Add tests

test/explorer/data_frame_test.exs

@@ -3526,6 +3526,18 @@ defmodule Explorer.DataFrameTest do
              }
     end
 
+    test "mode/1" do
+      df =
+        Datasets.iris()
+        |> DF.group_by(:species)
+        |> DF.summarise(petal_width_mode: mode(petal_width))
+
+      assert DF.to_columns(df) == %{
+               "petal_width_mode" => [[0.2], [1.3], [1.8]],
+               "species" => ["Iris-setosa", "Iris-versicolor", "Iris-virginica"]
+             }
+    end
+
     test "argmax/1 and argmin/1" do
       df =
         DF.new(

test/explorer/series_test.exs

@@ -3598,6 +3598,76 @@ defmodule Explorer.SeriesTest do
     end
   end
 
+  describe "mode/1" do
+    test "returns the mode of an integer series" do
+      s = Series.from_list([1, 2, 2, 3])
+      mode = Series.mode(s)
+      s2 = Series.from_list([2])
+      assert Series.equal(mode, s2)
+    end
+
+    test "returns the mode of an integer series when it is multiple values" do
+      s = Series.from_list([1, 2, 2, 3, 3])
+      mode = s |> Series.mode() |> Series.sort()
+      s2 = Series.from_list([2, 3])
+      assert Series.equal(mode, s2)
+    end
+
+    test "returns the mode of a float series" do
+      s = Series.from_list([1.0, 2.0, 2.0, 3.0])
+      mode = Series.mode(s)
+      s2 = Series.from_list([2.0])
+      assert Series.equal(mode, s2)
+    end
+
+    test "returns the mode of a string series" do
+      s = Series.from_list(["a", "b", "b", "c"])
+      mode = Series.mode(s)
+      s2 = Series.from_list(["b"])
+      assert Series.equal(mode, s2)
+    end
+
+    test "returns the mode of a date series" do
+      s =
+        Series.from_list([
+          ~D[2022-01-01],
+          ~D[2022-01-02],
+          ~D[2022-01-02],
+          ~D[2022-01-03]
+        ])
+
+      mode = Series.mode(s)
+      s2 = Series.from_list([~D[2022-01-02]])
+      assert Series.equal(mode, s2)
+    end
+
+    test "returns the mode of a datetime series" do
+      s =
+        Series.from_list([
+          ~N[2022-01-01 00:00:00],
+          ~N[2022-01-01 00:01:00],
+          ~N[2022-01-01 00:01:00]
+        ])
+
+      mode = Series.mode(s)
+      s2 = Series.from_list([~N[2022-01-01 00:01:00]])
+      assert Series.equal(mode, s2)
+    end
+
+    test "returns the mode of a boolean series" do
+      s = Series.from_list([true, false, false, true])
+      mode = Series.mode(s)
+      s2 = Series.from_list([false])
+      assert Series.equal(mode, s2)
+    end
+
+    test "returns the mode of a category series" do
+      s = Series.from_list(["EUA", "Brazil", "Brazil", "Poland"], dtype: :category)
+      mode = Series.mode(s)
+      assert Series.to_list(mode) == ["Brazil"]
+    end
+  end
+
   describe "sum/1" do
     test "sum of integers" do
       s = Series.from_list([1, 2, nil, 3])