comparison - Simple Comparing of two texts in R

Question

I want to compare two texts to similarity, therefore i need a simple function to list clearly and chronologically the words and phrases occurring in both texts. these words/sentences should be highlighted or underlined for better visualization)

on the base of @joris Meys ideas, i added an array to divide text into sentences and subordinate sentences.

this is how it looks like:

  textparts <- function (text){
  textparts <- c("\,", "\.")
  i <- 1
  while(i<=length(textparts)){
        text <- unlist(strsplit(text, textparts[i]))
        i <- i+1
  }
  return (text)
}

textparts1 <- textparts("This is a complete sentence, whereas this is a dependent clause. This thing works.")
textparts2 <- textparts("This could be a sentence, whereas this is a dependent clause. Plagiarism is not cool. This thing works.")

  commonWords <- intersect(textparts1, textparts2)
  commonWords <- paste("\<(",commonWords,")\>",sep="")


  for(x in commonWords){
    textparts1 <- gsub(x, "\1*", textparts1,ignore.case=TRUE)
    textparts2 <- gsub(x, "\1*", textparts2,ignore.case=TRUE)
  }
  return(list(textparts1,textparts2))

However, sometimes it works, sometimes it doesn't.

I WOULD like to have results like these:

>   return(list(textparts1,textparts2))
[[1]]
[1] "This is a complete sentence"         " whereas this is a dependent clause*" " This thing works*"                  

[[2]]
[1] "This could be a sentence"            " whereas this is a dependent clause*" " Plagiarism is not cool"             " This thing works*"           

whereas i get none results.

Answer 1

There are some problems with the answer of @Chase :

• differences in capitalization are not taken into account
• interpunction can mess up results
• if there is more than one word similar, then you get a lot of warnings due to the gsub call.

Based on his idea, there is the following solution that makes use of tolower() and some nice functionalities of regular expressions :

compareSentences <- function(sentence1, sentence2) {
  # split everything on "not a word" and put all to lowercase
  x1 <- tolower(unlist(strsplit(sentence1, "\W")))
  x2 <- tolower(unlist(strsplit(sentence2, "\W")))

  commonWords <- intersect(x1, x2)
  #add word beginning and ending and put words between ()
  # to allow for match referencing in gsub
  commonWords <- paste("\<(",commonWords,")\>",sep="")


  for(x in commonWords){ 
    # replace the match by the match with star added
    sentence1 <- gsub(x, "\1*", sentence1,ignore.case=TRUE)
    sentence2 <- gsub(x, "\1*", sentence2,ignore.case=TRUE)
  }
  return(list(sentence1,sentence2))      
}

This gives following result :

text1 <- "This is a test. Weather is fine"
text2 <- "This text is a test. This weather is fine. This blabalba This "

compareSentences(text1,text2)
[[1]]
[1] "This* is* a* test*. Weather* is* fine*"

[[2]]
[1] "This* text is* a* test*. This* weather* is* fine*. This* blabalba This* "