edponce commented on a change in pull request #11176:
URL: https://github.com/apache/arrow/pull/11176#discussion_r711814286



##########
File path: r/tests/testthat/test-dplyr.R
##########
@@ -989,19 +989,86 @@ test_that("sign()", {
   )
 })
 
-test_that("ceiling(), floor(), trunc()", {
+test_that("ceiling(), floor(), trunc(), round()", {
   df <- tibble(x = c(-1, -0.55, -0.5, -0.1, 0, 0.1, 0.5, 0.55, 1, NA, NaN))
 
   expect_dplyr_equal(
     input %>%
       mutate(
         c = ceiling(x),
         f = floor(x),
-        t = trunc(x)
+        t = trunc(x),
+        r = round(x)
       ) %>%
       collect(),
     df
   )
+
+  # with digits set to 1
+  expect_dplyr_equal(
+    input %>%
+      filter(x %% 0.5 == 0) %>%
+      mutate(r = round(x, 1)) %>%
+      collect(),
+    df
+  )
+
+  # with digits set to -1
+  expect_dplyr_equal(
+    input %>%
+      mutate(
+        rd = round(floor(x * 111), -1), # double
+        y = ifelse(is.nan(x), NA_integer_, x),
+        ri = round(as.integer(y * 111), -1) # integer (with the NaN removed)
+      ) %>%
+      collect(),
+    df
+  )
+
+  # round(x, -2) is equivalent to round_to_multiple(x, 100)
+  expect_equal(
+    Table$create(x = 1111.1) %>%
+      mutate(r = round(x, -2)) %>%
+      collect(),
+    Table$create(x = 1111.1) %>%
+      mutate(r = arrow_round_to_multiple(x, options = list(multiple = 100))) 
%>%
+      collect()
+  )
+
+  skip_on_os("windows") # float representation error might cause inconsistency
+
+  expect_dplyr_equal(
+    input %>%
+      mutate(r = round(x, 1)) %>%
+      collect(),
+    df
+  )
+
+  # verify that round mode HALF_TO_EVEN which is what the round() binding uses
+  # yields results consistent with R...
+  expect_equal(
+    as.vector(
+      call_function(
+        "round",
+        Array$create(df$x),
+        options = list(ndigits = 1L, round_mode = RoundMode$HALF_TO_EVEN)
+      )
+    ),
+    round(df$x, 1)
+  )
+  # ... but round mode HALF_TOWARDS_ZERO does not.

Review comment:
       @ianmcook Thanks for the clarification, I understand now the 
valid/invalid tests you set up. I previously thought that HALF_TOWARDS_ZERO was 
set for both approaches but errors were showing up. It was my mistake, I did 
not read carefully the comments.




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