# R matrices in C functions

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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; }

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