Add .vary parameter to expand_grid()

NEWS.md

@@ -1,5 +1,8 @@
 # tidyr (development version)
 
+* `expand_grid()` gains a new `.vary` argument, allowing users to control
+  whether the first column varies fastest or slowest (#1543, @JamesHWade).
+
 * `unite()` no longer errors if you provide a selection that doesn't select any
   columns. Instead, it returns a column containing the empty string (#1548,
   @catalamarti).

R/expand.R

@@ -162,30 +162,37 @@ nesting <- function(..., .name_repair = "check_unique") {
 #' `expand_grid()` is heavily motivated by [expand.grid()].
 #' Compared to `expand.grid()`, it:
 #'
-#' * Produces sorted output (by varying the first column the slowest, rather
-#'   than the fastest).
+#' * Produces sorted output by varying the first column the slowest by default.
 #' * Returns a tibble, not a data frame.
 #' * Never converts strings to factors.
 #' * Does not add any additional attributes.
 #' * Can expand any generalised vector, including data frames.
 #'
+#' @inheritParams vctrs::vec_expand_grid
+#'
 #' @param ... Name-value pairs. The name will become the column name in the
 #'   output.
-#' @inheritParams tibble::as_tibble
-#' @return A tibble with one column for each input in `...`. The output
-#'   will have one row for each combination of the inputs, i.e. the size
-#'   be equal to the product of the sizes of the inputs. This implies
-#'   that if any input has length 0, the output will have zero rows.
+#'
+#' @return A tibble with one column for each input in `...`. The output will
+#'   have one row for each combination of the inputs, i.e. the size will be
+#'   equal to the product of the sizes of the inputs. This implies that if any
+#'   input has length 0, the output will have zero rows. The ordering of the
+#'   output depends on the `.vary` argument.
+#'
 #' @export
 #' @examples
+#' # Default behavior varies the first column "slowest"
 #' expand_grid(x = 1:3, y = 1:2)
-#' expand_grid(l1 = letters, l2 = LETTERS)
+#'
+#' # Vary the first column "fastest", like `expand.grid()`
+#' expand_grid(x = 1:3, y = 1:2, .vary = "fastest")
 #'
 #' # Can also expand data frames
 #' expand_grid(df = tibble(x = 1:2, y = c(2, 1)), z = 1:3)
+#'
 #' # And matrices
 #' expand_grid(x1 = matrix(1:4, nrow = 2), x2 = matrix(5:8, nrow = 2))
-expand_grid <- function(..., .name_repair = "check_unique") {
+expand_grid <- function(..., .name_repair = "check_unique", .vary = "slowest") {
   out <- grid_dots(...)
 
   names <- names2(out)
@@ -202,6 +209,7 @@ expand_grid <- function(..., .name_repair = "check_unique") {
 
   out <- vec_expand_grid(
     !!!out,
+    .vary = .vary,
     .name_repair = "minimal",
     .error_call = current_env()
   )

tests/testthat/_snaps/expand.md

@@ -55,6 +55,14 @@
       * `x` -> `x...1`
       * `x` -> `x...2`
 
+# expand_grid() throws an error for invalid `.vary` parameter
+
+    Code
+      expand_grid(x = 1:2, y = 1:2, .vary = "invalid")
+    Condition
+      Error in `expand_grid()`:
+      ! `.vary` must be one of "slowest" or "fastest", not "invalid".
+
 # grid_dots() reject non-vector input
 
     Code

tests/testthat/test-expand.R

@@ -390,6 +390,32 @@ test_that("expand_grid() works with 0 row tibbles", {
   expect_identical(expand_grid(df, x = 1:2), tibble(x = integer()))
 })
 
+test_that("expand_grid() respects `.vary` parameter", {
+  # Slowest
+  expect_identical(
+    expand_grid(x = 1:2, y = 1:2),
+    tibble(
+      x = c(1L, 1L, 2L, 2L),
+      y = c(1L, 2L, 1L, 2L)
+    )
+  )
+
+  # Fastest
+  expect_identical(
+    expand_grid(x = 1:2, y = 1:2, .vary = "fastest"),
+    tibble(
+      x = c(1L, 2L, 1L, 2L),
+      y = c(1L, 1L, 2L, 2L)
+    )
+  )
+})
+
+test_that("expand_grid() throws an error for invalid `.vary` parameter", {
+  expect_snapshot(error = TRUE, {
+    expand_grid(x = 1:2, y = 1:2, .vary = "invalid")
+  })
+})
+
 # ------------------------------------------------------------------------------
 # grid_dots()