How to split a data.frame into its columns?

________________________________
From: David Winsemius
Sent: ?29-?08-?2016 11:59
To: Marius Hofert
Cc: R-help
Subject: Re: [R] How to split a data.frame into its columns?


On Aug 28, 2016, at 11:14 PM, Marius Hofert <marius.hofert at uwaterloo.ca>
wrote:

Hi,

I need a fast way to split a data.frame (and matrix) into a list of
columns.

This is a bit of a puzzle since data.frame objects are by definition "lists
of columns".

If you want a data.frame object (say it's name is dat) to _only be a list of
columns then

dat <- unclass(dat)

The split.data.frame function splits by rows since that is the most desired
and expected behavior and because the authors of S/R probably thought there
was no point in making the split "by columns" when it already was.

--
David.

For matrices, split(x, col(x)) works (which can then be done
in C for speed-up, if necessary), but for a data.frame? split(iris,
col(iris)) does not work as expected (?).
The outcome should be lapply(seq_len(ncol(iris)), function(j)
iris[,j]) and not require additional packages (if possible).

Thanks & cheers,
Marius

PS: Below is the C code for matrices. Not sure how easy it would be to
extend that to data.frames (?)

SEXP col_split(SEXP x)
{
   /* Setup */
   int *dims = INTEGER(getAttrib(x, R_DimSymbol));
   int n = dims[0], d = dims[1];
   SEXP res = PROTECT(allocVector(VECSXP, d));
   SEXP ref;
   int i = 0, j, k;

   /* Distinguish int/real matrices */
   switch (TYPEOF(x)) {
   case INTSXP:
   for(j = 0; j < d; j++) {
   SET_VECTOR_ELT(res, j, allocVector(INTSXP, n));
   int *e = INTEGER(VECTOR_ELT(res, j));
   for(k = 0 ; k < n ; i++, k++) {
   e[k] = INTEGER(x)[i];
   }
   }
   break;
   case REALSXP:
   for(j = 0; j < d; j++) {
   SET_VECTOR_ELT(res, j, allocVector(REALSXP, n));
   double *e = REAL(VECTOR_ELT(res, j));
   for(k = 0 ; k < n ; i++, k++) {
   e[k] = REAL(x)[i];
   }
   }
   break;
   case LGLSXP:
   for(j = 0; j < d; j++) {
   SET_VECTOR_ELT(res, j, allocVector(LGLSXP, n));
   int *e = LOGICAL(VECTOR_ELT(res, j));
   for(k = 0 ; k < n ; i++, k++) {
   e[k] = LOGICAL(x)[i];
   }
   }
   break;
   case STRSXP:
   for(j = 0; j < d; j++) {
ref = allocVector(STRSXP, n);
   SET_VECTOR_ELT(res, j, ref);
   ref = VECTOR_ELT(res, j);
   for(k = 0 ; k < n ; i++, k++) {
   SET_STRING_ELT(ref, k, STRING_ELT(x, i));
   }
   }
   break;
   default: error("Wrong type of 'x': %s",
CHAR(type2str_nowarn(TYPEOF(x))));
   }

   /* Return */
   UNPROTECT(1);
   return(res);
}

______________________________________________
R-help at r-project.org mailing list -- To UNSUBSCRIBE and more, see
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.

David Winsemius
Alameda, CA, USA

______________________________________________
R-help at r-project.org mailing list -- To UNSUBSCRIBE and more, see
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.