[Rcpp-devel] Efficient DataFrame access by row & column

[Yan Zhou]

On Feb 20, 2013, at 1:46 AM, Dirk Eddelbuettel <edd at debian.org> wrote:

> 
> On 20 February 2013 at 01:15, Yan Zhou wrote:
> | Hi Dirk,
> 
> I see nothing Rcpp-specific here.  It's more about how to avoid ambiguity
> within C++.  Might make a good question for the C++ tag on StackOverflow.
Yes, exactly. The problem itself is nothing Rcpp specific but the operator implementation was copied directly from Rcpp's proxy header. I will ask it on Stack later.
> 
> Dirk
> 
> | On Feb 20, 2013, at 1:11 AM, Dirk Eddelbuettel <edd at debian.org> wrote:
> | 
> | > 
> | > On 19 February 2013 at 23:46, Ken Williams wrote:
> | > | > From: Dirk Eddelbuettel [mailto:edd at debian.org]
> | > | > You are relying on an implicit conversion here to make df(.,.) a double. The
> | > | > compiler tells you that it is trying sugar operators.  Sugar is good to have in
> | > | > Rcpp, but occassionally there is a cost. This may be one of those times.
> | > | >
> | > | > Try
> | > | >
> | > | >       double a = df(j,i);
> | > | >       double b = df(j,last_i);
> | > | >         if (fabs(a - b) > thresh) {
> | > | >
> | > | > and see if that works.
> | > | 
> | > | Unfortunately no, I get a similar error when I try that:
> | > | 
> | > | window.cpp:68:24: error: conversion from 'Rcpp::Vector<19>::Proxy {aka Rcpp::internal::generic_proxy<19>}' to 'double' is ambiguous
> | > | window.cpp:68:24: note: candidates are:
> | > | C:/mybin/R/R-2.15.2/library/Rcpp/include/Rcpp/vector/string_proxy.h:273:4: note: Rcpp::internal::generic_proxy<RTYPE>::operator int() const [with int RTYPE = 19]
> | > | C:/mybin/R/R-2.15.2/library/Rcpp/include/Rcpp/vector/string_proxy.h:272:4: note: Rcpp::internal::generic_proxy<RTYPE>::operator bool() const [with int RTYPE = 19]
> | > | C:/mybin/R/R-2.15.2/library/Rcpp/include/Rcpp/vector/string_proxy.h:267:26: note: Rcpp::internal::generic_proxy<RTYPE>::operator U() const [with U = double, int RTYPE = 19]
> | > 
> | > Maybe try std::fabs() instead of fabs() ?
> | > 
> | > That is one of the reasons I do not like a global 'using namespace Rcpp' in
> | > my code.  Explicit namespaces are, well, explicit.
> | > 
> | > Dirk
> | > | 
> | > | 
> | > | >
> | > | > Not every conceivable operation is implemented in Rcpp,
> | > | 
> | > | Of course. =)
> | > | 
> | > | > but we are always
> | > | > open to patches to make it more complete.
> | > | 
> | > | If I can get myself up to speed with my understanding, I'll be happy to contribute what I can.
> | > | 
> | > |  -Ken
> | > | 
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> | > 
> | > -- 
> | > Dirk Eddelbuettel | edd at debian.org | http://dirk.eddelbuettel.com  
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> 
> -- 
> Dirk Eddelbuettel | edd at debian.org | http://dirk.eddelbuettel.com