mutate in order when using .by. Closes #166

R/mutate.R

@@ -74,7 +74,7 @@ mutate..data.frame <- function(.df, ..., .by = NULL) {
     # Faster with "by", since the "by" call isn't looped multiple times for each column added
     .by <- select_vec_chr(.df, !!.by)
 
-    needs_copy <- any(vec_in(names(dots), names(.df)))
+    needs_copy <- any(names(dots) %in% names(.df))
 
     if (needs_copy) .df <- copy(.df)
 
@@ -95,8 +95,14 @@ mutate..data.frame <- function(.df, ..., .by = NULL) {
     }
 
     if (length(dots) > 0) {
+
+      assign <- map2.(syms(names(dots)), dots, ~ call2("<-", .x, .y))
+      output <- call2("list", !!!syms(names(dots)))
+      expr <- call2("{", !!!assign, output)
+      j <- call2(":=", call2("c", !!!names(dots)), expr)
+
       eval_quo(
-        .df[, ':='(!!!dots), by = .by],
+        .df[, !!j, by = .by],
         new_data_mask(data_env), env = caller_env()
       )
     }

tests/testthat/test-mutate.R

@@ -105,6 +105,15 @@ test_that("can use .by", {
 
 })
 
+test_that("can mutate in order with .by", {
+  df <- tidytable(x = rep(1, 3), z = c("a", "a", "b"))
+
+  tidytable_df <- df %>%
+    mutate.(x = x + 1, y = x + 1, .by = z)
+
+  expect_equal(tidytable_df$y, rep(3, 3))
+})
+
 test_that("modify-by-reference doesn't occur with single val with .by", {
   df <- data.table(x = 1:3, y = 1:3)
   df %>%