R matrices in C functions

December 13, 2008
By

(This article was first published on Quantitative Ecology, and kindly contributed to R-bloggers)

Using the .C() function in R, you can only pass vectors. Since R stores matrices columnwise as vectors anyhow, they can be passed to your C function as vectors (along with the number of rows in the matrix) and then accessed in familiar [row,col] manner using the following C functions (idea from here):
int Cmatrix(int *row, int *col, int *nrows){
/* Converts row-col notation into base-zero vector notation, based on a column-wise conversion*/
int vector_loc;
vector_loc=(*row)-1 + ((*col)-1)*(*nrows);
return(vector_loc);
}

void Rmatrix(int *vector_loc,int *row, int *col, int *nrows){
/*Converts vector notation into row-col notation*/
/*vector_loc is the vector address of the matrix (base zero)*/
/*row and col are pointers to the row and col variables that will be updated */
*col=floor(*vector_loc / *nrows)+1;
*row=*vector_loc-((*col)-1)*(*nrows)+1;
}

To leave a comment for the author, please follow the link and comment on his blog: Quantitative Ecology.

R-bloggers.com offers daily e-mail updates about R news and tutorials on topics such as: visualization (ggplot2, Boxplots, maps, animation), programming (RStudio, Sweave, LaTeX, SQL, Eclipse, git, hadoop, Web Scraping) statistics (regression, PCA, time series, trading) and more...



If you got this far, why not subscribe for updates from the site? Choose your flavor: e-mail, twitter, RSS, or facebook...

Comments are closed.