Behaviour empty behaviour for complete() and expand()

Fixes #331

NEWS.md

@@ -1,5 +1,9 @@
 # tidyr 0.7.2.9000
 
+* `complete()` with zero-length completions returns original input (#331).
+
+* `expand()` with empty input gives empty data frame instead of `NULL` (#331).
+
 * `uncount()` performs the inverse operation of `dplyr::count()` (#279)
 
 * `spread()` now consistently returns 0 row outputs for 0 row inputs (#269).

R/complete.R

@@ -38,6 +38,9 @@ complete.default <- function(data, ..., fill = list()) {
 #' @export
 complete.data.frame <- function(data, ..., fill = list()) {
   full <- expand(data, ...)
+  if (is_empty(full)) {
+    return(data)
+  }
   full <- dplyr::left_join(full, data, by = names(full))
   full <- replace_na(full, replace = fill)
 

R/expand.R

@@ -124,6 +124,9 @@ crossing <- function(...) {
   stopifnot(is_list(x))
 
   x <- drop_empty(x)
+  if (length(x) == 0) {
+    return(data.frame())
+  }
 
   is_atomic <- map_lgl(x, is_atomic)
   is_df <- map_lgl(x, is.data.frame)

tests/testthat/test-complete.R

@@ -16,8 +16,18 @@ test_that("preserves grouping", {
 
 test_that("expands empty factors", {
   f <- factor(levels = c("a", "b", "c"))
-  df <- dplyr::data_frame(one = f, two = f)
+  df <- tibble(one = f, two = f)
 
   expect_equal(nrow(complete(df, one, two)), 9)
   expect_equal(ncol(complete(df, one, two)), 2)
 })
+
+test_that("empty expansion returns original", {
+  df <- tibble(x = character())
+  rs <- complete(df, y = integer())
+  expect_equal(rs, df)
+
+  df <- tibble(x = 1:4)
+  rs <- complete(df, y = integer())
+  expect_equal(rs, df)
+})

tests/testthat/test-expand.R

@@ -63,6 +63,12 @@ test_that("zero length numeric & character inputs are automatically dropped", {
   expect_equal(crossing(x = tb$x, y = character()), tb)
 })
 
+test_that("zero length input gives zero length output", {
+  tb <- tibble(x = character())
+  expect_equal(expand(tb, x), tibble())
+  expect_equal(expand(tb, y = NULL), tibble())
+})
+
 test_that("zero length factor inputs are completed by expand & crossing, dropped by nesting", {
   tb <- tibble::tibble(x = 1:2)
   emptyfactor <- factor(levels = c("a","b"))