In fct_cross() vary last factor levels fastest, not first (#373)

Fixes #360

Author: Adam-AKong

NEWS.md

@@ -1,5 +1,6 @@
 # forcats (development version)
 
+* `fct_cross()` now varies the levels in the last factor fastest (@Adam-AKong, #373).
 * Functions that create a new factor (e.g. `lvls_expand()`, `lvls_reorder()`, `fct_lump_n()`, `fct_drop()`) now preserves the class of the original object in addition to the attributes (#83).
 * forcats now requires R 4.1.
 

R/cross.R

@@ -32,8 +32,9 @@ fct_cross <- function(..., sep = ":", keep_empty = FALSE) {
   newf <- exec(paste, !!!.data, sep = sep)
 
   old_levels <- lapply(.data, levels)
-  grid <- exec(expand.grid, old_levels)
-  new_levels <- exec(paste, !!!grid, sep = sep)
+  # expand.grid varies inputs fastest instead of last
+  grid_rev <- exec(expand.grid, rev(old_levels))
+  new_levels <- exec(paste, !!!rev(grid_rev), sep = sep)
 
   if (!keep_empty) {
     new_levels <- intersect(new_levels, newf)

tests/testthat/test-cross.R

@@ -81,3 +81,11 @@ test_that("validates its inputs", {
     fct_cross("x", keep_empty = 1)
   })
 })
+
+test_that("varies last values fastest", {
+  f1 <- fct(c("a4", "a3", "a2", "a1"))
+  f2 <- fct(c("b4", "b3", "b2", "b1"), levels = c("b4", "b3", "b2", "b1"))
+
+  fcross <- fct_cross(f1, f2)
+  expect_equal(levels(fcross), c("a4:b4", "a3:b3", "a2:b2", "a1:b1"))
+})