Split Function Behavioral Building Blocks

We call functions that can used as either pre- or post processing functions in make_split_fun (via entry into the lists passed to pre and post, respectively) behavioral building blocks (BBBs). Behavioral building blocks are modular, typically atomic (as in they only do one narrow thing to the data or split result) functions which we can mix and match to construct complex custom behaviors in our split function.

There are two types of behavioral building blocks:

pre-processing - accept, modify, and return the incoming parent data, and
post-processing - accept, modify, and return a split result object representing facets generated by the core splitting machinery

Existing BBBs provided by `rtables`

rtables provides a small number of behavioral building blocks that are ready for use:

behavior	function	fun factory	usage notes
drop specific facets	`restrict_facets`	yes	`op = "drop"`
keep only specific facets	`restrict_facets`	yes	`op = "keep"`
reorder facets	`restrict_facets`	yes	`op = keep`, `reorder = TRUE`, pass all existing facet names
add combination facet	`add_combo_facet`	yes	create single combo facet, can be called repeatedly
add overall/total facet	`add_overall_facet`	yes
Trim levels of another var in each facet	`trim_levels_in_facet`	yes	equivalent to `trim_levels_in_group`
exclude facets for unobserved variable levels	`drop_facet_levels`	no

We refer to the help for each of these functions for examples of their usage and will not recreate single examples here.

Creating Post Processing BBBs

Post processing behavioral building blocks are functions which accept:

ret - split result object returned by the core splitting machinery or a previously applied post processing BBB
spl - Split object
fulldf - incoming full data which was split in this faceting step
.spl_context - optional split context object

and returns ret modified typically in one of four ways:

using add_to_split_result on ret to add facets manually to it,
manually removing facets from it (or wrapping logic around a restrict_facets call),
reordering the existing facets, or, rarely,
calling make_split_result to construct an entirely new split result.

Illustrative And/Or Useful Examples

Here we implement a number of simple behavioral building blocks that both illustrate how to create our own and may be useful in some circumstances.

Placing Specific Facets First And/Or Last

Here we want to specify certain facets (in practice, often combination facets whose order can’t be controlled via variable re-leveling) to appear first and or last amongst their direct siblings.

Recognizing that this is ultimately a reordering behavior, we can wrap a call to restrict_facets with reorder = TRUE after calculating the desired full ordering:

library(rtables)

put_facets_first_last <- function(first = NULL, last = NULL) {
  if (is.null(first) && is.null(last)) {
    stop("must speficify at least one facet to be placed first or last")
  }
  function(ret, spl, fulldf) {
    fac_names <- names(ret$values)
    all_speced <- c(first, last)

    if (!all(all_speced %in% fac_names)) {
      stop(
        "Facet(s) []",
        paste(setdiff(all_speced, fac_names), collapse = ", "),
        "] not found in incoming split result."
      )
    }
    tmpfun <- restrict_facets(c(first, setdiff(fac_names, all_speced), last), op = "keep", reorder = TRUE)
    tmpfun(ret, spl, fulldf)
  }
}

While this could be achieved by variable re-leveling, we show that this works by forcing the U and UNDIFFERENTIATED levels of SEX to be first and last, respectively:

fl_splfun <- make_split_fun(
  post = list(
    put_facets_first_last(first = "U", last = "UNDIFFERENTIATED")
  )
)

We can then compare two similar (column) layouts to see the effect of our BBB

lyt_basic <- basic_table() |>
  split_cols_by("SEX")

build_table(lyt_basic, ex_adsl)
#    F   M   U   UNDIFFERENTIATED
# ———————————————————————————————

lyt_fl <- basic_table() |>
  split_cols_by("SEX", split_fun = fl_splfun)

build_table(lyt_fl, ex_adsl)
#    U   F   M   UNDIFFERENTIATED
# ———————————————————————————————

Pre-sorting Or Pre-pruning Facets Based On Data Sparsity

Here we want to either reorder our facets or remove some facets based on how much data they represent.

presort_facets <- function(ret, spl, fulldf) {
  fac_names <- names(ret$values)
  fac_ns <- vapply(ret$datasplit, NROW, 1L)
  ord <- order(fac_ns, decreasing = TRUE)
  tmpfun <- restrict_facets(fac_names[ord], op = "keep", reorder = TRUE)
  tmpfun(ret, spl, fulldf)
}

Here we can see that using this building block gives our desired behavior:

presort_splfun <- make_split_fun(post = list(presort_facets))

lyt_presort <- basic_table(show_colcounts = TRUE) |>
  split_cols_by("STRATA1", split_fun = presort_splfun)

build_table(lyt_presort, ex_adsl)
#       C         B         A   
#    (N=143)   (N=135)   (N=122)
# ——————————————————————————————

And similarly here:

drop_sparse_facets <- function(ncutoff = 5) {
  function(ret, spl, fulldf) {
    fac_names <- names(ret$values)
    fac_ns <- vapply(ret$datasplit, NROW, 1L)
    keep_inds <- which(fac_ns >= ncutoff)
    tmpfun <- restrict_facets(fac_names[keep_inds], op = "keep", reorder = FALSE)
    tmpfun(ret, spl, fulldf)
  }
}

lyt_preprune1 <- basic_table(show_colcounts = TRUE) |>
  split_cols_by("SEX")

build_table(lyt_preprune1, ex_adsl)
#       F         M        U     UNDIFFERENTIATED
#    (N=222)   (N=166)   (N=9)        (N=3)      
# ———————————————————————————————————————————————

preprune_splfun2 <- make_split_fun(post = list(drop_sparse_facets()))
lyt_preprune2 <- basic_table(show_colcounts = TRUE) |>
  split_cols_by("SEX", split_fun = preprune_splfun2)

build_table(lyt_preprune2, ex_adsl)
#       F         M        U  
#    (N=222)   (N=166)   (N=9)
# ————————————————————————————

preprune_splfun3 <- make_split_fun(post = list(drop_sparse_facets(10)))
lyt_preprune3 <- basic_table(show_colcounts = TRUE) |>
  split_cols_by("SEX", split_fun = preprune_splfun3)

build_table(lyt_preprune3, ex_adsl)
#       F         M   
#    (N=222)   (N=166)
# ————————————————————

Creating Preprocessing BBBs

Custom pre-processing BBB requirements are rarer than post-processing ones, as most things are simpler to do over a small set of facets rather than a large set of incoming data. Furthermore, most things that could be done via a pre-processing BBB can also be done via a post-processing BBB.

That said, for illustrative purposes, we can recreate part of functionality of trim_levels_to_map in a preprocessing BBB (the restriction of data based on inner variable values), like so:

trim_facets_to_map <- function(map = NULL) {
  function(df, spl, vals, labels, .spl_context) {
    if (is.null(map)) {
      return(df)
    } # do nothing
    cur_outer_val <- tail(.spl_context$value, 1)
    inner_var <- names(map)[2]
    inner_vec <- df[[inner_var]]
    inner_keep <- map[map[[1]] == cur_outer_val, inner_var, drop = TRUE]
    df_out <- df[inner_vec %in% inner_keep, ]
    df_out[[inner_var]] <- factor(df_out[[inner_var]], levels = intersect(levels(inner_vec), inner_keep))
    df_out
  }
}

We use spl_variable to retrieve the variable name for the split, determine the levels of the inner variable to keep based on the map and the current level of the split context, restrict the data to rows where the inner variable is the desired value(s), and recreate the inner variable factor to drop unwanted levels.

Because we are doing this as factor re-leveling before the core splitting machinery is invoked, we will use this as a pre-processing BBB on the inner variable split; for a post-processing BBB we would do it on the split data of the outer variable split.

Note: If our map does not include at least one entry for each factor level defined by the incoming data, we need to restrict those at the previous split; trim_levels_to_map combines this behavior.

map <- data.frame(
  ARM = c("A: Drug X", "B: Placebo"),
  STRATA1 = c("B", "A")
)

map_splfun <- make_split_fun(pre = list(trim_facets_to_map(map)))

outer_splfun <- make_split_fun(post = list(restrict_facets("C: Combination", op = "exclude")))

lyt <- basic_table() |>
  split_cols_by("ARM", split_fun = outer_splfun) |>
  split_cols_by("STRATA1", split_fun = map_splfun)

build_table(lyt, ex_adsl)
#    A: Drug X   B: Placebo
#        B           A     
# —————————————————————————

This matches the core behavior of trim_levels_to_map:

lyt <- basic_table() |>
  split_cols_by("ARM", split_fun = trim_levels_to_map(map)) |>
  split_cols_by("STRATA1")

build_table(lyt, ex_adsl)
#    A: Drug X   B: Placebo
#        B           A     
# —————————————————————————

Advanced rtables - Writing Reusable Behavior Building Blocks

Contributed by Johnson & Johnson Innovative Medicine

Gabriel Becker

Dan Hofstaedter

2025-10-22

Split Function Behavioral Building Blocks

Existing BBBs provided by `rtables`

Creating Post Processing BBBs

Illustrative And/Or Useful Examples

Placing Specific Facets First And/Or Last

Pre-sorting Or Pre-pruning Facets Based On Data Sparsity

Creating Preprocessing BBBs

Advanced rtables - Writing Reusable Behavior Building Blocks

Contributed by Johnson & Johnson Innovative Medicine

Gabriel Becker

Dan Hofstaedter

2025-10-22

Split Function Behavioral Building Blocks

Existing BBBs provided by rtables

Creating Post Processing BBBs

Illustrative And/Or Useful Examples

Placing Specific Facets First And/Or Last

Pre-sorting Or Pre-pruning Facets Based On Data Sparsity

Creating Preprocessing BBBs

Existing BBBs provided by `rtables`