Inverse document-frequency scaling matrix
This function creates an inverse-document-frequency (IDF)
scaling matrix from a document-term matrix. The IDF is defined as follows:
idf = log(# documents in the corpus) / (# documents where the term
appears + 1)
get_idf(dtm, log_scale = log, smooth_idf = TRUE)
a document-term matrix of class
logicalsmooth IDF weights by adding one to document frequencies, as if an extra document was seen containing every term in the collection exactly once. This prevents division by zero.
ddiMatrix IDF scaling diagonal sparse matrix.