Convert dense objects to sparse representations
as.sparse(x, ...)
# S3 method for class 'data.frame'
as.sparse(x, row.names = NULL, ...)
# S3 method for class 'Matrix'
as.sparse(x, ...)
# S3 method for class 'matrix'
as.sparse(x, ...)
# S3 method for class 'ngCMatrix'
as.sparse(x, ...)
A sparse representation of the input data