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"

Ozan

What package is tm_map from? This seems to be dependent on some non-base package. Please consider including library statement for completeness. – Ta

@DanielKrizian : tm_map() is from tm package, and tolower() is from base – Cramped

Answer 1

Use the base R function tolower():

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

Electrolyte

Thanks. But any insight as to why I got that error? I might need to use other tm_map applications! – Ozan

The help file for tm_map (in the package tm) shows a list of usable transformation functions and tolower is not one of them. The transformations appear to be S3 methods that operate on objects of class 'Corpus'. So you can't use just any function with tm_map. – Electrolyte

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"

Chemash

Answer 3

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

print(myCorpus[[1]])

Mckenna

You should wrap tolower inside of content_transformer not to screw up the VCorpus object, like: tm_map(myCorpus, content_transformer(tolower)) – Chemash

@daroczig: Please make that an answer! – Cramped

@Cramped thanks for the idea, I've just submitted the above comment as a new answer below :) – Chemash

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)

Sheerness