How to use pi in RcppEigen?

pi, r, rcpp

Solution

First off, grep for `pi` in `/usr/share/R/include/` and you find eg

  #define M_PI        3.141592653589793238462643383280    /* pi */

so that you have where R is used, eg here with Rcpp and RcppEigen.

Example:

R> getpi <- cppFunction('double twopi() { return M_PI; } ')
R> getpi()
[1] 3.142
R> print(getpi(), digits=20)
[1] 3.141592653589793116
R>

I am sure this is also in the math headers. [ Goes checking: Yup, starting in `math.h`. ] Probably multiple times. Grep'ing through other sources can be fruitful too.

Problem

I am very new to `Rcpp`, or more specifically `RcppEigen`, and struggling with how to use `pi` as a constant in my code. The code runs numerous time in a MCMC algorithm, so any speed improvement would be perfect. Currently, I hand pi over every time I call the function, as in the following code: ``` require(RcppEigen) require(inline) I.Cpp <- " using Eigen::Map; using Eigen::MatrixXd; using Eigen::VectorXd; using Rcpp::NumericVector; const Map<MatrixXd> delta(as<Map<MatrixXd> >(delta0)); const Map<VectorXd> d(as<Map<VectorXd> >(DD)); const Rcpp::NumericVector tpi(pie); double pi = tpi[0]; const MatrixXd I = delta.transpose() * d.asDiagonal() * pi * pi; return wrap(I); " I.cpp <- cxxfunction(signature(delta0 = "matrix", DD = "numeric", pie = "numeric"), I.Cpp, plugin = "RcppEigen") delta0 <- matrix(rnorm(25), 5) DD <- rnorm(5) I.cpp(delta0, DD, pi) # this piece of code gets called multiple times? ``` My question: How can I use the constant `pi` within `RcppEigen` without passing it over at every call of `I.cpp`?

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