Series.row_index/1

lib/explorer/backend/lazy_series.ex

@@ -116,6 +116,7 @@ defmodule Explorer.Backend.LazySeries do
     covariance: 3,
     all: 1,
     any: 1,
+    row_index: 1,
     # Strings
     contains: 2,
     replace: 3,
@@ -1111,6 +1112,13 @@ defmodule Explorer.Backend.LazySeries do
     Backend.Series.new(data, :string)
   end
 
+  @impl true
+  def row_index(series) do
+    data = new(:row_index, [lazy_series!(series)], {:u, 32})
+
+    Backend.Series.new(data, {:u, 32})
+  end
+
   @remaining_non_lazy_operations [
     at: 2,
     at_every: 2,

lib/explorer/backend/series.ex

@@ -93,6 +93,7 @@ defmodule Explorer.Backend.Series do
   @callback covariance(s, s, ddof :: non_neg_integer()) :: float() | non_finite() | lazy_s() | nil
   @callback all?(s) :: boolean() | lazy_s()
   @callback any?(s) :: boolean() | lazy_s()
+  @callback row_index(s) :: s | lazy_s()
 
   # Cumulative
 

lib/explorer/polars_backend/expression.ex

@@ -161,6 +161,7 @@ defmodule Explorer.PolarsBackend.Expression do
     shift: 3,
     slice: 2,
     slice: 3,
+    row_index: 1,
     concat: 1,
     column: 1,
     correlation: 4,
@@ -296,6 +297,11 @@ defmodule Explorer.PolarsBackend.Expression do
     end
   end
 
+  def to_expr(%LazySeries{op: :row_index, args: [lazy_series]}) do
+    size_expr = Native.expr_size(to_expr(lazy_series))
+    Native.expr_int_range(to_expr(0), size_expr, 1, {:u, 32})
+  end
+
   for {op, arity} <- @all_expressions do
     args = Macro.generate_arguments(arity, __MODULE__)
 

lib/explorer/polars_backend/native.ex

@@ -210,6 +210,7 @@ defmodule Explorer.PolarsBackend.Native do
   def expr_describe_filter_plan(_df, _expr), do: err()
   def expr_float(_number), do: err()
   def expr_integer(_number), do: err()
+  def expr_int_range(_start, _end, _step, _dtype), do: err()
   def expr_series(_series), do: err()
   def expr_string(_string), do: err()
   def expr_struct(_map), do: err()
@@ -382,6 +383,7 @@ defmodule Explorer.PolarsBackend.Native do
   def s_rename(_s, _name), do: err()
   def s_reverse(_s), do: err()
   def s_round(_s, _decimals), do: err()
+  def s_row_index(_s), do: err()
   def s_floor(_s), do: err()
   def s_ceil(_s), do: err()
   def s_rstrip(_s, _string), do: err()

lib/explorer/polars_backend/series.ex

@@ -236,6 +236,9 @@ defmodule Explorer.PolarsBackend.Series do
   @impl true
   def any?(series), do: Shared.apply_series(series, :s_any)
 
+  @impl true
+  def row_index(series), do: Shared.apply_series(series, :s_row_index)
+
   # Cumulative
 
   @impl true

lib/explorer/series.ex

@@ -2909,6 +2909,43 @@ defmodule Explorer.Series do
   def any?(%Series{dtype: :boolean} = series), do: apply_series(series, :any?)
   def any?(%Series{dtype: dtype}), do: dtype_error("any?/1", dtype, [:boolean])
 
+  @doc """
+  Returns a series of indexes for each item (row) in the series, starting from 0.
+
+  ## Examples
+
+      iex> s = Series.from_list([nil, true, true])
+      iex> Series.row_index(s)
+      #Explorer.Series<
+        Polars[3]
+        u32 [0, 1, 2]
+      >
+
+  This function can be used to add a row index as the first column of a dataframe.
+  The resulting column is a regular column of type `:u32`.
+
+      iex> require Explorer.DataFrame, as: DF
+      iex> df = DF.new(a: [1, 3, 5], b: [2, 4, 6])
+      iex> DF.mutate(df, index: row_index(a)) |> DF.relocate("index", before: 0)
+      #Explorer.DataFrame<
+        Polars[3 x 3]
+        index u32 [0, 1, 2]
+        a s64 [1, 3, 5]
+        b s64 [2, 4, 6]
+      >
+      iex> df = DF.new(a: [1, 3, 5], b: [2, 4, 6])
+      iex> DF.mutate(df, id: row_index(a) + 1000)
+      #Explorer.DataFrame<
+        Polars[3 x 3]
+        a s64 [1, 3, 5]
+        b s64 [2, 4, 6]
+        id s64 [1000, 1001, 1002]
+      >
+  """
+  @doc type: :shape
+  @spec row_index(Series.t()) :: Series.t()
+  def row_index(%Series{} = series), do: apply_series(series, :row_index)
+
   # Cumulative
 
   @doc """

native/explorer/Cargo.toml

@@ -80,6 +80,7 @@ features = [
   "product",
   "peaks",
   "moment",
+  "range",
   "rank",
   "propagate_nans",
   "extract_jsonpath"

native/explorer/src/expressions.rs

@@ -1054,6 +1054,16 @@ pub fn expr_join(expr: ExExpr, sep: String) -> ExExpr {
     ExExpr::new(expr.list().join(sep.lit()))
 }
 
+#[rustler::nif]
+pub fn expr_int_range(start: ExExpr, end: ExExpr, step: i64, dtype: ExSeriesDtype) -> ExExpr {
+    let start = start.clone_inner();
+    let end = end.clone_inner();
+    let dtype = DataType::try_from(&dtype).unwrap();
+    let expr = dsl::int_range(start, end, step, dtype);
+
+    ExExpr::new(expr)
+}
+
 #[rustler::nif]
 pub fn expr_lengths(expr: ExExpr) -> ExExpr {
     let expr = expr.clone_inner();

native/explorer/src/lib.rs

@@ -166,6 +166,7 @@ rustler::init!(
         expr_float,
         expr_head,
         expr_integer,
+        expr_int_range,
         expr_peaks,
         expr_rank,
         expr_unary_not,
@@ -439,6 +440,7 @@ rustler::init!(
         s_rename,
         s_replace,
         s_reverse,
+        s_row_index,
         s_rstrip,
         s_sample_n,
         s_sample_frac,

native/explorer/src/series.rs

@@ -1864,3 +1864,10 @@ pub fn s_json_path_match(s: ExSeries, json_path: &str) -> Result<ExSeries, Explo
         .clone();
     Ok(ExSeries::new(s2))
 }
+
+#[rustler::nif]
+pub fn s_row_index(series: ExSeries) -> Result<ExSeries, ExplorerError> {
+    let len = u32::try_from(series.len())?;
+    let s = Series::new("row_index", 0..len);
+    Ok(ExSeries::new(s))
+}

test/explorer/data_frame_test.exs

@@ -4452,4 +4452,25 @@ defmodule Explorer.DataFrameTest do
              }
     end
   end
+
+  describe "row_index/1" do
+    test "works as row_count(), including offset" do
+      df = DF.new(a: [1, 3, 5], b: [2, 4, 6])
+      df1 = DF.mutate(df, index: row_index(a))
+
+      assert DF.to_columns(df1, atom_keys: true) == %{
+               a: [1, 3, 5],
+               b: [2, 4, 6],
+               index: [0, 1, 2]
+             }
+
+      df2 = DF.mutate(df, id: row_index(a) + 1000)
+
+      assert DF.to_columns(df2, atom_keys: true) == %{
+               a: [1, 3, 5],
+               b: [2, 4, 6],
+               id: [1000, 1001, 1002]
+             }
+    end
+  end
 end