title: PCA for sparse matrix
body: <p>I want to use PCA for a sparse matrix(which represents images),420000*1784 size ,about 2% non-zero entries,and the principal function tells me that there is a lot NA value in its egien values and egien matrix ,what is the reason for that? is there another function to deal with such situations ?Or I need to write a function to do this ?</p>