[R] apply --> data.frame

[David Winsemius]

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"

-- 
David Winsemius, MD
Alameda, CA, USA