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

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

A distribution using weighted sampling to pick a discretely selected item.

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

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`] doesn't allocate any memory.

Time complexity of sampling from `WeightedIndex` is `O(log N)` where `N` is the number of weights.

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 implementaiton 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);
    }

## Methods

### `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]

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.

## Trait Implementations

### `impl<X> Distribution<usize> for WeightedIndex<X> where
X: SampleUniform + PartialOrd,` [src]

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

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

####
ⓘImportant traits for DistIter<'a, D, R, T>

### Important traits for DistIter<'a, D, R, T>

`impl<'a, D, R, T> Iterator for DistIter<'a, D, R, T> where
D: Distribution<T>,
R: Rng + 'a, type Item = T;`
`fn sample_iter<'a, R>(&'a self, rng: &'a mut R) -> DistIter<'a, Self, R, T> where
Self: Sized,
R: Rng,` [src]

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

### `impl<X: Clone + SampleUniform + PartialOrd> Clone for WeightedIndex<X> where
X::Sampler: Clone,` [src]

#### `fn clone(&self) -> WeightedIndex<X>`[src]

Returns a copy of the value. Read more

#### `fn clone_from(&mut self, source: &Self)`1.0.0[src]

Performs copy-assignment from `source`. Read more

### `impl<X: Debug + SampleUniform + PartialOrd> Debug for WeightedIndex<X> where
X::Sampler: Debug,` [src]

#### `fn fmt(&self, f: &mut Formatter) -> Result`[src]

Formats the value using the given formatter. Read more

## Auto Trait Implementations

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

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

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

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

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

## Blanket Implementations

### `impl<T> ToOwned for T where
T: Clone,` [src]

#### `type Owned = T`

The resulting type after obtaining ownership.

#### `fn to_owned(&self) -> T`[src]

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

#### `fn clone_into(&self, target: &mut T)`[src]

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

recently added

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

### `impl<T, U> Into<U> for T where
U: From<T>,` [src]

#### `fn into(self) -> U`[src]

Performs the conversion.

### `impl<T> From<T> for T`[src]

#### `fn from(t: T) -> T`[src]

Performs the conversion.

### `impl<T, U> TryFrom<U> for T where
U: Into<T>,` [src]

#### `type Error = Infallible`

The type returned in the event of a conversion error.

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

Performs the conversion.

### `impl<T, U> TryInto<U> for T where
U: TryFrom<T>,` [src]

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

The type returned in the event of a conversion error.

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

Performs the conversion.

### `impl<T> BorrowMut<T> for T where
T: ?Sized,` [src]

#### `fn borrow_mut(&mut self) -> &mut T`[src]

Mutably borrows from an owned value. Read more

### `impl<T> Borrow<T> for T where
T: ?Sized,` [src]

#### `fn borrow(&self) -> &T`[src]

Immutably borrows from an owned value. Read more

### `impl<T> Any for T where
T: 'static + ?Sized,` [src]

#### `fn type_id(&self) -> TypeId`[src]

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

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