R/make.ngrams.R

In stylo: Stylometric Multivariate Analyses

# #################################################
# Function for combining single features (words
# or characters) into n-grams, or strings of n elements;
# e.g. character 2-grams of the sentence "This is a sentence"
# are as follows: "th", "hi", "is", "s ", " i", "is", etc.
# Required arguments: (1) name of the vector of words/chars,
# (2) size of the n-gram (default value is 1)
# #################################################

make.ngrams = function(input.text,ngram.size=1) {
  
  
  # sanity check
  if(ngram.size > length(input.text) ) {
    stop("something wrong with your sample: unable to make ",
          ngram.size,"-grams out of ", length(input.text), " element(s)") 
    }
  

    
  # 'stringi' is a great package for text manipulation; suggested by 'stylo';
  # if it is installed, it would be better to take advantage of it.
  # Thus, checking if stri_paste() from the package stringi is available:
  test.stringi = tryCatch(stringi::stri_paste("test", "test" ,sep = " "), 
                          error = function(e) NULL)



  # variable initialization
  txt = c()
  # checking if any n-grams are wanted (i.e. if n-gram size > 1)
  if(ngram.size > 1) {
    txt = input.text
    for(n in 2:ngram.size) {
    # agglutinating subsequent words/chars into n-grams
    # if the package 'stringi' could not be invoked, use paste()
      if(is.null(test.stringi) == TRUE) {
        # regular paste()
        txt = paste(txt[1:(length(txt)-1)],input.text[n:length(input.text)],sep = " ")
      } else {
        # stri_paste() from 'stringi'
        txt = stringi::stri_paste(txt[1:(length(txt)-1)],input.text[n:length(input.text)],sep = " ")
      }
    }
  } else {
  # if n-gram size is set to 1, then nothing will happen
  txt = input.text
  }
return(txt)
}