Error converting text to lowercase with tm_map(..., tolower)

Question

I tried using the tm_map. It gave the following error. How can I get around this?

 require(tm)
 byword<-tm_map(byword, tolower)

Error in UseMethod("tm_map", x) : 
  no applicable method for 'tm_map' applied to an object of class "character"

Answer 1

Use the base R function tolower():

tolower(c("THE quick BROWN fox"))
# [1] "the quick brown fox"

Answer 2

Expanding my comment to a more detailed answer here: you have to wrap tolower inside of content_transformer not to screw up the VCorpus object -- something like:

> library(tm)
> data('crude')
> crude[[1]]$content
[1] "Diamond Shamrock Corp said that\neffective today it had cut its contract prices for crude oil by\n1.50 dlrs a barrel.\n    The reduction brings its posted price for West Texas\nIntermediate to 16.00 dlrs a barrel, the copany said.\n    \"The price reduction today was made in the light of falling\noil product prices and a weak crude oil market,\" a company\nspokeswoman said.\n    Diamond is the latest in a line of U.S. oil companies that\nhave cut its contract, or posted, prices over the last two days\nciting weak oil markets.\n Reuter"
> tm_map(crude, content_transformer(tolower))[[1]]$content
[1] "diamond shamrock corp said that\neffective today it had cut its contract prices for crude oil by\n1.50 dlrs a barrel.\n    the reduction brings its posted price for west texas\nintermediate to 16.00 dlrs a barrel, the copany said.\n    \"the price reduction today was made in the light of falling\noil product prices and a weak crude oil market,\" a company\nspokeswoman said.\n    diamond is the latest in a line of u.s. oil companies that\nhave cut its contract, or posted, prices over the last two days\nciting weak oil markets.\n reuter"

Answer 3

myCorpus <- Corpus(VectorSource(byword))
myCorpus <- tm_map(myCorpus , tolower)

print(myCorpus[[1]])

Answer 4

using tolower in this way has an undesirable side effect: if you try to create a term document matrix out of the corpus later, it will fail. This is because of a recent change in tm that cannot handle the return type of tolower. Instead, use:

myCorpus <- tm_map(myCorpus, PlainTextDocument)