[Rcpp-devel] List of Lists to List of Vectors

wrote:

Check here
<https://github.com/RevolutionAnalytics/rmr2/blob/master/pkg/src/t-list.cpp>
for something similar to Tim's solution that preallocates all vectors to
avoid the costly push_back. Still needs the unlists in R, so it's expensive
in that dimension, the number of lists in the output.

Antonio

On Sun, May 3, 2015 at 4:22 PM, Tim Keitt <tkeitt at utexas.edu> wrote:

A slightly improved version:

library(Rcpp)

code = '
SEXP test(List a)
{
  auto l = Rf_length(a[0]);
  using svec = std::vector<SEXP>;
  std::vector<svec> x(l);
  for (List b : a)
  {
    if (b.size() != l)
      stop("Ragged input");
    for (int i = 0; i != l; ++i)
      x[i].push_back(b[i]);
  }
  return wrap(x);
}'

f = cppFunction(code = code, plugins = "cpp11")

res = lapply(f(list(list(T, 1, 'a'),
                    list(F, 2, 'b'))),
             unlist)


On Sun, May 3, 2015 at 11:57 AM, Tim Keitt <tkeitt at utexas.edu> wrote:

Here's a really bare-bones version. Not very pretty or complete, but it
does do the job. Could the 'unlist' part be converted to Rcpp?

THK

library(Rcpp)

code = '
SEXP test(List a)
{
  int l = Rf_length(a[0]);
  typedef std::vector<SEXP> svec;
  std::vector<svec> x(l);
  for (int i = 0; i != l; ++i)
    for (int j = 0; j != a.size(); ++j)
    {
      List b = a[j];
      x[i].push_back(b[i]);
    }
  return wrap(x);
}'

f = cppFunction(code = code)

res = lapply(f(list(list(1, 'a'), list(2, 'b'))), unlist)


On Sat, May 2, 2015 at 1:18 PM, Dirk Eddelbuettel <edd at debian.org>
wrote:

On 2 May 2015 at 10:49, William Dunlap wrote:
| Since translation from R to Rcpp is "seamless" I will leave that to
you.

One problem is with the strongly typed nature of C++.  Rearranging
dynamicly
growing objects can be done.  I think I used Boost's variant type a few
years.  I am sure there are other possibilities.  We should collect a
few and
compare.   But in C++ please :)

Dirk

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