title: incompatible matrix dimensions: 50x1000 and 50x1000 ? why?
body: <p>I'm doing a job with RcppArmadillo and trying to multiply a arma::cx_mat with a arma::mat, they are both size of 50x1000. but it raises a error: 
  error: matrix multiplication: incompatible matrix dimensions: 50x1000 and 50x1000 ?
  why it happens? and what should I do ?</p>