Printing a tibble: Control and data flow

Requirements

• Fit into pre-specified width, distributing across multiple tiers if necessary

• Optionally shrink and stretch individual columns

• Header, body and footer for the tibble

  • Avoid recomputation of information

• Custom components for the pillars in a tibble, top-aligned

  • The container, not the column vectors, determine the appearance

• Customization of the entire output and of the pillars

• Support for data frame columns (packed data frames) and matrix/array columns

• Pillars are always shown from left to right, no “holes” in the colonnade

  • If the first column consumes all available space, the remaining columns are not shown, even if they all would fit if the first column is omitted.

• Printing pillars should take time proportional to the number of characters printed, and be “fast enough”.

Overview

The overall control and data flow are illustrated in the diagram below. Boxes are functions and methods. Solid lines are function calls. Dotted lines represent information that a function obtains via argument or (in the case of options) queries actively.

The pillar package uses debugme for debugging. Activating debugging for pillar is another way to track the control flow, see vignette("debugme") for details.

Initialization

A tibble is a list of columns of class "tbl_df" and "tbl". Printing is designed to work for non-data-frame table-like objects such as lazy tables. The print.tbl() method calls format() for the object and prints the output.

tbl <- tibble::tibble(a = 1:3, b = tibble::tibble(c = 4:6, d = 7:9), e = 10:12)
print(tbl, width = 23)
#> # A tibble: 3 × 3
#>       a   b$c     e
#>   <int> <int> <int>
#> 1     1     4    10
#> 2     2     5    11
#> 3     3     6    12
#> # ℹ 1 more variable:
#> #   b$d <int>
str(tbl)
#> tibble [3 × 3] (S3: tbl_df/tbl/data.frame)
#>  $ a: int [1:3] 1 2 3
#>  $ b: tibble [3 × 2] (S3: tbl_df/tbl/data.frame)
#>   ..$ c: int [1:3] 4 5 6
#>   ..$ d: int [1:3] 7 8 9
#>  $ e: int [1:3] 10 11 12

print.tbl <- function (x, width = NULL, ..., n = NULL, max_extra_cols = NULL, 
    max_footer_lines = NULL) 
{
    print_tbl(x, width, ..., n = n, max_extra_cols = max_extra_cols, 
        max_footer_lines = max_footer_lines)
}

The format.tbl() method creates a setup object, and uses that object to format header, body and footer.

format.tbl <- function (x, width = NULL, ..., n = NULL, max_extra_cols = NULL, 
    max_footer_lines = NULL) 
{
    format_tbl(x, width, ..., n = n, max_extra_cols = max_extra_cols, 
        max_footer_lines = max_footer_lines)
}

While it’s possible to extend or override these methods for your "tbl" subclass, often overriding the more specialized methods shown below is sufficient.

Setup

Most of the work for formatting actually happens in tbl_format_setup(). The desired output width is baked into the setup object and must be available when calling. Setup objects print like a tibble but with a clear separation of header, body, and footer.

setup <- tbl_format_setup(tbl, width = 24)
setup
#> <pillar_tbl_format_setup>
#> <tbl_format_header(setup)>
#> # A tibble: 3 × 3
#> <tbl_format_body(setup)>
#>       a   b$c     e
#>   <int> <int> <int>
#> 1     1     4    10
#> 2     2     5    11
#> 3     3     6    12
#> <tbl_format_footer(setup)>
#> # ℹ 1 more variable:
#> #   b$d <int>

A setup object is required here to avoid computing information twice. For instance, the dimensions shown in the header or the extra columns displayed in the footer are available only after the body has been computed.

The generic dispatches over the container, so that you can override it if necessary. It is responsible for assigning default values to arguments before passing them on to the method.

tbl_format_setup <- function (x, width = NULL, ..., n = NULL, max_extra_cols = NULL, 
    max_footer_lines = NULL, focus = NULL) 
{
    "!!!!DEBUG tbl_format_setup()"
    width <- get_width_print(width)
    n <- get_n_print(n, tbl_nrow(x))
    max_extra_cols <- get_max_extra_cols(max_extra_cols)
    max_footer_lines <- get_max_footer_lines(max_footer_lines)
    out <- tbl_format_setup_dispatch(x, width, ..., n = n, max_extra_cols = max_extra_cols, 
        max_footer_lines = max_footer_lines, focus = focus)
    return(out)
    UseMethod("tbl_format_setup")
}

The default implementation converts the input to a data frame via as.data.frame(head(x)), and returns an object constructed with new_tbl_format_setup() that contains the data frame and additional information. If you override this method, e.g. to incorporate more information, you can add new items to the default setup object, but you should not overwrite existing items.

tbl_format_setup.tbl <- function (x, width, ..., n, max_extra_cols, max_footer_lines, 
    focus) 
{
    "!!!!DEBUG tbl_format_setup.tbl()"
    rows <- tbl_nrow(x)
    lazy <- is.na(rows)
    if (lazy) {
        df <- as.data.frame(head(x, n + 1))
        if (nrow(df) <= n) {
            rows <- nrow(df)
        }
        else {
            df <- vec_head(df, n)
        }
    }
    else {
        df <- df_head(x, n)
    }
    if (is.na(rows)) {
        needs_dots <- (nrow(df) >= n)
    }
    else {
        needs_dots <- (rows > n)
    }
    if (needs_dots) {
        rows_missing <- rows - n
    }
    else {
        rows_missing <- 0
    }
    tbl_sum <- tbl_sum(x)
    rownames(df) <- NULL
    colonnade <- ctl_colonnade(df, has_row_id = if (!lazy && 
        .row_names_info(x) > 0) 
        "*"
    else TRUE, width = width, controller = x, focus = focus)
    body <- colonnade$body
    extra_cols <- colonnade$extra_cols
    extra_cols_total <- length(extra_cols)
    if (extra_cols_total > max_extra_cols) {
        length(extra_cols) <- max_extra_cols
    }
    abbrev_cols <- colonnade$abbrev_cols
    new_tbl_format_setup(x = x, df = df, width = width, tbl_sum = tbl_sum, 
        body = body, rows_missing = rows_missing, rows_total = rows, 
        extra_cols = extra_cols, extra_cols_total = extra_cols_total, 
        max_footer_lines = max_footer_lines, abbrev_cols = abbrev_cols)
}

At the core, the internal function ctl_colonnade() composes the body. Its functionality and the customization points it offers are detailed in the “Colonnade” section below.

Header, body, footer

The components of a tibble are formatted with tbl_format_*() generics, which also dispatch on the container to allow extension or overrides. They return a character vector, with one element per line printed. The setup object is required.

tbl_format_header(tbl, setup)
#> [1] "# A tibble: 3 × 3"
tbl_format_body(tbl, setup)
#>       a   b$c     e
#>   <int> <int> <int>
#> 1     1     4    10
#> 2     2     5    11
#> 3     3     6    12
tbl_format_footer(tbl, setup)
#> [1] "# ℹ 1 more variable:" "#   b$d <int>"

(The body is returned as a classed object with a print() method, it is still a character() under the hood.)

class(tbl_format_body(tbl, setup))
#> [1] "glue"      "character"
typeof(tbl_format_body(tbl, setup))
#> [1] "character"

Since most of the work already has been carried out in tbl_format_setup(), the default implementations mostly consist of code that styles and wraps the output.

tbl_format_header.tbl <- function (x, setup, ...) 
{
    named_header <- setup$tbl_sum
    focus <- attr(x, "pillar_focus")
    if (!is.null(focus)) {
        named_header <- c(named_header, `Focus columns` = collapse(tick_if_needed(focus)))
    }
    if (all(names2(named_header) == "")) {
        header <- named_header
    }
    else {
        header <- paste0(align(paste0(names2(named_header), ":"), 
            space = NBSP), " ", named_header)
    }
    style_subtle(format_comment(header, width = setup$width))
}

tbl_format_body.tbl <- function (x, setup, ...) 
{
    force(setup)
    setup$body
}

tbl_format_footer.tbl <- function (x, setup, ...) 
{
    footer <- format_footer(x, setup)
    footer_comment <- wrap_footer_bullet(footer, setup)
    footer_advice <- format_footer_advice(x, setup)
    footer_advice_comment <- wrap_footer_bullet(footer_advice, 
        setup, lines = 1, ellipsis = FALSE, bullet = symbol$info)
    style_subtle(c(footer_comment, footer_advice_comment))
}

Colonnade

The internal function ctl_colonnade() composes the body. It performs the following tasks:

1. Create a pillar object for every top-level column that fits, using the minimum width, via ctl_new_pillar_list(), ctl_new_pillar() and ultimately pillar() and pillar_shaft()
2. Determine the number of tiers and the width for each tier
3. Distribute the pillars across the tiers, assigning a width to each pillar.
4. Format each pillar via its format() function, passing the now known width.
5. Combine the formatted pillars horizontally.
6. Combine the tiers vertically.
7. Return the formatted body, and the columns that could not fit.

In the following, the first and the fourth steps are discussed.

Creating pillar objects

The initial tibble is passed to ctl_new_pillar_list(), which eventually calls ctl_new_pillar() once or several times. For each top-level column, one pillar object is constructed. The loop is terminated when the available width is exhausted even considering the minimum width.

ctl_new_pillar_list(tbl, tbl$a, width = 20)
#> [[1]]
#> <pillar>
#> <int>
#>     1
#>     2
#>     3
#> 
#> attr(,"remaining_width")
#> [1] 14
#> attr(,"simple")
#> [1] TRUE
ctl_new_pillar_list(tbl, tbl$b, width = 20)
#> [[1]]
#> <pillar>
#>     c
#> <int>
#>     4
#>     5
#>     6
#> 
#> [[2]]
#> <pillar>
#>     d
#> <int>
#>     7
#>     8
#>     9
#> 
#> attr(,"extra")
#> character(0)
#> attr(,"remaining_width")
#> [1] 8
#> attr(,"simple")
#> [1] FALSE

ctl_new_pillar_list.tbl <- function (controller, x, width, ..., title = NULL, first_pillar = NULL) 
{
    "!!!!DEBUG ctl_new_pillar_list.tbl(`v(width)`, `v(title)`)"
    if (is.data.frame(x)) {
        new_data_frame_pillar_list(x, controller, width, title = title, 
            first_pillar = first_pillar)
    }
    else if (is.matrix(x) && !inherits(x, c("Surv", "Surv2"))) {
        new_matrix_pillar_list(x, controller, width, title = title, 
            first_pillar = first_pillar)
    }
    else if (is.array(x) && length(dim(x)) > 2) {
        new_array_pillar_list(x, controller, width, title = title, 
            first_pillar = first_pillar)
    }
    else {
        if (is.null(first_pillar)) {
            first_pillar <- ctl_new_pillar(controller, x, width, 
                ..., title = prepare_title(title))
        }
        new_single_pillar_list(first_pillar, width)
    }
}

ctl_new_pillar(tbl, tbl$a, width = 20)
#> <pillar>
#> <int>
#>     1
#>     2
#>     3

ctl_new_pillar.tbl <- function (controller, x, width, ..., title = NULL) 
{
    "!!!!DEBUG ctl_new_pillar.tbl(`v(width)`, `v(title)`)"
    pillar(x, title, if (!is.null(width)) 
        max0(width))
}

pillar <- function (x, title = NULL, width = NULL, ...) 
{
    "!!!!DEBUG pillar(`v(class(x))`, `v(title)`, `v(width)`)"
    pillar_from_shaft(new_pillar_title(title), new_pillar_type(x), 
        pillar_shaft(x, ...), width)
}

new_pillar <- function (components, ..., width = NULL, class = NULL, extra = deprecated()) 
{
    "!!!!DEBUG new_pillar(`v(width)`, `v(class)`)"
    if (is_present(extra)) {
        deprecate_warn("1.7.0", "pillar::new_pillar(extra = )")
    }
    check_dots_empty()
    if (length(components) > 0 && !is_named(components)) {
        abort("All components must have names.")
    }
    structure(components, width = width, class = c(class, "pillar"))
}

Formatting pillars

When a pillar object is constructed, it has a minimum and a desired (maximum) width. Because it depends on the number and width of other pillar objects that may not be even constructed, the final width is not known yet. It is passed to format(), which uses the desired width if empty:

format.pillar <- function (x, width = NULL, ...) 
{
    if (is.null(width)) {
        width <- get_width(x)
    }
    if (is.null(width)) {
        width <- pillar_get_width(x)
    }
    as_glue(pillar_format_parts_2(x, width)$aligned)
}