tm_map（...、tolower）でテキストを小文字に変換する際のエラー

Question

tm_mapを使ってみました。それは次のエラーを出しました。どうすればこの問題を回避できますか？

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

ベースR機能tolower()使用します。この方法でTOLOWERを使用して

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

Answer 2

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

print(myCorpus[[1]]) 

Answer 3

を望ましくない副作用を持っている：あなたは後でコーパスのうち、長期文書行列を作成しようとした場合、それは失敗するでしょう。これは、tolowerの戻り値の型を処理できないtmの最近の変更が原因です。代わりに、使用：

myCorpus <- tm_map(myCorpus, PlainTextDocument) 

Answer 4

は、ここではより詳細な回答に私のcommentを展開する：あなたはVCorpusオブジェクトを台無しにしないcontent_transformerの内側tolowerをラップする必要がある - のようなもの：

> 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"