What is the idiomatic way to get the index of a maximum or minimum floating point value in a slice or Vec in Rust?

Question

Assumption -- The Vec does not have any NaN values or exhibit any NaN behavior.

T

Answer 1

The reason why this is tricky is because f32 does not implement Ord. That is because NaN values prevent floating point numbers from forming a total order, which violates the contract of Ord.

There are 3rd party crates that work around this by defining a numeric type wrapper which is not allowed to contain a NaN. One example is ordered-float. If you use this crate to first prepare the collection to contain NotNan values, then you can write code very close to your original idea:

use ordered_float::NotNan;

let non_nan_floats: Vec<_> = nets.iter()
    .cloned()
    .map(NotNan::new)       // Attempt to convert each f32 to a NotNan
    .filter_map(Result::ok) // Unwrap the `NotNan`s and filter out the `NaN` values 
    .collect();

let max = non_nan_floats.iter().max().unwrap();
let index = non_nan_floats.iter().position(|element| element == max).unwrap();

Add this to Cargo.toml:

[dependencies]
ordered-float = "1.0.1"

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Bonus material: The type conversion can be made truly zero-cost (assuming you are really sure that there are no NaN values!), by taking advantage of the fact that NotNan has a transparent representation:

let non_nan_floats: Vec> = unsafe { mem::transmute(nets) };