stringslider <- function(stri, l) {
  # Returns a vector of unique strings of length l extracted from string
  # works through a sliding window from left to right. Checks if each returned
  # string is of length l.
  #
  # Args:
  #   stri: A string from on which the extraction occurs.
  #   l: length of the extracted strings.
  #
  # Returns:
  #   A vector of extracted strings with length l.
  strle <- nchar(stri)
  i <- 1
  subc <- c()

  while (i + l - 1 <= strle) {
    subc <- c(subc,
      substr(stri,
       start = i,
       stop  = i + l - 1))
   i <- i + 1
  }
  return(subc)
}