apply --> data.frame

On Aug 30, 2012, at 9:44 AM, Sam Steingold wrote:

* Sam Steingold <fqf at tah.bet> [2012-08-30 08:56:17 -0400]:

Is there a way for an apply-type function to return a data frame?
the closest thing I think of is

 foo <- as.data.frame(t(sapply(...)))
 names(foo) <- c(....)

alas, this has a problem of creating a "homogeneous" data frame, i.e.,
all the columns are numbers or characters, because the function passed
to sapply returns c(....) and

c(1,2,"a")

[1] "1" "2" "a"

e.g.,
as.data.frame(t(sapply(c("a,1","b,2","c,3"),function (n) 
strsplit(n,",")[[1]])))
   V1 V2
a,1  a  1
b,2  b  2
c,3  c  3

'data.frame':    3 obs. of  2 variables:
$ V1: Factor w/ 3 levels "a","b","c": 1 2 3
 ..- attr(*, "names")= chr  "a,1" "b,2" "c,3"
$ V2: Factor w/ 3 levels "1","2","3": 1 2 3
 ..- attr(*, "names")= chr  "a,1" "b,2" "c,3"

I wanted the V1 column to be a string, and V2 to be a number.
(I know stringsAsFactors=FALSE would replace factors with strings, but I
need a string and a number)

I could, of course, do ret$V2 <- as.numeric(ret$V2) but this would mean
a double conversion: from number to string first (by c()) and then back.

It is starting as a 'string' ('character' in R parlance) so you will 
need to coerce it to "numeric" at some point:

Consider this alternate route:

do.call(rbind, strsplit(c("a,1","b,2","c,3"), ",") )

     [,1] [,2]
[1,] "a"  "1"
[2,] "b"  "2"
[3,] "c"  "3"

as.data.frame( do.call(rbind, strsplit(c("a,1","b,2","c,3"), ",") ) )

  V1 V2
1  a  1
2  b  2
3  c  3

str( as.data.frame( do.call(rbind, strsplit(c("a,1","b,2","c,3"), 

",") ) , stringsAsFactors=FALSE) )
'data.frame':    3 obs. of  2 variables:
 $ V1: chr  "a" "b" "c"
 $ V2: chr  "1" "2" "3"