Sent from my phone. Please excuse my brevity.
On August 28, 2016 11:14:20 PM PDT, 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. 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);
>}
>
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