R: Calculate cosine distance from a term-document matrix with tm and proxy

Question

I want to calculate the cosine distance among authors of a corpus. Let's take a corpus of 20 documents.

require(tm)
data("crude")
length(crude)
# [1] 20

I want to find out the cosine distance (similarity) among these 20 documents. I create a term-document matrix with

tdm <- TermDocumentMatrix(crude,
                          control = list(removePunctuation = TRUE,
                                         stopwords = TRUE))

then I have to convert it to a matrix to pass it to dist() of the proxy package

tdm <- as.matrix(tdm)
require(proxy)
cosine_dist_mat <- as.matrix(dist(t(tdm), method = "cosine"))

Finally I remove the diagonal of my cosine distance matrix (since I am not interested in the distance between a document and itself) and compute the average distance between each document and the other 19 document of the corpus

diag(cosine_dist_mat) <- NA
cosine_dist <- apply(cosine_dist_mat, 2, mean, na.rm=TRUE)

cosine_dist
# 127       144       191       194 
# 0.6728505 0.6788326 0.7808791 0.8003223 
# 211       236       237       242 
# 0.8218699 0.6702084 0.8752164 0.7553570 
# 246       248       273       349 
# 0.8205872 0.6495110 0.7064158 0.7494145 
# 352       353       368       489 
# 0.6972964 0.7134836 0.8352642 0.7214411 
# 502       543       704       708 
# 0.7294907 0.7170188 0.8522494 0.8726240

So far so good (with small corpora). The problem is that this method doesn't scale well for larger corpora of documents. For once it seems inefficient because of the two calls to as.matrix(), to pass the tdm from tm to proxy and finally to calculate the average.

Is it possible to conceive a smarter way to obtain the same result?

Answer 1

Since tm's term document matrices are just sparse "simple triplet matrices" from the slam package, you could use the functions there to calculate the distances directly from the definition of cosine similarity:

library(slam)
cosine_dist_mat <- 1 - crossprod_simple_triplet_matrix(tdm)/(sqrt(col_sums(tdm^2) %*% t(col_sums(tdm^2))))

This takes advantage of sparse matrix multiplication. In my hands, a tdm with 2963 terms in 220 documents and 97% sparsity took barely a couple of seconds.

I haven't profiled this, so I have no idea if it's any faster than proxy::dist().

NOTE: for this to work, you should not coerce the tdm into a regular matrix, i.e don't do tdm <- as.matrix(tdm).