Error in text mining: "replacement has length zero" & "number of items to replace is not a multiple of replacement length"

Question

I was trying to extract multi-words from text using for loop.

The following line of code gives me an error which says replacement has length zero and number of items to replace is not a multiple of replacement length. To make it clear my question, consider the following situation.

library(tm)
library(stringr)
library(stringi)
mydata<-data.frame(id=c(1,2,3), 
          text=c("This is text mining exercise","Text analysis is bit confusing","Hint on this text 
          analysis?")) 
multiwords<-c("text","analysis","bit confusing")
txt<- freq<- list() 
for(i in 1:length(mydata$id)){ 
    txt[i]<-str_extract_all(mydata[i,], paste0(multiwords,collapse = "|")) freq[i]<-table(txt[i])
}

Note that every terms in multiwords does not necessarily appear at each iteration.

Answer 1

If we need the table on the entire extracted elements, use str_extract_all on the 'text' column after pasteing the 'multiwords' as pattern, then unlist the list and get the table

library(stringr)
lst1 <- str_extract_all(mydata$text, str_c(multiwords, collapse="|"))
table(unlist(lst1))
#    analysis bit confusing          text 
#           2             1             2 

If we need to apply table on each element of list

lapply(lst1, table)
#[[1]]

#text 
#   1 

#[[2]]

#     analysis bit confusing 
#            1             1 

#[[3]]

#analysis     text 
#       1        1