Creating a data.frame in C

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Introduction

data.frame objects are mostly list objects with some added constraints.

In general a data.frame consists of

a list (VECSXP)
each item in the list is a column
each element in the list must have the same length
a character vector of column names (optional, but always included in practice)
an (optional) character vector of row names
the class data.frame

Example: Create a data.frame in C

The following code generates a data.frame within C.

The C code is the equivalent of the following R code:

data.frame(
  idx = 1:10,
  x   = (0:9) +  10.0,
  y   = (0:9) + 100.0
)

#include <R.h>
#include <Rinternals.h>

//~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
// 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(void) {


  //~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  // Each column of the data.frame gets allocated separately
  //~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  int n = 10; // number of rows

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


  //~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  // Assign some dummy values into the columns
  //~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  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
  //~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  setAttrib(df_, R_ClassSymbol, mkString("data.frame"));

  //~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  // Set the names on the data.frame
  //~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  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 data.frame
  // 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_;
}

Click to show R code

code = r"(
#include <R.h>
#include <Rinternals.h>

//~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
// 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(void) {


  //~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  // Each column of the data.frame gets allocated separately
  //~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  int n = 10; // number of rows

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


  //~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  // Assign some dummy values into the columns
  //~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  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
  //~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  setAttrib(df_, R_ClassSymbol, mkString("data.frame"));

  //~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  // Set the names on the data.frame
  //~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  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 data.frame
  // 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_;
}
)"

callme::compile(code)

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

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