groupvalues function and Base.pairs method for GroupedDataFrame

src/groupeddataframe/grouping.jl

@@ -1214,3 +1214,48 @@ groupindices(gd::GroupedDataFrame) = replace(gd.groups, 0=>missing)
 Return a vector of column names in `parent(gd)` used for grouping.
 """
 groupvars(gd::GroupedDataFrame) = _names(gd)[gd.cols]
+
+"""
+    groupvalues(gd::GroupedDataFrame)
+
+Get a vector of values of the grouped columns for each group.
+
+### Returns
+
+Vector of tuples, where the `j`th element of the `i`th tuple is the value of the
+column `groupvars(gd)[j]` in `gd[i]`.
+
+Note: the values are always tuples, even if a single column was used in
+(`groupby`)[@ref].
+
+### Examples
+
+julia> df = DataFrame(a = repeat([:foo, :bar, :baz], outer=[2]),
+                      b = repeat([2, 1], outer=[3]),
+                      c = 1:6);
+
+julia> gd = groupby(df, :a)
+GroupedDataFrame with 3 groups based on key: a
+First Group (2 rows): a = :foo
+│ Row │ a      │ b     │ c     │
+│     │ Symbol │ Int64 │ Int64 │
+├─────┼────────┼───────┼───────┤
+│ 1   │ foo    │ 2     │ 1     │
+│ 2   │ foo    │ 1     │ 4     │
+⋮
+Last Group (2 rows): a = :baz
+│ Row │ a      │ b     │ c     │
+│     │ Symbol │ Int64 │ Int64 │
+├─────┼────────┼───────┼───────┤
+│ 1   │ baz    │ 2     │ 3     │
+│ 2   │ baz    │ 1     │ 6     │
+
+julia> groupvalues(gd)
+3-element Array{Tuple{Symbol},1}:
+ (:foo,)
+ (:bar,)
+ (:baz,)
+"""
+groupvalues(gd::GroupedDataFrame) = map(Tuple, eachrow(gb.parent[gb.idx[gb.starts], gb.cols]))
+
+Base.pairs(gd::GroupedDataFrame) = map(Pair, groupvalues(gd), gd)