# WeightedIndex in rand::distributions::weighted - Rust
A distribution using weighted sampling of discrete items
Keywords: rust, rustlang, rust-lang, WeightedIndex

# Struct rand::distributions::weighted::WeightedIndex

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    pub struct WeightedIndex<X: SampleUniform + PartialOrd> { /* private fields */ }

Expand description

A distribution using weighted sampling of discrete items

Sampling a `WeightedIndex` distribution returns the index of a randomly selected element from the iterator used when the `WeightedIndex` was created. The chance of a given element being picked is proportional to the value of the element. The weights can use any type `X` for which an implementation of `Uniform<X>` exists.

## Performance

Time complexity of sampling from `WeightedIndex` is `O(log N)` where `N` is the number of weights. As an alternative, `rand_distr::weighted_alias` supports `O(1)` sampling, but with much higher initialisation cost.

A `WeightedIndex<X>` contains a `Vec<X>` and a `Uniform<X>` and so its size is the sum of the size of those objects, possibly plus some alignment.

Creating a `WeightedIndex<X>` will allocate enough space to hold `N - 1` weights of type `X`, where `N` is the number of weights. However, since `Vec` doesn’t guarantee a particular growth strategy, additional memory might be allocated but not used. Since the `WeightedIndex` object also contains, this might cause additional allocations, though for primitive types, `Uniform<X>` doesn’t allocate any memory.

Sampling from `WeightedIndex` will result in a single call to `Uniform<X>::sample` (method of the `Distribution` trait), which typically will request a single value from the underlying `RngCore`, though the exact number depends on the implementation of `Uniform<X>::sample`.

## Example

    use rand::prelude::*;
    use rand::distributions::WeightedIndex;

    let choices = ['a', 'b', 'c'];
    let weights = [2,   1,   1];
    let dist = WeightedIndex::new(&weights).unwrap();
    let mut rng = thread_rng();
    for _ in 0..100 {
        // 50% chance to print 'a', 25% chance to print 'b', 25% chance to print 'c'
        println!("{}", choices[dist.sample(&mut rng)]);
    }

    let items = [('a', 0), ('b', 3), ('c', 7)];
    let dist2 = WeightedIndex::new(items.iter().map(|item| item.1)).unwrap();
    for _ in 0..100 {
        // 0% chance to print 'a', 30% chance to print 'b', 70% chance to print 'c'
        println!("{}", items[dist2.sample(&mut rng)].0);
    }

## Implementations

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### impl<X: SampleUniform + PartialOrd> WeightedIndex<X>

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#### pub fn new<I>(weights: I) -> Result<WeightedIndex<X>, WeightedError> where
I: IntoIterator,
I::Item: SampleBorrow<X>,
X: for<'a> AddAssign<&'a X> + Clone + Default,

Creates a new a `WeightedIndex` `Distribution` using the values in `weights`. The weights can use any type `X` for which an implementation of `Uniform<X>` exists.

Returns an error if the iterator is empty, if any weight is `< 0`, or if its total value is 0.

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#### pub fn update_weights(
&mut self,
new_weights: &[ ( usize, &X) ]
) -> Result<(), WeightedError> where
X: for<'a> AddAssign<&'a X> + for<'a> SubAssign<&'a X> + Clone + Default,

Update a subset of weights, without changing the number of weights.

`new_weights` must be sorted by the index.

Using this method instead of `new` might be more efficient if only a small number of weights is modified. No allocations are performed, unless the weight type `X` uses allocation internally.

In case of error, `self` is not modified.

## Trait Implementations

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### impl<X: Clone + SampleUniform + PartialOrd> Clone for WeightedIndex<X> where
X::Sampler: Clone,

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#### fn clone(&self) -> WeightedIndex<X>

Returns a copy of the value. Read more

1.0.0 · source

#### fn clone_from(&mut self, source: &Self)

Performs copy-assignment from `source`. Read more

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### impl<X: Debug + SampleUniform + PartialOrd> Debug for WeightedIndex<X> where
X::Sampler: Debug,

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#### fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter. Read more

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### impl<X> Distribution<usize> for WeightedIndex<X> where
X: SampleUniform + PartialOrd,

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#### fn sample<R: Rng + ?Sized>(&self, rng: &mut R) -> usize

Generate a random value of `T`, using `rng` as the source of randomness.

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#### fn sample_iter<R>(self, rng: R) -> DistIter<Self, R, T>ⓘNotable traits for DistIter<D, R, T>`impl<D, R, T> Iterator for DistIter<D, R, T> where
D: Distribution<T>,
R: Rng, type Item = T;` where
R: Rng,
Self: Sized,

Create an iterator that generates random values of `T`, using `rng` as the source of randomness. Read more

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#### fn map<F, S>(self, func: F) -> DistMap<Self, F, T, S> where
F: Fn(T) -> S,
Self: Sized,

Create a distribution of values of ‘S’ by mapping the output of `Self` through the closure `F` Read more

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### impl<X: PartialEq + SampleUniform + PartialOrd> PartialEq<WeightedIndex<X>> for WeightedIndex<X> where
X::Sampler: PartialEq,

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#### fn eq(&self, other: &WeightedIndex<X>) -> bool

This method tests for `self` and `other` values to be equal, and is used by `==`. Read more

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#### fn ne(&self, other: &WeightedIndex<X>) -> bool

This method tests for `!=`.

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### impl<X: SampleUniform + PartialOrd> StructuralPartialEq for WeightedIndex<X>

## Auto Trait Implementations

### impl<X> RefUnwindSafe for WeightedIndex<X> where
X: RefUnwindSafe,
<X as SampleUniform>::Sampler: RefUnwindSafe,

### impl<X> Send for WeightedIndex<X> where
X: Send,
<X as SampleUniform>::Sampler: Send,

### impl<X> Sync for WeightedIndex<X> where
X: Sync,
<X as SampleUniform>::Sampler: Sync,

### impl<X> Unpin for WeightedIndex<X> where
X: Unpin,
<X as SampleUniform>::Sampler: Unpin,

### impl<X> UnwindSafe for WeightedIndex<X> where
X: UnwindSafe,
<X as SampleUniform>::Sampler: UnwindSafe,

## Blanket Implementations

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### impl<T> Any for T where
T: 'static + ?Sized,

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#### fn type_id(&self) -> TypeId

Gets the `TypeId` of `self`. Read more

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### impl<T> Borrow<T> for T where
T: ?Sized,

const: unstable · source

#### fn borrow(&self) -> &T

Immutably borrows from an owned value. Read more

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### impl<T> BorrowMut<T> for T where
T: ?Sized,

const: unstable · source

#### fn borrow_mut(&mut self) -> &mut T

Mutably borrows from an owned value. Read more

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### impl<T> From<T> for T

const: unstable · source

#### fn from(t: T) -> T

Returns the argument unchanged.

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### impl<T, U> Into<U> for T where
U: From<T>,

const: unstable · source

#### fn into(self) -> U

Calls `U::from(self)`.

That is, this conversion is whatever the implementation of `From<T> for U` chooses to do.

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### impl<T> ToOwned for T where
T: Clone,

#### type Owned = T

The resulting type after obtaining ownership.

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#### fn to_owned(&self) -> T

Creates owned data from borrowed data, usually by cloning. Read more

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#### fn clone_into(&self, target: &mut T)

🔬 This is a nightly-only experimental API. (`toowned_clone_into`)

Uses borrowed data to replace owned data, usually by cloning. Read more

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### impl<T, U> TryFrom<U> for T where
U: Into<T>,

#### type Error = Infallible

The type returned in the event of a conversion error.

const: unstable · source

#### fn try_from(value: U) -> Result<T, <T as TryFrom<U>>::Error>

Performs the conversion.

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### impl<T, U> TryInto<U> for T where
U: TryFrom<T>,

#### type Error = <U as TryFrom<T>>::Error

The type returned in the event of a conversion error.

const: unstable · source

#### fn try_into(self) -> Result<U, <U as TryFrom<T>>::Error>

Performs the conversion.

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### impl<V, T> VZip<V> for T where
V: MultiLane<T>,

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#### fn vzip(self) -> V