w2l_split: Reshape Wide Data to Long Format and Split into a Named List

In mintyr: Grouped and Nested Data Pipelines Built on 'data.table'

Reshape Wide Data to Long Format and Split into a Named List

Description

w2l_split reshapes a wide-format data.frame or data.table into long format, then splits the result into a named list keyed by the pivoted column identifier (variable) and any optional grouping variables supplied via by. List element names are derived directly from the grouping key combinations produced by split(), guaranteeing name-to-content alignment.

Usage

w2l_split(data, cols = NULL, by = NULL, out_type = "dt", sep = "_")

Arguments

data	data.frame or data.table. Wide-format input dataset. The input object is never modified.
cols	numeric or character. Columns to pivot from wide to long, specified as integer indices or column names. Default NULL: when NULL, by must be provided and the function splits the data as-is without melting.
by	numeric or character. Additional grouping variables used as secondary split keys, specified as integer indices or column names. Default NULL.
out_type	character. Class of each list element: "dt" for data.table (default) or "df" for data.frame.
sep	character. Separator used when concatenating multiple grouping key values into a single list-element name. Default "_".

Details

Name safety: list names are built from each element's own key values (joined by sep), so names always match contents. This also makes sep work on every data.table version (split(sep = ) is only honoured from data.table 1.16.0 on).

Column resolution: both cols and by accept integer column positions or character column names. Out-of-bounds indices and unknown names are caught early with informative error messages.

Overlap guard: columns appearing in both cols and by raise an error before melting to prevent id.vars / measure.vars conflicts.

Factor-free melting: melt() is called with variable.factor = FALSE so the variable column is always character, keeping split() sort order consistent with lexicographic expectations.

Memory efficiency: the elements returned by split() are independent objects, so for out_type = "df" they are converted in place with setDF() and no column vector is duplicated. The input itself is never modified.

Value

A named list of data.table or data.frame objects (controlled by out_type). Names reflect the key combination of variable (and by levels if provided), joined by sep.

• If by is NULL, the list is keyed by the pivoted column names only.

• If by is specified, the list is keyed by variable and all by level combinations.

Note

• An empty input table (0 rows) triggers a warning() and returns an empty list immediately.

• cols and by must not overlap; shared columns raise an error.

• out_type values other than "dt" or "df" raise an error.

See Also

tidytable::group_split() for a tidyverse-style equivalent.

Examples

# Example: Wide to long format splitting demonstrations

# Example 1: Basic splitting by Species
w2l_split(
  data = iris,                    # Input dataset
  by = "Species"                  # Split by Species column
) |> 
  lapply(head)                    # Show first 6 rows of each split

# Example 2: Split specific columns using numeric indices
w2l_split(
  data = iris,                    # Input dataset
  cols = 1:3,                   # Select first 3 columns to split
  by = 5                          # Split by column index 5 (Species)
) |> 
  lapply(head)                    # Show first 6 rows of each split

# Example 3: Split specific columns using column names
list_res <- w2l_split(
  data = iris,                    # Input dataset
  cols = c("Sepal.Length",        # Select columns by name
           "Sepal.Width"),
  by = "Species"                  # Split by Species column
)
lapply(list_res, head)            # Show first 6 rows of each split
# Returns similar structure to Example 2

mintyr documentation built on Oct. 5, 2026, 5:08 p.m.