# R matrices in C functions

December 13, 2008
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(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;}