R: tm Textmining package: Doc-Level metadata generation is slow - performance

Question

I have a list of documents to process, and for each record I want to attach some metadata to the document "member" inside the "corpus" data structure that tm, the R package, generates (from reading in text files).
This for-loop works but it is very slow,
Performance seems to degrade as a function f ~ 1/n_docs.
for (i in seq(from= 1, to=length(corpus), by=1)){
if(opts$options$verbose == TRUE || i %% 50 == 0){
print(paste(i, " ", substr(corpus[[i]], 1, 140), sep = " "))
}
DublinCore(corpus[[i]], "title") = csv[[i,10]]
DublinCore(corpus[[i]], "Publisher" ) = csv[[i,16]] #institutions
}
This may do something to the corpus variable but I don't know what.
But when I put it inside a tm_map() (similar to lapply() function), it runs much faster, but the changes are not made persistent:
i = 0
corpus = tm_map(corpus, function(x){
i <<- i + 1
if(opts$options$verbose == TRUE){
print(paste(i, " ", substr(x, 1, 140), sep = " "))
}
meta(x, tag = "Heading") = csv[[i,10]]
meta(x, tag = "publisher" ) = csv[[i,16]]
})
Variable corpus has empty metadata fields after exiting the tm_map function. It should be filled. I have a few other things to do with the collection.
The R documentation for the meta() function says this:
Examples:
data("crude")
meta(crude[[1]])
DublinCore(crude[[1]])
meta(crude[[1]], tag = "Topics")
meta(crude[[1]], tag = "Comment") <- "A short comment."
meta(crude[[1]], tag = "Topics") <- NULL
DublinCore(crude[[1]], tag = "creator") <- "Ano Nymous"
DublinCore(crude[[1]], tag = "Format") <- "XML"
DublinCore(crude[[1]])
meta(crude[[1]])
meta(crude)
meta(crude, type = "corpus")
meta(crude, "labels") <- 21:40
meta(crude)
I tried many of these calls (with var "corpus" instead of "crude"), but they do not seem to work.
Someone else once seemed to have had the same problem with a similar data set (forum post from 2009, no response)

Answer 1

Here's a bit of benchmarking...
With the for loop :
expr.for <- function() {
for (i in seq(from= 1, to=length(corpus), by=1)){
DublinCore(corpus[[i]], "title") = LETTERS[round(runif(26))]
DublinCore(corpus[[i]], "Publisher" ) = LETTERS[round(runif(26))]
}
}
microbenchmark(expr.for())
# Unit: milliseconds
# expr min lq median uq max
# 1 expr.for() 21.50504 22.40111 23.56246 23.90446 70.12398
With tm_map :
corpus <- crude
expr.map <- function() {
tm_map(corpus, function(x) {
meta(x, "title") = LETTERS[round(runif(26))]
meta(x, "Publisher" ) = LETTERS[round(runif(26))]
x
})
}
microbenchmark(expr.map())
# Unit: milliseconds
# expr min lq median uq max
# 1 expr.map() 5.575842 5.700616 5.796284 5.886589 8.753482
So the tm_map version, as you noticed, seems to be about 4 times faster.
In your question you say that the changes in the tm_map version are not persistent, it is because you don't return x at the end of your anonymous function. In the end it should be :
meta(x, tag = "Heading") = csv[[i,10]]
meta(x, tag = "publisher" ) = csv[[i,16]]
x