R/vroom_lines.R

In vroom: Read and Write Rectangular Text Data Quickly

#' Read lines from a file
#'
#' `vroom_lines()` is similar to `readLines()`, however it reads the lines
#' lazily like [vroom()], so operations like `length()`, `head()`, `tail()` and `sample()`
#' can be done much more efficiently without reading all the data into R.
#' @inheritParams vroom
#' @examples
#' lines <- vroom_lines(vroom_example("mtcars.csv"))
#'
#' length(lines)
#' head(lines, n = 2)
#' tail(lines, n = 2)
#' sample(lines, size = 2)
#' @export
vroom_lines <- function(
  file,
  n_max = Inf,
  skip = 0,
  na = character(),
  skip_empty_rows = FALSE,
  locale = default_locale(),
  altrep = TRUE,
  num_threads = vroom_threads(),
  progress = vroom_progress()
) {
  check_number_decimal(n_max)

  file <- standardise_path(file)

  if (!is_ascii_compatible(locale$encoding)) {
    file <- reencode_file(file, locale$encoding)
    locale$encoding <- "UTF-8"
  }

  if (n_max < 0 || is.infinite(n_max)) {
    n_max <- -1
  }

  if (length(file) == 0 || n_max == 0) {
    return(character())
  }

  col_select <- quo(NULL)

  # delim = "\1" sets the delimiter to be start of header, which should never
  # appear in modern text. This essentially means the only record breaks will
  # be newlines. Ideally this would be "\0", but R doesn't let you have nulls
  # in character vectors.
  out <- vroom_(
    file,
    delim = "\1",
    col_names = "V1",
    col_types = cols(col_character()),
    id = NULL,
    skip = skip,
    col_select = col_select,
    name_repair = "minimal",
    na = na,
    quote = "",
    trim_ws = FALSE,
    escape_double = FALSE,
    escape_backslash = FALSE,
    comment = "",
    skip_empty_rows = skip_empty_rows,
    locale = locale,
    guess_max = 0,
    n_max = n_max,
    altrep = vroom_altrep(altrep),
    num_threads = num_threads,
    progress = progress
  )
  if (length(out) == 0) {
    return(character())
  }

  out[[1]]
}