The R Project SVN R

/*
 *  R : A Computer Language for Statistical Data Analysis
 *  Copyright (C) 1995, 1996  Robert Gentleman and Ross Ihaka
 *  Copyright (C) 1997-2007   The R Development Core Team
 *
 *  This program is free software; you can redistribute it and/or modify
 *  it under the terms of the GNU General Public License as published by
 *  the Free Software Foundation; either version 2 of the License, or
 *  (at your option) any later version.
 *
 *  This program is distributed in the hope that it will be useful,
 *  but WITHOUT ANY WARRANTY; without even the implied warranty of
 *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 *  GNU General Public License for more details.
 *
 *  You should have received a copy of the GNU General Public License
 *  along with this program; if not, a copy is available at
 *  http://www.r-project.org/Licenses/
 *
 *
 *  Vector and List Subsetting
 *
 *  There are three kinds of subscripting [, [[, and $.
 *  We have three different functions to compute these.
 *
 *
 *  Note on Matrix Subscripts
 *
 *  The special [ subscripting where dim(x) == ncol(subscript matrix)
 *  is handled inside VectorSubset. The subscript matrix is turned
 *  into a subscript vector of the appropriate size and then
 *  VectorSubset continues.  This provides coherence especially
 *  regarding attributes etc. (it would be quicker to handle this case
 *  separately, but then we would have more to keep in step.
 */

#ifdef HAVE_CONFIG_H
#include <config.h>
#endif

#include "Defn.h"

/* ExtractSubset does the transfer of elements from "x" to "result" */
/* according to the integer subscripts given in "indx". */

static SEXP ExtractSubset(SEXP x, SEXP result, SEXP indx, SEXP call)
{
    int i, ii, n, nx, mode;
    SEXP tmp, tmp2;
    mode = TYPEOF(x);
    n = LENGTH(indx);
    nx = length(x);
    tmp = result;

    if (x == R_NilValue)
    return x;

    for (i = 0; i < n; i++) {
    ii = INTEGER(indx)[i];
    if (ii != NA_INTEGER)
        ii--;
    switch (mode) {
    case LGLSXP:
        if (0 <= ii && ii < nx && ii != NA_LOGICAL)
        LOGICAL(result)[i] = LOGICAL(x)[ii];
        else
        LOGICAL(result)[i] = NA_INTEGER;
        break;
    case INTSXP:
        if (0 <= ii && ii < nx && ii != NA_INTEGER)
        INTEGER(result)[i] = INTEGER(x)[ii];
        else
        INTEGER(result)[i] = NA_INTEGER;
        break;
    case REALSXP:
        if (0 <= ii && ii < nx && ii != NA_INTEGER)
        REAL(result)[i] = REAL(x)[ii];
        else
        REAL(result)[i] = NA_REAL;
        break;
    case CPLXSXP:
        if (0 <= ii && ii < nx && ii != NA_INTEGER) {
        COMPLEX(result)[i] = COMPLEX(x)[ii];
        }
        else {
        COMPLEX(result)[i].r = NA_REAL;
        COMPLEX(result)[i].i = NA_REAL;
        }
        break;
    case STRSXP:
        if (0 <= ii && ii < nx && ii != NA_INTEGER)
        SET_STRING_ELT(result, i, STRING_ELT(x, ii));
        else
        SET_STRING_ELT(result, i, NA_STRING);
        break;
    case VECSXP:
    case EXPRSXP:
        if (0 <= ii && ii < nx && ii != NA_INTEGER)
        SET_VECTOR_ELT(result, i, VECTOR_ELT(x, ii));
        else
        SET_VECTOR_ELT(result, i, R_NilValue);
        break;
    case LISTSXP:
        /* cannot happen: pairlists are coerced to lists */
    case LANGSXP:
        if (0 <= ii && ii < nx && ii != NA_INTEGER) {
        tmp2 = nthcdr(x, ii);
        SETCAR(tmp, CAR(tmp2));
        SET_TAG(tmp, TAG(tmp2));
        }
        else
        SETCAR(tmp, R_NilValue);
        tmp = CDR(tmp);
        break;
    case RAWSXP:
        if (0 <= ii && ii < nx && ii != NA_INTEGER)
        RAW(result)[i] = RAW(x)[ii];
        else
        RAW(result)[i] = (Rbyte) 0;
        break;
    default:
        errorcall(call, R_MSG_ob_nonsub, type2char(mode));
    }
    }
    return result;
}


/* This is for all cases with a single index, including 1D arrays and
   matrix indexing of arrays */
static SEXP VectorSubset(SEXP x, SEXP s, SEXP call)
{
    int n, mode, stretch = 1;
    SEXP indx, result, attrib, nattrib;

    if (s == R_MissingArg) return duplicate(x);

    PROTECT(s);
    attrib = getAttrib(x, R_DimSymbol);

    /* Check to see if we have special matrix subscripting. */
    /* If we do, make a real subscript vector and protect it. */

    if (isMatrix(s) && isArray(x) && (isInteger(s) || isReal(s)) &&
        ncols(s) == length(attrib)) {
    s = mat2indsub(attrib, s, call);
    UNPROTECT(1);
    PROTECT(s);
    }

    /* Convert to a vector of integer subscripts */
    /* in the range 1:length(x). */

    PROTECT(indx = makeSubscript(x, s, &stretch, call));
    n = LENGTH(indx);

    /* Allocate the result. */

    mode = TYPEOF(x);
    /* No protection needed as ExtractSubset does not allocate */
    result = allocVector(mode, n);
    if (mode == VECSXP || mode == EXPRSXP)
    /* we do not duplicate the values when extracting the subset,
       so to be conservative mark the result as NAMED = 2 */
    SET_NAMED(result, 2);

    PROTECT(result = ExtractSubset(x, result, indx, call));
    if (result != R_NilValue) {
    if (
        ((attrib = getAttrib(x, R_NamesSymbol)) != R_NilValue) ||
        ( /* here we might have an array.  Use row names if 1D */
        isArray(x) && LENGTH(getAttrib(x, R_DimNamesSymbol)) == 1 &&
        (attrib = getAttrib(x, R_DimNamesSymbol)) != R_NilValue &&
        (attrib = GetRowNames(attrib)) != R_NilValue
        )
        ) {
        nattrib = allocVector(TYPEOF(attrib), n);
        PROTECT(nattrib); /* seems unneeded */
        nattrib = ExtractSubset(attrib, nattrib, indx, call);
        setAttrib(result, R_NamesSymbol, nattrib);
        UNPROTECT(1);
    }
    if ((attrib = getAttrib(x, R_SrcrefSymbol)) != R_NilValue &&
        TYPEOF(attrib) == VECSXP) {
        nattrib = allocVector(VECSXP, n);
        PROTECT(nattrib); /* seems unneeded */
        nattrib = ExtractSubset(attrib, nattrib, indx, call);
        setAttrib(result, R_SrcrefSymbol, nattrib);
        UNPROTECT(1);
    }
    if(IS_S4_OBJECT(x)) { /* e.g. contains = "list" */
        setAttrib(result, R_ClassSymbol, getAttrib(x, R_ClassSymbol));
        SET_S4_OBJECT(result);
    }
    }
    UNPROTECT(3);
    return result;
}


static SEXP MatrixSubset(SEXP x, SEXP s, SEXP call, int drop)
{
    SEXP attr, result, sr, sc, dim;
    int nr, nc, nrs, ncs;
    int i, j, ii, jj, ij, iijj;

    nr = nrows(x);
    nc = ncols(x);

    /* Note that "s" is protected on entry. */
    /* The following ensures that pointers remain protected. */
    dim = getAttrib(x, R_DimSymbol);

    sr = SETCAR(s, arraySubscript(0, CAR(s), dim, getAttrib,
                  (STRING_ELT), x));
    sc = SETCADR(s, arraySubscript(1, CADR(s), dim, getAttrib,
                   (STRING_ELT), x));
    nrs = LENGTH(sr);
    ncs = LENGTH(sc);
    PROTECT(sr);
    PROTECT(sc);
    result = allocVector(TYPEOF(x), nrs*ncs);
    PROTECT(result);
    for (i = 0; i < nrs; i++) {
    ii = INTEGER(sr)[i];
    if (ii != NA_INTEGER) {
        if (ii < 1 || ii > nr)
        errorcall(call, R_MSG_subs_o_b);
        ii--;
    }
    for (j = 0; j < ncs; j++) {
        jj = INTEGER(sc)[j];
        if (jj != NA_INTEGER) {
        if (jj < 1 || jj > nc)
            errorcall(call, R_MSG_subs_o_b);
        jj--;
        }
        ij = i + j * nrs;
        if (ii == NA_INTEGER || jj == NA_INTEGER) {
        switch (TYPEOF(x)) {
        case LGLSXP:
        case INTSXP:
            INTEGER(result)[ij] = NA_INTEGER;
            break;
        case REALSXP:
            REAL(result)[ij] = NA_REAL;
            break;
        case CPLXSXP:
            COMPLEX(result)[ij].r = NA_REAL;
            COMPLEX(result)[ij].i = NA_REAL;
            break;
        case STRSXP:
            SET_STRING_ELT(result, ij, NA_STRING);
            break;
        case VECSXP:
            SET_VECTOR_ELT(result, ij, R_NilValue);
            break;
        case RAWSXP:
            RAW(result)[ij] = (Rbyte) 0;
            break;
        default:
            errorcall(call, _("matrix subscripting not handled for this type"));
            break;
        }
        }
        else {
        iijj = ii + jj * nr;
        switch (TYPEOF(x)) {
        case LGLSXP:
            LOGICAL(result)[ij] = LOGICAL(x)[iijj];
            break;
        case INTSXP:
            INTEGER(result)[ij] = INTEGER(x)[iijj];
            break;
        case REALSXP:
            REAL(result)[ij] = REAL(x)[iijj];
            break;
        case CPLXSXP:
            COMPLEX(result)[ij] = COMPLEX(x)[iijj];
            break;
        case STRSXP:
            SET_STRING_ELT(result, ij, STRING_ELT(x, iijj));
            break;
        case VECSXP:
            SET_VECTOR_ELT(result, ij, VECTOR_ELT(x, iijj));
            break;
        case RAWSXP:
            RAW(result)[ij] = RAW(x)[iijj];
            break;
        default:
            errorcall(call, _("matrix subscripting not handled for this type"));
            break;
        }
        }
    }
    }
    if(nrs >= 0 && ncs >= 0) {
    PROTECT(attr = allocVector(INTSXP, 2));
    INTEGER(attr)[0] = nrs;
    INTEGER(attr)[1] = ncs;
    setAttrib(result, R_DimSymbol, attr);
    UNPROTECT(1);
    }

    /* The matrix elements have been transferred.  Now we need to */
    /* transfer the attributes.  Most importantly, we need to subset */
    /* the dimnames of the returned value. */

    if (nrs >= 0 && ncs >= 0) {
    SEXP dimnames, dimnamesnames, newdimnames;
    dimnames = getAttrib(x, R_DimNamesSymbol);
    dimnamesnames = getAttrib(dimnames, R_NamesSymbol);
    if (!isNull(dimnames)) {
        PROTECT(newdimnames = allocVector(VECSXP, 2));
        if (TYPEOF(dimnames) == VECSXP) {
          SET_VECTOR_ELT(newdimnames, 0,
            ExtractSubset(VECTOR_ELT(dimnames, 0),
                  allocVector(STRSXP, nrs), sr, call));
          SET_VECTOR_ELT(newdimnames, 1,
            ExtractSubset(VECTOR_ELT(dimnames, 1),
                  allocVector(STRSXP, ncs), sc, call));
        }
        else {
          SET_VECTOR_ELT(newdimnames, 0,
            ExtractSubset(CAR(dimnames),
                  allocVector(STRSXP, nrs), sr, call));
          SET_VECTOR_ELT(newdimnames, 1,
            ExtractSubset(CADR(dimnames),
                  allocVector(STRSXP, ncs), sc, call));
        }
        setAttrib(newdimnames, R_NamesSymbol, dimnamesnames);
        setAttrib(result, R_DimNamesSymbol, newdimnames);
        UNPROTECT(1);
    }
    }
    /*  Probably should not do this:
    copyMostAttrib(x, result); */
    if (drop)
    DropDims(result);
    UNPROTECT(3);
    return result;
}


static SEXP ArraySubset(SEXP x, SEXP s, SEXP call, int drop)
{
    int i, j, k, ii, jj, mode, n;
    int **subs, *indx, *offset, *bound;
    SEXP dimnames, dimnamesnames, p, q, r, result, xdims;
    void *vmaxsave;

    mode = TYPEOF(x);
    xdims = getAttrib(x, R_DimSymbol);
    k = length(xdims);

    vmaxsave = vmaxget();
    subs = (int**)R_alloc(k, sizeof(int*));
    indx = (int*)R_alloc(k, sizeof(int));
    offset = (int*)R_alloc(k, sizeof(int));
    bound = (int*)R_alloc(k, sizeof(int));

    /* Construct a vector to contain the returned values. */
    /* Store its extents. */

    n = 1;
    r = s;
    for (i = 0; i < k; i++) {
    SETCAR(r, arraySubscript(i, CAR(r), xdims, getAttrib,
                 (STRING_ELT), x));
    bound[i] = LENGTH(CAR(r));
    n *= bound[i];
    r = CDR(r);
    }
    PROTECT(result = allocVector(mode, n));
    r = s;
    for (i = 0; i < k; i++) {
    indx[i] = 0;
    subs[i] = INTEGER(CAR(r));
    r = CDR(r);
    }
    offset[0] = 1;
    for (i = 1; i < k; i++)
    offset[i] = offset[i - 1] * INTEGER(xdims)[i - 1];

    /* Transfer the subset elements from "x" to "a". */

    for (i = 0; i < n; i++) {
    ii = 0;
    for (j = 0; j < k; j++) {
        jj = subs[j][indx[j]];
        if (jj == NA_INTEGER) {
        ii = NA_INTEGER;
        goto assignLoop;
        }
        if (jj < 1 || jj > INTEGER(xdims)[j])
        errorcall(call, R_MSG_subs_o_b);
        ii += (jj - 1) * offset[j];
    }

      assignLoop:
    switch (mode) {
    case LGLSXP:
        if (ii != NA_INTEGER)
        LOGICAL(result)[i] = LOGICAL(x)[ii];
        else
        LOGICAL(result)[i] = NA_LOGICAL;
        break;
    case INTSXP:
        if (ii != NA_INTEGER)
        INTEGER(result)[i] = INTEGER(x)[ii];
        else
        INTEGER(result)[i] = NA_INTEGER;
        break;
    case REALSXP:
        if (ii != NA_INTEGER)
        REAL(result)[i] = REAL(x)[ii];
        else
        REAL(result)[i] = NA_REAL;
        break;
    case CPLXSXP:
        if (ii != NA_INTEGER) {
        COMPLEX(result)[i] = COMPLEX(x)[ii];
        }
        else {
        COMPLEX(result)[i].r = NA_REAL;
        COMPLEX(result)[i].i = NA_REAL;
        }
        break;
    case STRSXP:
        if (ii != NA_INTEGER)
        SET_STRING_ELT(result, i, STRING_ELT(x, ii));
        else
        SET_STRING_ELT(result, i, NA_STRING);
        break;
    case VECSXP:
        if (ii != NA_INTEGER)
        SET_VECTOR_ELT(result, i, VECTOR_ELT(x, ii));
        else
        SET_VECTOR_ELT(result, i, R_NilValue);
        break;
    case RAWSXP:
        if (ii != NA_INTEGER)
        RAW(result)[i] = RAW(x)[ii];
        else
        RAW(result)[i] = (Rbyte) 0;
        break;
    default:
        errorcall(call, _("array subscripting not handled for this type"));
        break;
    }
    if (n > 1) {
        j = 0;
        while (++indx[j] >= bound[j]) {
        indx[j] = 0;
        j = (j + 1) % k;
        }
    }
    }

    PROTECT(xdims = allocVector(INTSXP, k));
    for(i = 0 ; i < k ; i++)
    INTEGER(xdims)[i] = bound[i];
    setAttrib(result, R_DimSymbol, xdims);
    UNPROTECT(1);

    /* The array elements have been transferred. */
    /* Now we need to transfer the attributes. */
    /* Most importantly, we need to subset the */
    /* dimnames of the returned value. */

    dimnames = getAttrib(x, R_DimNamesSymbol);
    dimnamesnames = getAttrib(dimnames, R_NamesSymbol);
    if (dimnames != R_NilValue) {
    /*SEXP xdims;
    int */ j = 0;
    PROTECT(xdims = allocVector(VECSXP, k));
    if (TYPEOF(dimnames) == VECSXP) {
        r = s;
        for (i = 0; i < k ; i++) {
        if (bound[i] > 0) {
          SET_VECTOR_ELT(xdims, j++,
            ExtractSubset(VECTOR_ELT(dimnames, i),
                      allocVector(STRSXP, bound[i]),
                      CAR(r), call));
        } else { /* 0-length dims have NULL dimnames */
            SET_VECTOR_ELT(xdims, j++, R_NilValue);
        }
        r = CDR(r);
        }
    }
    else {
        p = dimnames;
        q = xdims;
        r = s;
        for(i = 0 ; i < k; i++) {
        SETCAR(q, allocVector(STRSXP, bound[i]));
        SETCAR(q, ExtractSubset(CAR(p), CAR(q), CAR(r), call));
        p = CDR(p);
        q = CDR(q);
        r = CDR(r);
        }
    }
    setAttrib(xdims, R_NamesSymbol, dimnamesnames);
    setAttrib(result, R_DimNamesSymbol, xdims);
    UNPROTECT(1);
    }
    /* This was removed for matrices in 1998
       copyMostAttrib(x, result); */
    /* Free temporary memory */
    vmaxset(vmaxsave);
    if (drop)
    DropDims(result);
    UNPROTECT(1);
    return result;
}


/* Returns and removes a named argument from argument list args.
   The search ends as soon as a matching argument is found.  If
   the argument is not found, the argument list is not modified
   and R_NilValue is returned.
 */
static SEXP ExtractArg(SEXP args, SEXP arg_sym)
{
    SEXP arg, prev_arg;
    int found = 0;

    for (arg = prev_arg = args; arg != R_NilValue; arg = CDR(arg)) {
    if(TAG(arg) == arg_sym) {
        if (arg == prev_arg) /* found at head of args */
        args = CDR(args);
        else
        SETCDR(prev_arg, CDR(arg));
        found = 1;
        break;
    }
    else  prev_arg = arg;
    }
    return found ? CAR(arg) : R_NilValue;
}

/* Extracts the drop argument, if present, from the argument list.
   The object being subsetted must be the first argument. */
static void ExtractDropArg(SEXP el, int *drop)
{
    SEXP dropArg = ExtractArg(el, R_DropSymbol);
    *drop = asLogical(dropArg);
    if (*drop == NA_LOGICAL) *drop = 1;
}


/* Extracts and, if present, removes the 'exact' argument from the
   argument list.  An integer code giving the desired exact matching
   behavior is returned:
       0  not exact
       1  exact
      -1  not exact, but warn when partial matching is used
 */
static int ExtractExactArg(SEXP args)
{
    SEXP argval = ExtractArg(args, R_ExactSymbol);
    int exact;
    if(isNull(argval)) return 1; /* Default is true as from R 2.7.0 */
    exact = asLogical(argval);
    if (exact == NA_LOGICAL) exact = -1;
    return exact;
}


/* The "[" subset operator.
 * This provides the most general form of subsetting. */

SEXP attribute_hidden do_subset(SEXP call, SEXP op, SEXP args, SEXP rho)
{
    SEXP ans;

    /* If the first argument is an object and there is an */
    /* approriate method, we dispatch to that method, */
    /* otherwise we evaluate the arguments and fall through */
    /* to the generic code below.  Note that evaluation */
    /* retains any missing argument indicators. */

    if(DispatchOrEval(call, op, "[", args, rho, &ans, 0, 0))
    return(ans);

    /* Method dispatch has failed, we now */
    /* run the generic internal code. */
    return do_subset_dflt(call, op, ans, rho);
}

SEXP attribute_hidden do_subset_dflt(SEXP call, SEXP op, SEXP args, SEXP rho)
{
    SEXP ans, ax, px, x, subs;
    int drop, i, nsubs, type;

    /* By default we drop extents of length 1 */

    /* Handle case of extracting a single element from a simple vector
       directly to improve speed for this simple case. */
    if (CDDR(args) == R_NilValue) {
    int i;
    SEXP x = CAR(args);
    SEXP s = CADR(args);
    if (ATTRIB(x) == R_NilValue && ATTRIB(s) == R_NilValue) {
        switch (TYPEOF(x)) {
        case REALSXP:
        switch (TYPEOF(s)) {
        case REALSXP: i = (LENGTH(s) == 1) ? REAL(s)[0] : -1; break;
        case INTSXP: i = (LENGTH(s) == 1) ? INTEGER(s)[0] : -1; break;
        default:  i = -1;
        }
        if (i >= 1 && i <= LENGTH(x))
            return ScalarReal( REAL(x)[i-1] );
        break;
        case INTSXP:
        switch (TYPEOF(s)) {
        case REALSXP: i = (LENGTH(s) == 1) ? REAL(s)[0] : -1; break;
        case INTSXP: i = (LENGTH(s) == 1) ? INTEGER(s)[0] : -1; break;
        default:  i = -1;
        }
        if (i >= 1 && i <= LENGTH(x))
            return ScalarInteger( INTEGER(x)[i-1] );
        break;
        default: break;
        }
    }
    }

    PROTECT(args);

    drop = 1;
    ExtractDropArg(args, &drop);
    x = CAR(args);

    /* This was intended for compatibility with S, */
    /* but in fact S does not do this. */
    /* FIXME: replace the test by isNull ... ? */

    if (x == R_NilValue) {
    UNPROTECT(1);
    return x;
    }
    subs = CDR(args);
    nsubs = length(subs);
    type = TYPEOF(x);

    /* Here coerce pair-based objects into generic vectors. */
    /* All subsetting takes place on the generic vector form. */

    ax = x;
    if (isVector(x))
    PROTECT(ax);
    else if (isPairList(x)) {
    SEXP dim = getAttrib(x, R_DimSymbol);
    int ndim = length(dim);
    if (ndim > 1) {
        PROTECT(ax = allocArray(VECSXP, dim));
        setAttrib(ax, R_DimNamesSymbol, getAttrib(x, R_DimNamesSymbol));
        setAttrib(ax, R_NamesSymbol, getAttrib(x, R_DimNamesSymbol));
    }
    else {
        PROTECT(ax = allocVector(VECSXP, length(x)));
        setAttrib(ax, R_NamesSymbol, getAttrib(x, R_NamesSymbol));
    }
    for(px = x, i = 0 ; px != R_NilValue ; px = CDR(px))
        SET_VECTOR_ELT(ax, i++, CAR(px));
    }
    else errorcall(call, R_MSG_ob_nonsub, type2char(TYPEOF(x)));

    /* This is the actual subsetting code. */
    /* The separation of arrays and matrices is purely an optimization. */

    if(nsubs < 2) {
    SEXP dim = getAttrib(x, R_DimSymbol);
    int ndim = length(dim);
    PROTECT(ans = VectorSubset(ax, (nsubs == 1 ? CAR(subs) : R_MissingArg),
                   call));
    /* one-dimensional arrays went through here, and they should
       have their dimensions dropped only if the result has
       length one and drop == TRUE
    */
    if(ndim == 1) {
        SEXP attr, attrib, nattrib;
        int len = length(ans);

        if(!drop || len > 1) {
        PROTECT(attr = allocVector(INTSXP, 1));
        INTEGER(attr)[0] = length(ans);
        setAttrib(ans, R_DimSymbol, attr);
        UNPROTECT(1);
        if((attrib = getAttrib(x, R_DimNamesSymbol)) != R_NilValue) {
            /* reinstate dimnames, include names of dimnames */
            PROTECT(nattrib = duplicate(attrib));
            SET_VECTOR_ELT(nattrib, 0,
                   getAttrib(ans, R_NamesSymbol));
            setAttrib(ans, R_DimNamesSymbol, nattrib);
            setAttrib(ans, R_NamesSymbol, R_NilValue);
            UNPROTECT(1);
        }
        }
    }
    } else {
    if (nsubs != length(getAttrib(x, R_DimSymbol)))
        errorcall(call, _("incorrect number of dimensions"));
    if (nsubs == 2)
        ans = MatrixSubset(ax, subs, call, drop);
    else
        ans = ArraySubset(ax, subs, call, drop);
    PROTECT(ans);
    }

    /* Note: we do not coerce back to pair-based lists. */
    /* They are "defunct" in this version of R. */

    if (type == LANGSXP) {
    ax = ans;
    PROTECT(ans = allocList(LENGTH(ax)));
    if ( LENGTH(ax) > 0 )
        SET_TYPEOF(ans, LANGSXP);
    for(px = ans, i = 0 ; px != R_NilValue ; px = CDR(px))
        SETCAR(px, VECTOR_ELT(ax, i++));
    setAttrib(ans, R_DimSymbol, getAttrib(ax, R_DimSymbol));
    setAttrib(ans, R_DimNamesSymbol, getAttrib(ax, R_DimNamesSymbol));
    setAttrib(ans, R_NamesSymbol, getAttrib(ax, R_NamesSymbol));
    SET_NAMED(ans, NAMED(ax)); /* PR#7924 */
    }
    else {
    PROTECT(ans);
    }
    if (ATTRIB(ans) != R_NilValue) {
    setAttrib(ans, R_TspSymbol, R_NilValue);
    if(!IS_S4_OBJECT(x))
        setAttrib(ans, R_ClassSymbol, R_NilValue);
    }
    UNPROTECT(4);
    return ans;
}


/* The [[ subset operator.  It needs to be fast. */
/* The arguments to this call are evaluated on entry. */

SEXP attribute_hidden do_subset2(SEXP call, SEXP op, SEXP args, SEXP rho)
{
    SEXP ans;

    /* If the first argument is an object and there is */
    /* an approriate method, we dispatch to that method, */
    /* otherwise we evaluate the arguments and fall */
    /* through to the generic code below.  Note that */
    /* evaluation retains any missing argument indicators. */

    if(DispatchOrEval(call, op, "[[", args, rho, &ans, 0, 0))
    return(ans);

    /* Method dispatch has failed. */
    /* We now run the generic internal code. */

    return do_subset2_dflt(call, op, ans, rho);
}

SEXP attribute_hidden do_subset2_dflt(SEXP call, SEXP op, SEXP args, SEXP rho)
{
    SEXP ans, dims, dimnames, indx, subs, x;
    int i, ndims, nsubs, offset = 0;
    int drop = 1, pok, exact = -1;
    int named_x;

    PROTECT(args);
    ExtractDropArg(args, &drop);
    /* Is partial matching ok?  When the exact arg is NA, a warning is
       issued if partial matching occurs.
     */
    exact = ExtractExactArg(args);
    if (exact == -1)
    pok = exact;
    else
    pok = !exact;

    x = CAR(args);

    /* This code was intended for compatibility with S, */
    /* but in fact S does not do this.  Will anyone notice? */

    if (x == R_NilValue) {
    UNPROTECT(1);
    return x;
    }

    /* Get the subscripting and dimensioning information */
    /* and check that any array subscripting is compatible. */

    subs = CDR(args);
    if(0 == (nsubs = length(subs)))
    errorcall(call, _("no index specified"));
    dims = getAttrib(x, R_DimSymbol);
    ndims = length(dims);
    if(nsubs > 1 && nsubs != ndims)
    errorcall(call, _("incorrect number of subscripts"));

    /* split out ENVSXP for now */
    if( TYPEOF(x) == ENVSXP ) {
      if( nsubs != 1 || !isString(CAR(subs)) || length(CAR(subs)) != 1 )
    errorcall(call, _("wrong arguments for subsetting an environment"));
      ans = findVarInFrame(x, install(translateChar(STRING_ELT(CAR(subs), 0))));
      if( TYPEOF(ans) == PROMSXP ) {
        PROTECT(ans);
        ans = eval(ans, R_GlobalEnv);
        UNPROTECT(1);
      } else {
        SET_NAMED(ans, 2);
      }

      UNPROTECT(1);
      if(ans == R_UnboundValue )
      return(R_NilValue);
      return(ans);
    }

    /* back to the regular program */
    if (!(isVector(x) || isList(x) || isLanguage(x)))
    errorcall(call, R_MSG_ob_nonsub, type2char(TYPEOF(x)));

    named_x = NAMED(x);  /* x may change below; save this now.  See PR#13411 */
    
    if(nsubs == 1) { /* vector indexing */
    SEXP thesub = CAR(subs);
    int i = -1, len = length(thesub);

    /* new case in 1.7.0, one vector index for a list */
    if(isVectorList(x) && length(CAR(subs)) > 1) {
        for(i = 0; i < len - 1; i++) {
        if(!isVectorList(x))
            errorcall(call, _("recursive indexing failed at level %d\n"), i+1);
        offset = get1index(CAR(subs), getAttrib(x, R_NamesSymbol),
                   length(x), pok, i, call);
        if(offset < 0 || offset >= length(x))
            errorcall(call, _("no such index at level %d\n"), i+1);
        x = VECTOR_ELT(x, offset);
        }
    }
    offset = get1index(CAR(subs), getAttrib(x, R_NamesSymbol),
               length(x), pok, i, call);
    if (offset < 0 || offset >= length(x)) {
        /* a bold attempt to get the same behaviour for $ and [[ */
        if (offset < 0 && (isNewList(x) ||
                   isExpression(x) ||
                   isList(x) ||
                   isLanguage(x))) {
        UNPROTECT(1);
        return R_NilValue;
        }
        else errorcall(call, R_MSG_subs_o_b);
    }
    } else { /* matrix indexing */
    /* Here we use the fact that: */
    /* CAR(R_NilValue) = R_NilValue */
    /* CDR(R_NilValue) = R_NilValue */

    int ndn; /* Number of dimnames. Unlikely to be anything but
            0 or nsubs, but just in case... */

    PROTECT(indx = allocVector(INTSXP, nsubs));
    dimnames = getAttrib(x, R_DimNamesSymbol);
    ndn = length(dimnames);
    for (i = 0; i < nsubs; i++) {
        INTEGER(indx)[i] =
        get1index(CAR(subs), (i < ndn) ? VECTOR_ELT(dimnames, i) :
              R_NilValue,
              INTEGER(indx)[i], pok, -1, call);
        subs = CDR(subs);
        if (INTEGER(indx)[i] < 0 ||
        INTEGER(indx)[i] >= INTEGER(dims)[i])
        errorcall(call, R_MSG_subs_o_b);
    }
    offset = 0;
    for (i = (nsubs - 1); i > 0; i--)
        offset = (offset + INTEGER(indx)[i]) * INTEGER(dims)[i - 1];
    offset += INTEGER(indx)[0];
    UNPROTECT(1);
    }

    if(isPairList(x)) {
    ans = CAR(nthcdr(x, offset));
    if (named_x > NAMED(ans))
        SET_NAMED(ans, named_x);
    } else if(isVectorList(x)) {
    /* did unconditional duplication before 2.4.0 */
    ans = VECTOR_ELT(x, offset);
    if (named_x > NAMED(ans))
        SET_NAMED(ans, named_x);
    } else {
    ans = allocVector(TYPEOF(x), 1);
    switch (TYPEOF(x)) {
    case LGLSXP:
    case INTSXP:
        INTEGER(ans)[0] = INTEGER(x)[offset];
        break;
    case REALSXP:
        REAL(ans)[0] = REAL(x)[offset];
        break;
    case CPLXSXP:
        COMPLEX(ans)[0] = COMPLEX(x)[offset];
        break;
    case STRSXP:
        SET_STRING_ELT(ans, 0, STRING_ELT(x, offset));
        break;
    case RAWSXP:
        RAW(ans)[0] = RAW(x)[offset];
        break;
    default:
        UNIMPLEMENTED_TYPE("do_subset2", x);
    }
    }
    UNPROTECT(1);
    return ans;
}


enum pmatch {
    NO_MATCH,
    EXACT_MATCH,
    PARTIAL_MATCH
};

/* A helper to partially match tags against a candidate.
   Tags are always in the native charset.
 */
/* Returns: */
static
enum pmatch
pstrmatch(SEXP target, SEXP input, int slen)
{
    const char *st = "";

    if(target == R_NilValue)
    return NO_MATCH;

    switch (TYPEOF(target)) {
    case SYMSXP:
    st = CHAR(PRINTNAME(target));
    break;
    case CHARSXP:
    st = translateChar(target);
    break;
    }
    if(strncmp(st, translateChar(input), slen) == 0)
    return (strlen(st) == slen) ?  EXACT_MATCH : PARTIAL_MATCH;
    else return NO_MATCH;
}


/* The $ subset operator.
   We need to be sure to only evaluate the first argument.
   The second will be a symbol that needs to be matched, not evaluated.
*/
SEXP attribute_hidden do_subset3(SEXP call, SEXP op, SEXP args, SEXP env)
{
    SEXP input, nlist, ans;

    checkArity(op, args);

    /* first translate CADR of args into a string so that we can
       pass it down to DispatchorEval and have it behave correctly */
    input = PROTECT(allocVector(STRSXP, 1));

    nlist = CADR(args);
    if(isSymbol(nlist) )
    SET_STRING_ELT(input, 0, PRINTNAME(nlist));
    else if(isString(nlist) )
    SET_STRING_ELT(input, 0, STRING_ELT(nlist, 0));
    else {
    errorcall(call,_("invalid subscript type '%s'"),
          type2char(TYPEOF(nlist)));
    }

    /* replace the second argument with a string */

    /* Previously this was SETCADR(args, input); */
    /* which could cause problems when nlist was */
    /* ..., as in PR#8718 */
    PROTECT(args = CONS(CAR(args), CONS(input, R_NilValue)));

    /* If the first argument is an object and there is */
    /* an approriate method, we dispatch to that method, */
    /* otherwise we evaluate the arguments and fall */
    /* through to the generic code below.  Note that */
    /* evaluation retains any missing argument indicators. */

    if(DispatchOrEval(call, op, "$", args, env, &ans, 0, 0)) {
    UNPROTECT(2);
    return(ans);
    }

    UNPROTECT(2);
    return R_subset3_dflt(CAR(ans), STRING_ELT(input, 0), call);
}

/* used in eval.c */
SEXP attribute_hidden R_subset3_dflt(SEXP x, SEXP input, SEXP call)
{
    SEXP y, nlist;
    int slen;

    PROTECT(x);
    PROTECT(input);

    /* Optimisation to prevent repeated recalculation */
    slen = strlen(translateChar(input));

    /* If this is not a list object we return NULL. */
    /* Or should this be allocVector(VECSXP, 0)? */

    if (isPairList(x)) {
    SEXP xmatch = R_NilValue;
    int havematch;
    UNPROTECT(2);
    havematch = 0;
    for (y = x ; y != R_NilValue ; y = CDR(y)) {
        switch(pstrmatch(TAG(y), input, slen)) {
        case EXACT_MATCH:
        y = CAR(y);
        if (NAMED(x) > NAMED(y)) SET_NAMED(y, NAMED(x));
        return y;
        case PARTIAL_MATCH:
        havematch++;
        xmatch = y;
        break;
        case NO_MATCH:
        break;
        }
    }
    if (havematch == 1) { /* unique partial match */
        if(R_warn_partial_match_dollar) {
        const char *st = "";
        SEXP target = TAG(y);
        switch (TYPEOF(target)) {
        case SYMSXP:
            st = CHAR(PRINTNAME(target));
            break;
        case CHARSXP:
            st = translateChar(target);
            break;
        }
        warningcall(call, _("partial match of '%s' to '%s'"),
                translateChar(input), st);
        }
        y = CAR(xmatch);
        if (NAMED(x) > NAMED(y)) SET_NAMED(y, NAMED(x));
        return y;
    }
    return R_NilValue;
    }
    else if (isVectorList(x)) {
    int i, n, havematch, imatch=-1;
    nlist = getAttrib(x, R_NamesSymbol);
    UNPROTECT(2);
    n = length(nlist);
    havematch = 0;
    for (i = 0 ; i < n ; i = i + 1) {
        switch(pstrmatch(STRING_ELT(nlist, i), input, slen)) {
        case EXACT_MATCH:
        y = VECTOR_ELT(x, i);
        if (NAMED(x) > NAMED(y))
            SET_NAMED(y, NAMED(x));
        return y;
        case PARTIAL_MATCH:
        havematch++;
        if (havematch == 1) {
            /* partial matches can cause aliasing in eval.c:evalseq
               This is overkill, but alternative ways to prevent
               the aliasing appear to be even worse */
            y = VECTOR_ELT(x,i);
            SET_NAMED(y,2);
            SET_VECTOR_ELT(x,i,y);
        }
        imatch = i;
        break;
        case NO_MATCH:
        break;
        }
    }
    if(havematch == 1) { /* unique partial match */
        if(R_warn_partial_match_dollar) {
        const char *st = "";
        SEXP target = STRING_ELT(nlist, imatch);
        switch (TYPEOF(target)) {
        case SYMSXP:
            st = CHAR(PRINTNAME(target));
            break;
        case CHARSXP:
            st = translateChar(target);
            break;
        }
        warningcall(call, _("partial match of '%s' to '%s'"),
                translateChar(input), st);
        }
        y = VECTOR_ELT(x, imatch);
        if (NAMED(x) > NAMED(y)) SET_NAMED(y, NAMED(x));
        return y;
    }
    return R_NilValue;
    }
    else if( isEnvironment(x) ){
    y = findVarInFrame(x, install(translateChar(input)));
    if( TYPEOF(y) == PROMSXP ) {
        PROTECT(y);
        y = eval(y, R_GlobalEnv);
        UNPROTECT(1);
    }
    UNPROTECT(2);
    if( y != R_UnboundValue ) {
        if (NAMED(x) > NAMED(y))
        SET_NAMED(y, NAMED(x));
        return(y);
    }
      return R_NilValue;
    }
    else if( isVectorAtomic(x) ){
    errorcall(call, "$ operator is invalid for atomic vectors");
    }
    else if( IS_S4_OBJECT(x) ){
    errorcall(call, "$ operator not defined for this S4 class");
    }
    else /* e.g. a function */
    errorcall(call, R_MSG_ob_nonsub, type2char(TYPEOF(x)));
    UNPROTECT(2);
    return R_NilValue;
}