)
        cat("\n")
    }
    invisible(x)
}

Maybe I am wrong but It seems to me that in this code there might be useful informations about why that happens, specifically when the if (length(nn) != ll) is stated.