R/rot2mat.R

In Monneret/bandsolve: Fast solver for symetric band matrix

#' @useDynLib bandsolve
#' @export
#' 
#' @description Get back a symmetric square matrix based on his rotated row-wised version. 
#' The rotated form of the input is such each column correspond to a diagonal, where the first column is the main diagonal and next ones are the upper/lower-diagonal.
#' To match dimension, last element of these columns are discarded.
#' @title rot2mat
#' @title Get back a symmetric square matrix based on his rotated row-wised version.
#' @param R Rotated matrix.
#' @return Band square matrix.
#' @examples
#' 
#' D0=1:5;
#' D1=c(0,1,0,0);
#' D2=rep(2,3);
#' 
#' A=rot2mat(cbind(D0,c(D1,0),c(D2,0,0)))
#' A
#' mat2rot(rot.mat(cbind(D0,c(D1,0),c(D2,0,0))))
#' 


rot2mat <- function(R){
  N=nrow(R)
  l=ncol(R)
  M=matrix(0,N,N)
  if (l>1){
  for (i in 1:(l-1)){
    if (i==N-1){
      M[-c(1:i),-c(N:(N-i+1))]=R[-c(N:(N-i+1)),i+1]
      M[-c(N:(N-i+1)),-c(1:i)]=R[-c(N:(N-i+1)),i+1]
    } else {
      diag(M[-c(1:i),-c(N:(N-i+1))])=R[-c(N:(N-i+1)),i+1]
      diag(M[-c(N:(N-i+1)),-c(1:i)])=R[-c(N:(N-i+1)),i+1]
    }
  }
  diag(M)=R[,1];
  }
  return(M)
}