How to apply a closure to rust polars filter?

I'm processing data with polars in rust. I need to filter out some values depend on previous line or item in other columns. I have read the documents but they seem to use internal methods to filter each single va开发者_如何转开发lue, such as gt(), is_not_null() etc. Here are my needs:

1. filter non-monotonically increasing value in a column
2. filter a value depend on other values in the same row.
3. modify some values in a row depend on some values in current row.

I think I have to apply a closure to generate a boolean value when filtering, but I can't find suitable functions in document. So I want to know how to apply a closure to filter? Are there other methods could meet my needs? Thank in advance.

I'm only gonna answer '1.' since '2.' doesn't really make sense and '3.' is an unrelated question. You can filter monotonically by creating a mask from the series:

use polars::prelude::*;
use polars::df;

fn main() {
    let f = df!{
        "a" => [  1,   2,   2,   3,   4,   3],
        "b" => ["a", "a", "b", "a", "c", "d"],
        "c" => [  5,   4,   3,   2,   1,   0],
    }.unwrap();
    let mut previous = i32::MIN;
    let filter_monoton = f["a"].i32().unwrap().into_iter().map(|x| {
        let ret = if x.unwrap() > previous { true } else { false };
        previous = x.unwrap();
        ret
    }).collect();
    let mono = f.filter(&filter_monoton).unwrap();
    dbg!(&f);
    dbg!(&mono);
}

will output:

[src/main.rs:17] &f = shape: (6, 3)
┌─────┬─────┬─────┐
│ a   ┆ b   ┆ c   │
│ --- ┆ --- ┆ --- │
│ i32 ┆ str ┆ i32 │
╞═════╪═════╪═════╡
│ 1   ┆ a   ┆ 5   │
├╌╌╌╌╌┼╌╌╌╌╌┼╌╌╌╌╌┤
│ 2   ┆ a   ┆ 4   │
├╌╌╌╌╌┼╌╌╌╌╌┼╌╌╌╌╌┤
│ 2   ┆ b   ┆ 3   │
├╌╌╌╌╌┼╌╌╌╌╌┼╌╌╌╌╌┤
│ 3   ┆ a   ┆ 2   │
├╌╌╌╌╌┼╌╌╌╌╌┼╌╌╌╌╌┤
│ 4   ┆ c   ┆ 1   │
├╌╌╌╌╌┼╌╌╌╌╌┼╌╌╌╌╌┤
│ 3   ┆ d   ┆ 0   │
└─────┴─────┴─────┘
[src/main.rs:18] &mono = shape: (4, 3)
┌─────┬─────┬─────┐
│ a   ┆ b   ┆ c   │
│ --- ┆ --- ┆ --- │
│ i32 ┆ str ┆ i32 │
╞═════╪═════╪═════╡
│ 1   ┆ a   ┆ 5   │
├╌╌╌╌╌┼╌╌╌╌╌┼╌╌╌╌╌┤
│ 2   ┆ a   ┆ 4   │
├╌╌╌╌╌┼╌╌╌╌╌┼╌╌╌╌╌┤
│ 3   ┆ a   ┆ 2   │
├╌╌╌╌╌┼╌╌╌╌╌┼╌╌╌╌╌┤
│ 4   ┆ c   ┆ 1   │
└─────┴─────┴─────┘