Custom formatting

How to customize the printed output of a "tbl" subclass? This vignette shows the various customization options. Customizing the formatting of a vector class in a tibble is described in vignette("pillar", package = "vctrs"). An overview over the control and data flow is given in vignette("printing").

This vignette assumes that the reader is familiar with S3 classes, methods, and inheritance. The “S3” chapter of Hadley Wickham’s “Advanced R” is a good start.

To make use of pillar’s printing capabilities, create a class that inherits from "tbl", like tibble (classes "tbl_df" and "tbl"), dbplyr lazy tables ("tbl_lazy" and "tbl") and sf spatial data frames ("sf", "tbl_df" and "tbl"). Because we are presenting various customization options, we create a constructor for an example data frame with arbitrary subclass.

The "default" class doesn’t have any customizations yet, and prints like a regular tibble.

Header

Tweak header

The easiest customization consists of tweaking the header. Implement a tbl_sum() method to extend or replace the information shown in the header, keeping the original formatting.

tbl_sum.default_header_extend <- function(x, ...) {
  default_header <- NextMethod()
  c(default_header, "New" = "A new header")
}

example_tbl("default_header_extend")
#> # A data frame: 3 × 2
#> # New:          A new header
#>   a       b$c    $d
#>   <chr> <int> <dbl>
#> 1 a         1   4.5
#> 2 b         2   5.5
#> 3 c         3   6.5

tbl_sum.default_header_replace <- function(x, ...) {
  c("Override" = "Replace all headers")
}

example_tbl("default_header_replace")
#> # Override: Replace all headers
#>   a       b$c    $d
#>   <chr> <int> <dbl>
#> 1 a         1   4.5
#> 2 b         2   5.5
#> 3 c         3   6.5

Restyle header

To style the header in a different way, implement a tbl_format_header() method. The implementation is responsible for the entire formatting and styling, including the leading hash.

tbl_format_header.custom_header_replace <- function(x, setup, ...) {
  cli::style_italic(names(setup$tbl_sum), " = ", setup$tbl_sum)
}

example_tbl("custom_header_replace")
#> A data frame = 3 × 2
#>   a       b$c    $d
#>   <chr> <int> <dbl>
#> 1 a         1   4.5
#> 2 b         2   5.5
#> 3 c         3   6.5

Footer

Restyle footer

Similarly, to add information the footer, or to replace it entirely, implement a tbl_format_footer() method. Here, as in all tbl_format_*() methods, you can use the information contained in the setup object, see ?new_tbl_format_setup for the available fields. Again, the implementation is responsible for the entire formatting and styling, including the leading hash if needed.

tbl_format_footer.custom_footer_extend <- function(x, setup, ...) {
  default_footer <- NextMethod()

  extra_info <- "and with extra info in the footer"
  extra_footer <- style_subtle(paste0("# ", cli::symbol$ellipsis, " ", extra_info))

  c(default_footer, extra_footer)
}

print(example_tbl("custom_footer_extend"), n = 2)
#> # A data frame: 3 × 2
#>   a       b$c    $d
#>   <chr> <int> <dbl>
#> 1 a         1   4.5
#> 2 b         2   5.5
#> # ℹ 1 more row
#> # … and with extra info in the footer

tbl_format_footer.custom_footer_replace <- function(x, setup, ...) {
  paste0("The table has ", setup$rows_total, " rows in total.")
}

print(example_tbl("custom_footer_replace"), n = 2)
#> # A data frame: 3 × 2
#>   a       b$c    $d
#>   <chr> <int> <dbl>
#> 1 a         1   4.5
#> 2 b         2   5.5
#> The table has 3 rows in total.

Compute additional info beforehand

If the same information needs to be displayed in several parts (e.g., in both header and footer), it is useful to compute it in tbl_format_setup() and store it in the setup object. New elements may be added to the setup object, existing elements should not be overwritten. Exception: the tbl_sum element contains the output of tbl_sum() and can be enhanced with additional elements.

tbl_format_setup.extra_info <- function(x, width, ...) {
  setup <- NextMethod()
  cells <- prod(dim(x))
  setup$cells <- cells
  setup$tbl_sum <- c(setup$tbl_sum, "Cells" = as.character(cells))
  setup
}

tbl_format_footer.extra_info <- function(x, setup, ...) {
  paste0("The table has ", setup$cells, " cells in total.")
}

example_tbl("extra_info")
#> # A data frame: 3 × 2
#> # Cells:        6
#>   a       b$c    $d
#>   <chr> <int> <dbl>
#> 1 a         1   4.5
#> 2 b         2   5.5
#> 3 c         3   6.5
#> The table has 6 cells in total.

Row IDs

By implementing the generic ctl_new_rowid_pillar(), printing of the row ID column can be customized. In order to print Roman instead of Arabic numerals, one could use utils::as.roman() to generate the corresponding sequence and build up a row ID pillar using new_pillar() and associated methods as has been introduced previously.

ctl_new_rowid_pillar.pillar_roman <- function(controller, x, width, ...) {
  out <- NextMethod()
  rowid <- utils::as.roman(seq_len(nrow(x)))
  width <- max(nchar(as.character(rowid)))
  new_pillar(
    list(
      title = out$title,
      type = out$type,
      data = pillar_component(
        new_pillar_shaft(list(row_ids = rowid),
          width = width,
          class = "pillar_rif_shaft"
        )
      )
    ),
    width = width
  )
}

example_tbl("pillar_roman")
#> # A data frame: 3 × 2
#>     a       b$c    $d
#>     <chr> <int> <dbl>
#> I   a         1   4.5
#> II  b         2   5.5
#> III c         3   6.5

Body

Tweak pillar composition

Pillars consist of components, see ?new_pillar_component for details. Extend or override the ctl_new_pillar() method to alter the appearance. The example below adds table rules of constant width to the output.

ctl_new_pillar.pillar_rule <- function(controller, x, width, ..., title = NULL) {
  out <- NextMethod()
  new_pillar(list(
    top_rule = new_pillar_component(list("========"), width = 8),
    title = out$title,
    type = out$type,
    mid_rule = new_pillar_component(list("--------"), width = 8),
    data = out$data,
    bottom_rule = new_pillar_component(list("========"), width = 8)
  ))
}

example_tbl("pillar_rule")
#> # A data frame: 3 × 2
#>   ======== ======== ========
#>   a             b$c       $d
#>   <chr>       <int>    <dbl>
#>   -------- -------- --------
#> 1 a               1      4.5
#> 2 b               2      5.5
#> 3 c               3      6.5
#>   ======== ======== ========

To make the width adaptive, we implement a "rule" class with a format() method that formats rules to prespecified widths.

rule <- function(char = "-") {
  stopifnot(nchar(char) == 1)
  structure(char, class = "rule")
}

format.rule <- function(x, width, ...) {
  paste(rep(x, width), collapse = "")
}

ctl_new_pillar.pillar_rule_adaptive <- function(controller, x, width, ..., title = NULL) {
  out <- NextMethod()
  if (is.null(out)) {
    return(NULL)
  }

  new_pillar(list(
    top_rule = new_pillar_component(list(rule("=")), width = 1),
    title = out$title,
    type = out$type,
    mid_rule = new_pillar_component(list(rule("-")), width = 1),
    data = out$data,
    bottom_rule = new_pillar_component(list(rule("=")), width = 1)
  ))
}

example_tbl("pillar_rule_adaptive")
#> # A data frame: 3 × 2
#>   =         =     =
#>   a       b$c    $d
#>   <chr> <int> <dbl>
#>   -         -     -
#> 1 a         1   4.5
#> 2 b         2   5.5
#> 3 c         3   6.5
#>   =         =     =

Tweak display of compound pillars

Compound pillars are created by ctl_new_pillar_list() for columns that contain a data frame, a matrix or an array. The default implementation also calls ctl_new_pillar() shown above. The (somewhat artificial) example hides all data frame columns in a column with the type "<hidden>".

ctl_new_pillar_list.hide_df <- function(controller, x, width, ..., title = NULL) {
  if (!is.data.frame(x)) {
    return(NextMethod())
  }

  if (width < 8) {
    return(NULL)
  }

  list(new_pillar(
    list(
      title = pillar_component(new_pillar_title(title)),
      type = new_pillar_component(list("<hidden>"), width = 8),
      data = new_pillar_component(list(""), width = 1)
    ),
    width = 8
  ))
}

example_tbl("hide_df")
#> # A data frame: 3 × 2
#>   <hidden>
#>           
#> 1 <hidden>
#> 2         
#> 3 <hidden>

Restyle body

Last but not least, it is also possible to completely alter the display of the body by overriding tbl_format_body(). The example below uses plain data frame output for a tibble.

tbl_format_body.oldskool <- function(x, setup, ...) {
  capture.output(print.data.frame(setup$df))
}

print(example_tbl("oldskool"), n = 2)
#> # A data frame: 3 × 2
#>   a b.c b.d
#> 1 a   1 4.5
#> 2 b   2 5.5
#> # ℹ 1 more row

Note that default printed output is computed in tbl_format_setup(), this takes a considerable amount of time. If you really need to change the output for the entire body, consider providing your own tbl_format_setup() method.