相关疑难解决方法(0)

我试过用了tm_map.它给出了以下错误.我怎么能绕过这个？

 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"