In Rust, how to rename all columns of a Polars Dataframe?

Question

Rust newbie here, I want to use Polars to summarize a dataset that didn't come with column names. If I just use df directly, the error was: cannot borrow df as mutable because it is also borrowed as immutable. How to fix it?

use polars::prelude::*;

fn main() {
    let mut df = df! [
        "names" => ["a", "b", "c"],
        "values" => [1, 2, 3],
    ].unwrap();

    let columns = vec![
        "name_chg",
        "value_chg",
    ];

    df
    .get_column_names()
    .iter()
    .zip(columns)
    .for_each(|(old, new)| {df.rename(old, new);}); // cannot borrow `df` as mutable because it is also borrowed as immutable

    println!("{df}");
}

Answer 1

If you want to rename all columns, you can use df.set_column_names.

use polars::prelude::*;

fn main() {
    let mut df = df! [
        "names" => ["a", "b", "c"],
        "values" => [1, 2, 3],
    ]
    .unwrap();

    println!("BEFORE: {df}");
    
    let columns = vec!["name_chg", "value_chg"];    
    df.set_column_names(columns.as_slice()).unwrap();

    println!("AFTER: {df}");
}

This will produce following output:

BEFORE: shape: (3, 2)
┌───────┬────────┐
│ names ┆ values │
│ ---   ┆ ---    │
│ str   ┆ i32    │
╞═══════╪════════╡
│ a     ┆ 1      │
│ b     ┆ 2      │
│ c     ┆ 3      │
└───────┴────────┘
AFTER: shape: (3, 2)
┌──────────┬───────────┐
│ name_chg ┆ value_chg │
│ ---      ┆ ---       │
│ str      ┆ i32       │
╞══════════╪═══════════╡
│ a        ┆ 1         │
│ b        ┆ 2         │
│ c        ┆ 3         │
└──────────┴───────────┘

set_column_names API will zip the current columns and the column names you passed in. And then perform the .rename operation. It's equivalent to

pub fn set_column_names<S: AsRef<str>>(&mut self, names: &[S]) -> PolarsResult<()> {
        // Skipping other part of the code for brevity
        let columns = mem::take(&mut self.columns);
        self.columns = columns
            .into_iter()
            .zip(names)
            .map(|(s, name)| {
                let mut s = s;
                s.rename(name.as_ref());
                s
            })
            .collect();
        Ok(())
    }