# Struct rand::distributions::weighted::WeightedIndex [−][src]

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

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

[src]

`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,

[src]

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.

`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,

[src]

&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

`impl<X: Clone + SampleUniform + PartialOrd> Clone for WeightedIndex<X> where`

X::Sampler: Clone,

[src]

X::Sampler: Clone,

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

[src]

`pub fn clone_from(&mut self, source: &Self)`

1.0.0[src]

`impl<X: Debug + SampleUniform + PartialOrd> Debug for WeightedIndex<X> where`

X::Sampler: Debug,

[src]

X::Sampler: Debug,

`impl<X> Distribution<usize> for WeightedIndex<X> where`

X: SampleUniform + PartialOrd,

[src]

X: SampleUniform + PartialOrd,

## Auto Trait Implementations

`impl<X> RefUnwindSafe for WeightedIndex<X> where`

X: RefUnwindSafe,

<X as SampleUniform>::Sampler: RefUnwindSafe,

X: RefUnwindSafe,

<X as SampleUniform>::Sampler: RefUnwindSafe,

`impl<X> Send for WeightedIndex<X> where`

X: Send,

<X as SampleUniform>::Sampler: Send,

X: Send,

<X as SampleUniform>::Sampler: Send,

`impl<X> Sync for WeightedIndex<X> where`

X: Sync,

<X as SampleUniform>::Sampler: Sync,

X: Sync,

<X as SampleUniform>::Sampler: Sync,

`impl<X> Unpin for WeightedIndex<X> where`

X: Unpin,

<X as SampleUniform>::Sampler: Unpin,

X: Unpin,

<X as SampleUniform>::Sampler: Unpin,

`impl<X> UnwindSafe for WeightedIndex<X> where`

X: UnwindSafe,

<X as SampleUniform>::Sampler: UnwindSafe,

X: UnwindSafe,

<X as SampleUniform>::Sampler: UnwindSafe,

## Blanket Implementations

`impl<T> Any for T where`

T: 'static + ?Sized,

[src]

T: 'static + ?Sized,

`impl<T> Borrow<T> for T where`

T: ?Sized,

[src]

T: ?Sized,

`impl<T> BorrowMut<T> for T where`

T: ?Sized,

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T: ?Sized,

`pub fn borrow_mut(&mut self) -> &mut T`

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`impl<T> From<T> for T`

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`impl<T, U> Into<U> for T where`

U: From<T>,

[src]

U: From<T>,

`impl<T> ToOwned for T where`

T: Clone,

[src]

T: Clone,

`type Owned = T`

The resulting type after obtaining ownership.

`pub fn to_owned(&self) -> T`

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`pub fn clone_into(&self, target: &mut T)`

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`impl<T, U> TryFrom<U> for T where`

U: Into<T>,

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U: Into<T>,

`type Error = Infallible`

The type returned in the event of a conversion error.

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

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`impl<T, U> TryInto<U> for T where`

U: TryFrom<T>,

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U: TryFrom<T>,

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

The type returned in the event of a conversion error.

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

[src]

`impl<V, T> VZip<V> for T where`

V: MultiLane<T>,

[src]

V: MultiLane<T>,