# sample in rand::seq::index - Rust
Randomly sample exactly `amount` distinct indices from `0..length`, and return them in random order (fully shuffled).
Keywords: rust, rustlang, rust-lang, sample

# Function rand::seq::index::sample

source · [−]

    pub fn sample<R>(rng: &mut R, length: usize, amount: usize) -> IndexVec where
        R: Rng + ?Sized,

Expand description

Randomly sample exactly `amount` distinct indices from `0..length`, and return them in random order (fully shuffled).

This method is used internally by the slice sampling methods, but it can sometimes be useful to have the indices themselves so this is provided as an alternative.

The implementation used is not specified; we automatically select the fastest available algorithm for the `length` and `amount` parameters (based on detailed profiling on an Intel Haswell CPU). Roughly speaking, complexity is `O(amount)`, except that when `amount` is small, performance is closer to `O(amount^2)`, and when `length` is close to `amount` then `O(length)`.

Note that performance is significantly better over `u32` indices than over `u64` indices. Because of this we hide the underlying type behind an abstraction, `IndexVec`.

If an allocation-free `no_std` function is required, it is suggested to adapt the internal `sample_floyd` implementation.

Panics if `amount > length`.