dataframe - Renaming a single column with unknown name in Polars-Rust - Stack Overflow

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The following renames a column in a Polars-Rust dataframe:

#![allow(unused_variables)]
use polars::prelude::*;

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

    println!("{:?}", df);

    let new_name = <PlSmallStr>::from_str("letters");
    let _ = df.rename("names", new_name);
    println!("{:?}", df);
}

Suppose now that the name of the first column is unknown (e.g., the dataframe is read from a csv/excel file, where it has no name), and I would like to rename it for the sake of future use:

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

    println!("{:?}", df);

    let old_name = df.get_column_names_str()[0];
    // let old_name = df.get_column_names_str()[0].clone();
    // let old_name = &mut df.get_column_names_str()[0].clone();
    // let mut old_name = &mut df.get_column_names_str()[0].clone();
    let new_name = <PlSmallStr>::from_str("letters");
    let _ = df.rename(old_name, new_name);
    println!("{:?}", df);
}

This results in an error

error[E0502]: cannot borrow `df` as mutable because it is also borrowed as immutable
  --> src/main.rs:32:13
   |
27 |     let old_name = df.get_column_names_str()[0];
   |                    -- immutable borrow occurs here
...
32 |     let _ = df.rename(old_name, new_name);
   |             ^^^------^^^^^^^^^^^^^^^^^^^^
   |             |  |
   |             |  immutable borrow later used by call
   |             mutable borrow occurs here

For more information about this error, try `rustc --explain E0502`.
error: could not compile `rename_col` (bin "rename_col") due to 1 previous error

It is more or less clear why the error occurs, but not clear how to fix it...
The commented lines show my various unsuccessful attempts at resolving this issue.

Related:
In Rust, how to rename all columns of a Polars Dataframe?
How to rename column names with first row in polars?

The following renames a column in a Polars-Rust dataframe:

#![allow(unused_variables)]
use polars::prelude::*;

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

    println!("{:?}", df);

    let new_name = <PlSmallStr>::from_str("letters");
    let _ = df.rename("names", new_name);
    println!("{:?}", df);
}

Suppose now that the name of the first column is unknown (e.g., the dataframe is read from a csv/excel file, where it has no name), and I would like to rename it for the sake of future use:

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

    println!("{:?}", df);

    let old_name = df.get_column_names_str()[0];
    // let old_name = df.get_column_names_str()[0].clone();
    // let old_name = &mut df.get_column_names_str()[0].clone();
    // let mut old_name = &mut df.get_column_names_str()[0].clone();
    let new_name = <PlSmallStr>::from_str("letters");
    let _ = df.rename(old_name, new_name);
    println!("{:?}", df);
}

This results in an error

error[E0502]: cannot borrow `df` as mutable because it is also borrowed as immutable
  --> src/main.rs:32:13
   |
27 |     let old_name = df.get_column_names_str()[0];
   |                    -- immutable borrow occurs here
...
32 |     let _ = df.rename(old_name, new_name);
   |             ^^^------^^^^^^^^^^^^^^^^^^^^
   |             |  |
   |             |  immutable borrow later used by call
   |             mutable borrow occurs here

For more information about this error, try `rustc --explain E0502`.
error: could not compile `rename_col` (bin "rename_col") due to 1 previous error

It is more or less clear why the error occurs, but not clear how to fix it...
The commented lines show my various unsuccessful attempts at resolving this issue.

Related:
In Rust, how to rename all columns of a Polars Dataframe?
How to rename column names with first row in polars?

• dataframe
• rust
• rust-polars

Share Improve this question Follow edited Feb 24 at 13:29 Roger V. asked Feb 24 at 13:28 Roger V.Roger V. 73555 silver badges1818 bronze badges 3

• 1 The simplest is probably to just turn the old name into an owned String. – cafce25 Commented Feb 24 at 13:29
• @cafce25 could you spell it code? – Roger V. Commented Feb 24 at 13:30
• stackoverflow/questions/41034635/… – cafce25 Commented Feb 24 at 13:32

Add a comment  | 

2 Answers 2

Sorted by: Reset to default 2

Alternatively you can use df.get_column_names_owned() function which will return Vec<PlSmallStr>.

let column_names = df.get_column_names_owned();
let old_name = column_names.first().unwrap();
let new_name = <PlSmallStr>::from_str("letters");
let _ = df.rename(old_name, new_name);

OR

let old_name = &df.get_column_names_owned()[0];
let new_name = <PlSmallStr>::from_str("letters");
let _ = df.rename(old_name, new_name);
println!("{:?}", df);

The following works (thanks to a hint from @cafce25):

#![allow(unused_variables)]
use polars::prelude::*;

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

    println!("{:?}", df);

    let old_name = df.get_column_names_str()[0].to_owned();
    let new_name = <PlSmallStr>::from_str("letters");
    let _ = df.rename(&old_name, new_name);
    println!("{:?}", df);
}

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