polars.Expr.bottom_k_by#

Return the elements corresponding to the k smallest elements of the by column(s).

This has time complexity:

\[O(n + k \log{n})\]

Parameters:

by: Column(s) used to determine the smallest elements. Accepts expression input. Strings are parsed as column names.
k: Number of elements to return.
descending: Consider the k largest elements of the by column(s) (instead of the k smallest). This can be specified per column by passing a sequence of booleans.
nulls_last: Place null values last.
maintain_order: Whether the order should be maintained if elements are equal.

Deprecated since version 0.20.31: This parameter will be removed in the next breaking release. There will be no guarantees about the order of the output.
multithreaded: Sort using multiple threads.

Deprecated since version 0.20.31: This parameter will be removed in the next breaking release. Polars itself will determine whether to use multithreading or not.

See also

top_k
top_k_by
bottom_k

Examples

>>> df = pl.DataFrame(
...     {
...         "a": [1, 2, 3, 4, 5, 6],
...         "b": [6, 5, 4, 3, 2, 1],
...         "c": ["Apple", "Orange", "Apple", "Apple", "Banana", "Banana"],
...     }
... )
>>> df
shape: (6, 3)
┌─────┬─────┬────────┐
│ a   ┆ b   ┆ c      │
│ --- ┆ --- ┆ ---    │
│ i64 ┆ i64 ┆ str    │
╞═════╪═════╪════════╡
│ 1   ┆ 6   ┆ Apple  │
│ 2   ┆ 5   ┆ Orange │
│ 3   ┆ 4   ┆ Apple  │
│ 4   ┆ 3   ┆ Apple  │
│ 5   ┆ 2   ┆ Banana │
│ 6   ┆ 1   ┆ Banana │
└─────┴─────┴────────┘

Get the bottom 2 rows by column a or b.

>>> df.select(
...     pl.all().bottom_k_by("a", 2).name.suffix("_btm_by_a"),
...     pl.all().bottom_k_by("b", 2).name.suffix("_btm_by_b"),
... )
shape: (2, 6)
┌────────────┬────────────┬────────────┬────────────┬────────────┬────────────┐
│ a_btm_by_a ┆ b_btm_by_a ┆ c_btm_by_a ┆ a_btm_by_b ┆ b_btm_by_b ┆ c_btm_by_b │
│ ---        ┆ ---        ┆ ---        ┆ ---        ┆ ---        ┆ ---        │
│ i64        ┆ i64        ┆ str        ┆ i64        ┆ i64        ┆ str        │
╞════════════╪════════════╪════════════╪════════════╪════════════╪════════════╡
│ 1          ┆ 6          ┆ Apple      ┆ 6          ┆ 1          ┆ Banana     │
│ 2          ┆ 5          ┆ Orange     ┆ 5          ┆ 2          ┆ Banana     │
└────────────┴────────────┴────────────┴────────────┴────────────┴────────────┘

Get the bottom 2 rows by multiple columns with given order.

>>> df.select(
...     pl.all()
...     .bottom_k_by(["c", "a"], 2, descending=[False, True])
...     .name.suffix("_by_ca"),
...     pl.all()
...     .bottom_k_by(["c", "b"], 2, descending=[False, True])
...     .name.suffix("_by_cb"),
... )
shape: (2, 6)
┌─────────┬─────────┬─────────┬─────────┬─────────┬─────────┐
│ a_by_ca ┆ b_by_ca ┆ c_by_ca ┆ a_by_cb ┆ b_by_cb ┆ c_by_cb │
│ ---     ┆ ---     ┆ ---     ┆ ---     ┆ ---     ┆ ---     │
│ i64     ┆ i64     ┆ str     ┆ i64     ┆ i64     ┆ str     │
╞═════════╪═════════╪═════════╪═════════╪═════════╪═════════╡
│ 4       ┆ 3       ┆ Apple   ┆ 1       ┆ 6       ┆ Apple   │
│ 3       ┆ 4       ┆ Apple   ┆ 3       ┆ 4       ┆ Apple   │
└─────────┴─────────┴─────────┴─────────┴─────────┴─────────┘

Get the bottom 2 rows by column a in each group.

>>> (
...     df.group_by("c", maintain_order=True)
...     .agg(pl.all().bottom_k_by("a", 2))
...     .explode(pl.all().exclude("c"))
... )
shape: (5, 3)
┌────────┬─────┬─────┐
│ c      ┆ a   ┆ b   │
│ ---    ┆ --- ┆ --- │
│ str    ┆ i64 ┆ i64 │
╞════════╪═════╪═════╡
│ Apple  ┆ 1   ┆ 6   │
│ Apple  ┆ 3   ┆ 4   │
│ Orange ┆ 2   ┆ 5   │
│ Banana ┆ 5   ┆ 2   │
│ Banana ┆ 6   ┆ 1   │
└────────┴─────┴─────┘