Creating a data.frame in C

2020-09-16

simplecall - demo package for calling C code with .Call()

simplecall is a small demo package showing how C code could be included in a package and called with .Call().

This package now includes demo code for:

  • creating a list in C
  • creating a data.frame in C

There are probably more succinct ways of doing this, and if you know such a way, please hit me up on twitter

Installation

You can install from GitHub with:

# install.package('remotes')
remotes::install_github('coolbutuseless/simplecall)

Creating a R List within C

Example code is included to create a list completely within C and return it to R

//~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
// Creating a list within C and returning it to R
//
//  1. Create individual integer/real/whatever vectors
//  2. allocate a VECSXP of the correct size
//  3. assign each member into the list
//  4. create names and assign them to the list
//~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
SEXP create_list_in_c_() {


  //~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  // Each member of the list gets allocated separately
  //~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  int n = 10;

  SEXP idx_ = PROTECT(allocVector(INTSXP , n));
  SEXP x_   = PROTECT(allocVector(REALSXP, n));
  SEXP y_   = PROTECT(allocVector(REALSXP, n));


  //~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  // Assign some dummy values into the members
  //~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  for (int i = 0; i < n; i++) {
    INTEGER(idx_)[i] = i + 1;
    REAL(x_)[i] = i + 10;
    REAL(y_)[i] = i + 100;
  }

  //~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  // Allocate a list with 3 members
  //~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  SEXP list_ = PROTECT(allocVector(VECSXP, 3));

  //~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  // Add members to the list
  //~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  SET_VECTOR_ELT(list_,  0, idx_);
  SET_VECTOR_ELT(list_,  1, x_);
  SET_VECTOR_ELT(list_,  2, y_);

  //~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  // Set the names on the list.
  //~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  SEXP names = PROTECT(allocVector(STRSXP, 3));
  SET_STRING_ELT(names,  0, mkChar("idx"));
  SET_STRING_ELT(names,  1, mkChar("x"));
  SET_STRING_ELT(names,  2, mkChar("y"));
  setAttrib(list_, R_NamesSymbol, names);

  UNPROTECT(5);
  return list_;
}
simplecall::create_list_in_c()
$idx
 [1]  1  2  3  4  5  6  7  8  9 10

$x
 [1] 10 11 12 13 14 15 16 17 18 19

$y
 [1] 100 101 102 103 104 105 106 107 108 109

Creating a R data.frame within C

Example code is included to create a data.frame completely within C and return it to R

//~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
// Creating a data.frame within C and returning it to R
//
//  1. Create individual integer/real/whatever vectors
//  2. allocate a VECSXP of the correct size
//  3. assign each member into the data.frame
//  4. create names and assign them to the data.frame
//  5. set the class to "data.frame"
//  6. set rownames on the data.frame
//~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
SEXP create_data_frame_in_c_() {


  //~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  // Each member of the list gets allocated separately
  //~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  int n = 10;

  SEXP idx_ = PROTECT(allocVector(INTSXP , n));
  SEXP x_   = PROTECT(allocVector(REALSXP, n));
  SEXP y_   = PROTECT(allocVector(REALSXP, n));


  //~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  // Assign some dummy values into the members
  //~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  for (int i = 0; i < n; i++) {
    INTEGER(idx_)[i] = i + 1;
    REAL(x_)[i] = i + 10;
    REAL(y_)[i] = i + 100;
  }

  //~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  // Allocate a data.frame
  //~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  SEXP df_ = PROTECT(allocVector(VECSXP, 3));

  //~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  // Add columns to the data.frame
  //~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  SET_VECTOR_ELT(df_,  0, idx_);
  SET_VECTOR_ELT(df_,  1, x_);
  SET_VECTOR_ELT(df_,  2, y_);

  //~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  // Treat the VECSXP as a data.frame rather than a list
  //~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  SET_CLASS(df_, mkString("data.frame"));

  //~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  // Set the names on the list.
  //~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  SEXP names = PROTECT(allocVector(STRSXP, 3));
  SET_STRING_ELT(names,  0, mkChar("idx"));
  SET_STRING_ELT(names,  1, mkChar("x"));
  SET_STRING_ELT(names,  2, mkChar("y"));
  setAttrib(df_, R_NamesSymbol, names);

  //~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  // Set the row.names on the list.
  // Use the shortcut as used in .set_row_names() in R
  // i.e. set rownames to c(NA_integer, -len) and it will
  // take care of the rest. This is equivalent to rownames(x) <- NULL
  //~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  SEXP rownames = PROTECT(allocVector(INTSXP, 2));
  SET_INTEGER_ELT(rownames, 0, NA_INTEGER);
  SET_INTEGER_ELT(rownames, 1, -n);
  setAttrib(df_, R_RowNamesSymbol, rownames);

  UNPROTECT(6);
  return df_;
}
simplecall::create_data_frame_in_c()
   idx  x   y
1    1 10 100
2    2 11 101
3    3 12 102
4    4 13 103
5    5 14 104
6    6 15 105
7    7 16 106
8    8 17 107
9    9 18 108
10  10 19 109

Similar packages

This is one of a series of small demo packages for
calling other languages from R:

Installation

Acknowledgements

  • R Core for developing and maintaining such a wonderful language.
  • CRAN maintainers, for patiently shepherding packages onto CRAN and maintaining the repository