AMR/R/freq.R

# ==================================================================== #
# TITLE                                                                #
# Antimicrobial Resistance (AMR) Analysis                              #
#                                                                      #
# AUTHORS                                                              #
# Berends MS (m.s.berends@umcg.nl), Luz CF (c.f.luz@umcg.nl)           #
#                                                                      #
# LICENCE                                                              #
# This program is free software; you can redistribute it and/or modify #
# it under the terms of the GNU General Public License version 2.0,    #
# as published by the Free Software Foundation.                        #
#                                                                      #
# This program is distributed in the hope that it will be useful,      #
# but WITHOUT ANY WARRANTY; without even the implied warranty of       #
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the        #
# GNU General Public License for more details.                         #
# ==================================================================== #

#' Frequency table
#'
#' Create a frequency table of a vector with items or a data frame. Supports quasiquotation and markdown for reports. \code{top_freq} can be used to get the top/bottom \emph{n} items of a frequency table, with counts as names.
#' @param x vector of any class or a \code{\link{data.frame}}, \code{\link{tibble}} or \code{\link{table}}
#' @param ... up to nine different columns of \code{x} when \code{x} is a \code{data.frame} or \code{tibble}, to calculate frequencies from - see Examples
#' @param sort.count sort on count, i.e. frequencies. This will be \code{TRUE} at default for everything except for factors.
#' @param nmax number of row to print. The default, \code{15}, uses \code{\link{getOption}("max.print.freq")}. Use \code{nmax = 0}, \code{nmax = Inf}, \code{nmax = NULL} or \code{nmax = NA} to print all rows.
#' @param na.rm a logical value indicating whether \code{NA} values should be removed from the frequency table. The header (if set) will always print the amount of \code{NA}s.
#' @param row.names a logical value indicating whether row indices should be printed as \code{1:nrow(x)}
#' @param markdown a logical value indicating whether the frequency table should be printed in markdown format. This will print all rows and is default behaviour in non-interactive R sessions (like when knitting RMarkdown files).
#' @param digits how many significant digits are to be used for numeric values in the header (not for the items themselves, that depends on \code{\link{getOption}("digits")})
#' @param quote a logical value indicating whether or not strings should be printed with surrounding quotes
#' @param header a logical value indicating whether an informative header should be printed
#' @param title text to show above frequency table, at default to tries to coerce from the variables passed to \code{x}
#' @param na a character string to should be used to show empty (\code{NA}) values (only useful when \code{na.rm = FALSE})
#' @param sep a character string to separate the terms when selecting multiple columns
#' @param f a frequency table
#' @param n number of top \emph{n} items to return, use -n for the bottom \emph{n} items. It will include more than \code{n} rows if there are ties.
#' @details Frequency tables (or frequency distributions) are summaries of the distribution of values in a sample. With the `freq` function, you can create univariate frequency tables. Multiple variables will be pasted into one variable, so it forces a univariate distribution. This package also has a vignette available to explain the use of this function further, run \code{browseVignettes("AMR")} to read it.
#'
#' For numeric values of any class, these additional values will all be calculated with \code{na.rm = TRUE} and shown into the header:
#' \itemize{
#'   \item{Mean, using \code{\link[base]{mean}}}
#'   \item{Standard Deviation, using \code{\link[stats]{sd}}}
#'   \item{Coefficient of Variation (CV), the standard deviation divided by the mean}
#'   \item{Mean Absolute Deviation (MAD), using \code{\link[stats]{mad}}}
#'   \item{Tukey Five-Number Summaries (minimum, Q1, median, Q3, maximum), using \code{\link[stats]{fivenum}}}
#'   \item{Interquartile Range (IQR) calculated as \code{Q3 - Q1} using the Tukey Five-Number Summaries, i.e. \strong{not} using the \code{\link[stats]{quantile}} function}
#'   \item{Coefficient of Quartile Variation (CQV, sometimes called coefficient of dispersion), calculated as \code{(Q3 - Q1) / (Q3 + Q1)} using the Tukey Five-Number Summaries}
#'   \item{Outliers (total count and unique count), using \code{\link[grDevices]{boxplot.stats}}}
#' }
#'
#' For dates and times of any class, these additional values will be calculated with \code{na.rm = TRUE} and shown into the header:
#' \itemize{
#'   \item{Oldest, using \code{\link{min}}}
#'   \item{Newest, using \code{\link{max}}, with difference between newest and oldest}
#'   \item{Median, using \code{\link[stats]{median}}, with percentage since oldest}
#' }
#'
#'
#' The function \code{top_freq} uses \code{\link[dplyr]{top_n}} internally and will include more than \code{n} rows if there are ties.
#' @importFrom stats fivenum sd mad
#' @importFrom grDevices boxplot.stats
#' @importFrom dplyr %>% select pull n_distinct group_by arrange desc mutate summarise n_distinct tibble
#' @importFrom utils browseVignettes installed.packages
#' @importFrom hms is.hms
#' @importFrom crayon red green silver
#' @keywords summary summarise frequency freq
#' @rdname freq
#' @name freq
#' @return A \code{data.frame} with an additional class \code{"frequency_tbl"}
#' @export
#' @examples
#' library(dplyr)
#'
#' # this all gives the same result:
#' freq(septic_patients$hospital_id)
#' freq(septic_patients[, "hospital_id"])
#' septic_patients$hospital_id %>% freq()
#' septic_patients[, "hospital_id"] %>% freq()
#' septic_patients %>% freq("hospital_id")
#' septic_patients %>% freq(hospital_id)  #<- easiest to remember when you're used to tidyverse
#'
#' # you could also use `select` or `pull` to get your variables
#' septic_patients %>%
#'   filter(hospital_id == "A") %>%
#'   select(mo) %>%
#'   freq()
#'
#' # multiple selected variables will be pasted together
#' septic_patients %>%
#'   left_join_microorganisms %>%
#'   filter(hospital_id == "A") %>%
#'   freq(genus, species)
#'
#' # get top 10 bugs of hospital A as a vector
#' septic_patients %>%
#'   filter(hospital_id == "A") %>%
#'   freq(mo) %>%
#'   top_freq(10)
#'
#' # save frequency table to an object
#' years <- septic_patients %>%
#'   mutate(year = format(date, "%Y")) %>%
#'   freq(year)
#'
#' # show only the top 5
#' years %>% print(nmax = 5)
#'
#' # save to an object with formatted percentages
#' years <- format(years)
#'
#' # print a histogram of numeric values
#' septic_patients %>%
#'   freq(age) %>%
#'   hist()
#'
#' # or print all points to a regular plot
#' septic_patients %>%
#'   freq(age) %>%
#'   plot()
#'
#' # transform to a data.frame or tibble
#' septic_patients %>%
#'   freq(age) %>%
#'   as.data.frame()
#'
#' # or transform (back) to a vector
#' septic_patients %>%
#'   freq(age) %>%
#'   as.vector()
#'
#' identical(septic_patients %>%
#'             freq(age) %>%
#'             as.vector() %>%
#'             sort(),
#'           sort(septic_patients$age)) # TRUE
#'
#' # it also supports `table` objects:
#' table(septic_patients$gender,
#'       septic_patients$age) %>%
#'   freq(sep = " **sep** ")
#'
#' # check differences between frequency tables
#' diff(freq(septic_patients$trim),
#'      freq(septic_patients$trsu))
frequency_tbl <- function(x,
                          ...,
                          sort.count = TRUE,
                          nmax = getOption("max.print.freq"),
                          na.rm = TRUE,
                          row.names = TRUE,
                          markdown = !interactive(),
                          digits = 2,
                          quote = FALSE,
                          header = !markdown,
                          title = NULL,
                          na = "<NA>",
                          sep = " ") {

  mult.columns <- 0

  x.name <- NULL
  cols <- NULL
  if (any(class(x) == 'list')) {
    cols <- names(x)
    x <- as.data.frame(x, stringsAsFactors = FALSE)
    x.name <- "a list"
  } else if (any(class(x) == 'matrix')) {
    x <- as.data.frame(x, stringsAsFactors = FALSE)
    x.name <- "a matrix"
    cols <- colnames(x)
    if (all(cols %like% 'V[0-9]')) {
      cols <- NULL
    }
  }

  if (any(class(x) == 'data.frame')) {
    if (is.null(x.name)) {
      x.name <- deparse(substitute(x))
    }
    if (x.name == ".") {
      x.name <- NULL
    }
    dots <- base::eval(base::substitute(base::alist(...)))
    ndots <- length(dots)

    if (ndots < 10) {
      cols <- as.character(dots)
      if (!all(cols %in% colnames(x))) {
        stop("one or more columns not found: `", paste(cols, collapse = "`, `"), '`', call. = FALSE)
      }
      if (length(cols) > 0) {
        x <- x[, cols]
      }
    } else if (ndots >= 10) {
      stop('A maximum of 9 columns can be analysed at the same time.', call. = FALSE)
    } else {
      cols <- NULL
    }
  } else if (any(class(x) == 'table')) {
    x <- as.data.frame(x, stringsAsFactors = FALSE)
    # now this DF contains 3 columns: the 2 vars and a Freq column
    # paste the first 2 cols and repeat them Freq times:
    x <- rep(x = do.call(paste, c(x[colnames(x)[1:2]], sep = sep)),
             times = x$Freq)
    x.name <- "a `table` object"
    cols <- NULL
    #mult.columns <- 2
  } else {
    x.name <- NULL
    cols <- NULL
  }

  if (!is.null(ncol(x))) {
    if (ncol(x) == 1 & any(class(x) == 'data.frame')) {
      x <- x %>% pull(1)
    } else if (ncol(x) < 10) {
      mult.columns <- ncol(x)
      x <- do.call(paste, c(x[colnames(x)], sep = sep))
    } else {
      stop('A maximum of 9 columns can be analysed at the same time.', call. = FALSE)
    }
  }

  if (mult.columns > 1) {
    NAs <- x[is.na(x) | x == trimws(strrep('NA ', mult.columns))]
  } else {
    NAs <- x[is.na(x)]
  }

  if (na.rm == TRUE) {
    x_class <- class(x)
    x <- x[!x %in% NAs]
    class(x) <- x_class
  }

  if (sort.count == FALSE & 'factor' %in% class(x)) {
    # warning("Sorting a factor sorts on factor level, not necessarily alphabetically.", call. = FALSE)
  }

  header_txt <- character(0)

  markdown_line <- ''
  if (markdown == TRUE) {
    markdown_line <- '\n'
  }
  x_align <- 'l'

  if (mult.columns > 0) {
    header_txt <- header_txt %>% paste0(markdown_line, 'Columns:   ', mult.columns)
  } else {
    header_txt <- header_txt %>% paste0(markdown_line, 'Class:     ', class(x) %>% rev() %>% paste(collapse = " > "))
    if (!mode(x) %in% class(x)) {
      header_txt <- header_txt %>% paste0(silver(paste0(" (", mode(x), ")")))
    }
  }

  na_txt <- paste0(NAs %>% length() %>% format(), ' = ',
                   (NAs %>% length() / (NAs %>% length() + x %>% length())) %>% percent(force_zero = TRUE, round = digits) %>%
                     sub('NaN', '0', ., fixed = TRUE))
  if (!na_txt %like% "^0 =") {
    na_txt <- red(na_txt)
  } else {
    na_txt <- green(na_txt)
  }
  header_txt <- header_txt %>% paste0(markdown_line, '\nLength:    ', (NAs %>% length() + x %>% length()) %>% format(),
                                      ' (of which NA: ', na_txt, ')')
  header_txt <- header_txt %>% paste0(markdown_line, '\nUnique:    ', x %>% n_distinct() %>% format())

  if (NROW(x) > 0 & any(class(x) == "character")) {
    header_txt <- header_txt %>% paste0('\n')
    header_txt <- header_txt %>% paste0(markdown_line, '\nShortest:  ', x %>% base::nchar() %>% base::min(na.rm = TRUE))
    header_txt <- header_txt %>% paste0(markdown_line, '\nLongest:   ', x %>% base::nchar() %>% base::max(na.rm = TRUE))
  }

  if (NROW(x) > 0 & any(class(x) == "difftime")) {
    header_txt <- header_txt %>% paste0('\n')
    header_txt <- header_txt %>% paste(markdown_line, '\nUnits:    ', attributes(x)$units)
    x <- as.double(x)
    # after this, the numeric header_txt continues
  }

  if (NROW(x) > 0 & any(class(x) %in% c('double', 'integer', 'numeric', 'raw', 'single'))) {
    # right align number
    Tukey_five <- stats::fivenum(x, na.rm = TRUE)
    x_align <- 'r'
    header_txt <- header_txt %>% paste0('\n')
    header_txt <- header_txt %>% paste(markdown_line, '\nMean:     ', x %>% base::mean(na.rm = TRUE) %>% format(digits = digits))
    header_txt <- header_txt %>% paste0(markdown_line, '\nStd. dev.: ', x %>% stats::sd(na.rm = TRUE) %>% format(digits = digits),
                                        ' (CV: ', x %>% cv(na.rm = TRUE) %>% format(digits = digits),
                                        ', MAD: ', x %>% stats::mad(na.rm = TRUE) %>% format(digits = digits), ')')
    header_txt <- header_txt %>% paste0(markdown_line, '\nFive-Num:  ', Tukey_five %>% format(digits = digits) %>% trimws() %>% paste(collapse = ' | '),
                                        ' (IQR: ', (Tukey_five[4] - Tukey_five[2]) %>% format(digits = digits),
                                        ', CQV: ', x %>% cqv(na.rm = TRUE) %>% format(digits = digits), ')')
    outlier_length <- length(boxplot.stats(x)$out)
    header_txt <- header_txt %>% paste0(markdown_line, '\nOutliers:  ', outlier_length)
    if (outlier_length > 0) {
      header_txt <- header_txt %>% paste0(' (unique count: ', boxplot.stats(x)$out %>% n_distinct(), ')')
    }
  }
  if (NROW(x) > 0 & any(class(x) == "rsi")) {
    header_txt <- header_txt %>% paste0('\n')
    cnt_S <- sum(x == "S", na.rm = TRUE)
    cnt_IR <- sum(x %in% c("I", "R"), na.rm = TRUE)
    header_txt <- header_txt %>% paste(markdown_line, '\n%IR:      ',
                                       (cnt_IR / sum(!is.na(x), na.rm = TRUE)) %>% percent(force_zero = TRUE, round = digits),
                                       paste0('(ratio S : IR = 1.0 : ', (cnt_IR / cnt_S) %>% format(digits = 1, nsmall = 1), ")"))
    if (NROW(x) < 30) {
      header_txt <- header_txt %>% paste(markdown_line, red('\nToo few isolates for reliable resistance interpretation.'))
    }
  }

  formatdates <- "%e %B %Y" # = d mmmm yyyy
  if (is.hms(x)) {
    x <- x %>% as.POSIXlt()
    formatdates <- "%H:%M:%S"
  }
  if (NROW(x) > 0 & any(class(x) %in% c('Date', 'POSIXct', 'POSIXlt'))) {
    header_txt <- header_txt %>% paste0('\n')
    mindate <- x %>% min(na.rm = TRUE)
    maxdate <- x %>% max(na.rm = TRUE)
    maxdate_days <- difftime(maxdate, mindate, units = 'auto') %>% as.double()
    mediandate <- x %>% median(na.rm = TRUE)
    median_days <- difftime(mediandate, mindate, units = 'auto') %>% as.double()

    if (formatdates == "%H:%M:%S") {
      # hms
      header_txt <- header_txt %>% paste0(markdown_line, '\nEarliest:  ', mindate %>% format(formatdates) %>% trimws())
      header_txt <- header_txt %>% paste0(markdown_line, '\nLatest:    ', maxdate %>% format(formatdates) %>% trimws(),
                                          ' (+', difftime(maxdate, mindate, units = 'mins') %>% as.double() %>% format(digits = digits), ' min.)')
    } else {
      # other date formats
      header_txt <- header_txt %>% paste0(markdown_line, '\nOldest:    ', mindate %>% format(formatdates) %>% trimws())
      header_txt <- header_txt %>% paste0(markdown_line, '\nNewest:    ', maxdate %>% format(formatdates) %>% trimws(),
                                          ' (+', difftime(maxdate, mindate, units = 'auto') %>% as.double() %>% format(digits = digits), ')')
    }
    header_txt <- header_txt %>% paste0(markdown_line, '\nMedian:    ', mediandate %>% format(formatdates) %>% trimws(),
                                        ' (~', percent(median_days / maxdate_days, round = 0), ')')
  }
  if (any(class(x) == 'POSIXlt')) {
    x <- x %>% format(formatdates)
  }

  nmax.set <- !missing(nmax)
  if (!nmax.set & is.null(nmax) & is.null(base::getOption("max.print.freq", default = NULL))) {
    # default for max print setting
    nmax <- 15
  } else if (is.null(nmax)) {
    nmax <- length(x)
  }

  if (nmax %in% c(0, Inf, NA, NULL)) {
    nmax <- length(x)
  }

  # create table with counts and percentages
  column_names <- c('Item', 'Count', 'Percent', 'Cum. Count', 'Cum. Percent')
  column_names_df <- c('item', 'count', 'percent', 'cum_count', 'cum_percent')


  df <- tibble(item = x) %>%
    group_by(item) %>%
    summarise(count = n())
  column_align <- c(x_align, 'r', 'r', 'r', 'r')

  if (df$item %>% paste(collapse = ',') %like% '\033') {
    # remove escape char
    # see https://en.wikipedia.org/wiki/Escape_character#ASCII_escape_character
    df <- df %>% mutate(item = item %>% gsub('\033', ' ', ., fixed = TRUE))
  }

  # sort according to setting
  if (sort.count == TRUE) {
    df <- df %>% arrange(desc(count), item)
  } else {
    df <- df %>% arrange(item)
  }

  if (quote == TRUE) {
    df$item <- paste0('"', df$item, '"')
  }

  df <- as.data.frame(df, stringsAsFactors = FALSE)

  df$percent <- df$count / base::sum(df$count, na.rm = TRUE)
  df$cum_count <- base::cumsum(df$count)
  df$cum_percent <- df$cum_count / base::sum(df$count, na.rm = TRUE)

  colnames(df) <- column_names_df

  if (markdown == TRUE) {
    tbl_format <- 'markdown'
  } else {
    tbl_format <- 'pandoc'
  }

  if (!is.null(title)) {
    x.name <- trimws(gsub("^Frequency table of", "", title[1L], ignore.case = TRUE))
    cols <- NULL
  }

  structure(.Data = df,
            class = c('frequency_tbl', class(df)),
            opt = list(data = x.name,
                       vars = cols,
                       header = header,
                       header_txt = header_txt,
                       row_names = row.names,
                       column_names = column_names,
                       column_align = column_align,
                       tbl_format = tbl_format,
                       na = na,
                       nmax = nmax,
                       nmax.set = nmax.set))
}

#' @rdname freq
#' @export
freq <- frequency_tbl

#' @rdname freq
#' @export
#' @importFrom dplyr top_n pull
top_freq <- function(f, n) {
  if (!'frequency_tbl' %in% class(f)) {
    stop('top_freq can only be applied to frequency tables', call. = FALSE)
  }
  if (!is.numeric(n) | length(n) != 1L) {
    stop('For top_freq, `nmax` must be a number of length 1', call. = FALSE)
  }
  top <- f %>% top_n(n, count)
  vect <- top %>% pull(item)
  names(vect) <- top %>% pull(count)
  if (length(vect) > abs(n)) {
    message("top_freq: selecting ", length(vect), " items instead of ", abs(n), ", because of ties")
  }
  vect
}

#' @noRd
#' @exportMethod diff.frequency_tbl
#' @importFrom dplyr %>% full_join mutate
#' @export
diff.frequency_tbl <- function(x, y, ...) {
  # check classes
  if (!"frequency_tbl" %in% class(x)
      | !"frequency_tbl" %in% class(y)) {
    stop("Both x and y must be a frequency table.")
  }

  cat("Differences between frequency tables")
  if (identical(x, y)) {
    cat("\n\nNo differences found.\n")
    return(invisible())
  }

  x.attr <- attributes(x)$opt

  # only keep item and count
  x <- x[, 1:2]
  y <- y[, 1:2]

  x <- x %>%
    full_join(y,
              by = colnames(x)[1],
              suffix = c(".x", ".y")) %>%
    mutate(
      diff = case_when(
        is.na(count.y) ~ -count.x,
        is.na(count.x) ~ count.y,
        TRUE ~ count.y - count.x)) %>%
    mutate(
      diff.percent = percent(
        diff / count.x,
        force_zero = TRUE)) %>%
    mutate(diff = ifelse(diff %like% '^-',
                         diff,
                         paste0("+", diff)),
           diff.percent = ifelse(diff.percent %like% '^-',
                                 diff.percent,
                                 paste0("+", diff.percent)))

  print(
    knitr::kable(x,
                 format = x.attr$tbl_format,
                 col.names = c("Item", "Count #1", "Count #2", "Difference", "Diff. percent"),
                 align = paste0(x.attr$column_align[1], "rrrr"),
                 padding = 1)
  )
}

#' @rdname freq
#' @exportMethod print.frequency_tbl
#' @importFrom knitr kable
#' @importFrom dplyr n_distinct
#' @importFrom crayon bold silver
#' @export
print.frequency_tbl <- function(x, nmax = getOption("max.print.freq", default = 15), ...) {

  opt <- attr(x, 'opt')

  if (length(opt$vars) == 0) {
    opt$vars <- NULL
  }

  if (!is.null(opt$data) & !is.null(opt$vars)) {
    title <- paste0("of `", paste0(opt$vars, collapse = "` and `"), "` from ", opt$data)
  } else if (!is.null(opt$data) & is.null(opt$vars)) {
    title <- paste("of", opt$data)
  } else if (is.null(opt$data) & !is.null(opt$vars)) {
    title <- paste0("of `", paste0(opt$vars, collapse = "` and `"), "`")
  } else {
    title <- ""
  }

  if (!missing(nmax)) {
    opt$nmax <- nmax
    opt$nmax.set <- TRUE
  }
  dots <- list(...)
  if ("markdown" %in% names(dots)) {
    if (dots$markdown == TRUE) {
      opt$tbl_format <- "markdown"
    } else {
      opt$tbl_format <- "pandoc"
    }
  }

  title <- paste("Frequency table", trimws(title))

  # bold title
  if (opt$tbl_format == "pandoc") {
    title <- bold(title)
  } else if (opt$tbl_format == "markdown") {
    title <- paste0("\n**", title, "**")
  }

  if (opt$header == TRUE) {
    cat(title, "\n")
    if (!is.null(opt$header_txt)) {
      cat(opt$header_txt)
    }
  } else if (opt$tbl_format == "markdown") {
    # do print title as caption in markdown
    cat("\n", title, sep = "")
  }

  if (NROW(x) == 0) {
    cat('\n\nNo observations.\n')
    return(invisible())
  }

  if (all(x$count == 1)) {
    warning('All observations are unique.', call. = FALSE)
  }

  # save old NA setting for kable
  opt.old <- options()$knitr.kable.NA
  if (is.null(opt$na)) {
    opt$na <- "<NA>"
  }
  options(knitr.kable.NA = opt$na)

  if (nrow(x) > opt$nmax & opt$tbl_format != "markdown") {

    x.rows <- nrow(x)
    x.unprinted <- base::sum(x[(opt$nmax + 1):nrow(x), 'count'], na.rm = TRUE)
    x.printed <- base::sum(x$count) - x.unprinted

    if (opt$nmax.set == TRUE) {
      nmax <- opt$nmax
    } else {
      nmax <- getOption("max.print.freq", default = 15)
    }

    x <- x[1:nmax,]

    if (opt$nmax.set == TRUE) {
      footer <- paste('[ reached `nmax = ', opt$nmax, '`', sep = '')
    } else {
      footer <- '[ reached getOption("max.print.freq")'
    }
    footer <- paste(footer,
                    ' -- omitted ',
                    format(x.rows - opt$nmax),
                    ' entries, n = ',
                    format(x.unprinted),
                    ' (',
                    (x.unprinted / (x.unprinted + x.printed)) %>% percent(force_zero = TRUE),
                    ') ]\n', sep = '')
    if (opt$tbl_format == "pandoc") {
      footer <- silver(footer) # only silver in regular printing
    }
  } else {
    footer <- NULL
  }

  if (any(class(x$item) %in% c('double', 'integer', 'numeric', 'raw', 'single'))) {
    x$item <- format(x$item)
  }
  x$count <- format(x$count)
  x$percent <- percent(x$percent, force_zero = TRUE)
  x$cum_count <- format(x$cum_count)
  x$cum_percent <- percent(x$cum_percent, force_zero = TRUE)

  if (opt$tbl_format == "markdown") {
    cat("\n")
  }

  print(
    knitr::kable(x,
                 format = opt$tbl_format,
                 row.names = opt$row_names,
                 col.names = opt$column_names,
                 align = opt$column_align,
                 padding = 1)
  )

  if (!is.null(footer)) {
    cat(footer)
  }

  if (opt$tbl_format == "markdown") {
    cat("\n\n")
  } else {
    cat('\n')
  }

  # reset old kable setting
  options(knitr.kable.NA = opt.old)
  return(invisible())

}

#' @noRd
#' @exportMethod as.data.frame.frequency_tbl
#' @export
as.data.frame.frequency_tbl <- function(x, ...) {
  attr(x, 'package') <- NULL
  attr(x, 'opt') <- NULL
  as.data.frame.data.frame(x, ...)
}

#' @noRd
#' @exportMethod as_tibble.frequency_tbl
#' @export
#' @importFrom dplyr as_tibble
as_tibble.frequency_tbl <- function(x, validate = TRUE, ..., rownames = NA) {
  attr(x, 'package') <- NULL
  attr(x, 'opt') <- NULL
  as_tibble(x = as.data.frame(x), validate = validate, ..., rownames = rownames)
}

#' @noRd
#' @exportMethod hist.frequency_tbl
#' @export
#' @importFrom graphics hist
hist.frequency_tbl <- function(x, breaks = "Sturges", main = NULL, ...) {
  opt <- attr(x, 'opt')
  if (!class(x$item) %in% c("numeric", "double", "integer", "Date")) {
    stop("'x' must be numeric or Date.", call. = FALSE)
  }
  if (!is.null(opt$vars)) {
    title <- opt$vars
  } else if (!is.null(opt$data)) {
    title <- opt$data
  } else {
    title <- "frequency table"
  }
  if (class(x$item) == "Date") {
    x <- as.Date(as.vector(x), origin = "1970-01-01")
  } else {
    x <- as.vector(x)
  }
  if (is.null(main)) {
    main <- paste("Histogram of", title)
  }
  hist(x, main = main, xlab = title, ...)
}

#' @noRd
#' @exportMethod plot.frequency_tbl
#' @export
plot.frequency_tbl <- function(x, y, ...) {
  opt <- attr(x, 'opt')
  if (!is.null(opt$vars)) {
    title <- opt$vars
  } else {
    title <- ""
  }
  plot(x = x$item, y = x$count, ylab = "Count", xlab = title, ...)
}

#' @noRd
#' @exportMethod as.vector.frequency_tbl
#' @export
as.vector.frequency_tbl <- function(x, mode = "any") {
  as.vector(rep(x$item, x$count), mode = mode)
}

#' @noRd
#' @exportMethod format.frequency_tbl
#' @export
format.frequency_tbl <- function(x, digits = 1, ...) {
  opt <- attr(x, 'opt')
  if (opt$nmax.set == TRUE) {
    nmax <- opt$nmax
  } else {
    nmax <- getOption("max.print.freq", default = 15)
  }

  x <- x[1:nmax,]
  x$percent <- percent(x$percent, round = digits, force_zero = TRUE)
  x$cum_percent <- percent(x$cum_percent, round = digits, force_zero = TRUE)
  base::format.data.frame(x, ...)
}