[R] How to split a data.frame into its columns?

Marius Hofert
Mon, 29 Aug 2016 13:54:42 -0000

Hi David and Jeff,

Thanks for your quick help, unclass() was precisely what I was looking for.

Cheers,
M

On Mon, Aug 29, 2016 at 10:39 AM, aditya pant <adityapant1 at gmail.com>
wrote:
>
> ^^??????),,,
> ________________________________
> 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
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>> PLEASE do read the posting guide
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>> and provide commented, minimal, self-contained, reproducible code.
>
> David Winsemius
> Alameda, CA, USA