Quick conversion of a list of lists into a data frame

Data frames are one of R’s distinguishing features. Exposing a list of lists as an array of cases, they make many formal operations such as regression or optimization easy to represent.

The R data.frame operation for lists is quite slow, in large part because it exposes a vast amount of functionality. This sample shows one way to write a much faster data.frame creator in C++ if one is willing to forego that generality.

#include <Rcpp.h>

using namespace Rcpp;

// [[Rcpp::export]]
List CheapDataFrameBuilder(List a) {
    List returned_frame = clone(a);
    GenericVector sample_row = returned_frame(0);

    StringVector row_names(sample_row.length());
    for (int i = 0; i < sample_row.length(); ++i) {
        char name[5];
        sprintf(&(name[0]), "%d", i);
        row_names(i) = name;
    }
    returned_frame.attr("row.names") = row_names;

    StringVector col_names(returned_frame.length());
    for (int j = 0; j < returned_frame.length(); ++j) {
        char name[6];
        sprintf(&(name[0]), "X.%d", j);
        col_names(j) = name;
    }
    returned_frame.attr("names") = col_names;
    returned_frame.attr("class") = "data.frame";

    return returned_frame;
}

Here is the result of comparing the native function to this version.

library(rbenchmark)
a <- replicate(250, 1:100, simplify=FALSE)

res <- benchmark(as.data.frame(a), 
                 CheapDataFrameBuilder(a), 
                 order="relative", replications=500)
res[,1:4]


                      test replications elapsed relative
2 CheapDataFrameBuilder(a)          500   0.104      1.0
1         as.data.frame(a)          500  16.730    160.9

There are some subtleties in this code:

— It turns out that one can’t send super-large data frames to it because of possible buffer overflows. I’ve never seen that problem when I’ve written Rcpp functions which exchanged SEXPs with R, but this one uses Rcpp:export in order to use sourceCpp.

— Notice the invocation of clone() in the first line of the code. If you don’t do that, you wind up side-effecting the parameter, which is not what most people would expect.