实现泛型类型的均值函数

问题描述：

我正在尝试编写一个函数以返回Vector的均值.我希望它可以与泛型类型一起使用，但是在实现它时遇到了一些困难.

I'm trying to write a function to return the mean of a Vector. I want it to work with generic types but I'm having some difficulty implementing it.

extern crate num; // 0.2.0

use num::Zero;
use std::ops::{Add, Div};

pub struct Vector<T> {
    pub size: usize,
    pub data: Vec<T>,
}

impl<T: Copy + Zero + Add<T, Output = T>> Vector<T> {
    pub fn sum(&self) -> T {
        self.data.iter().fold(T::zero(), |sum, &val| sum + val)
    }
}

impl<T: Copy + Zero + Add<T, Output = T> + Div<T, Output = T>> Vector<T> {
    pub fn mean(&self) -> T {
        let sum = self.sum();
        sum / self.data.len()
    }
}

游乐场.

上面的示例未编译，因为self.data.len()是usize，而sum是T类型:

The above example doesn't compile as self.data.len() is a usize and sum is of type T:

error[E0308]: mismatched types
  --> src/lib.rs:20:15
   |
20 |         sum / self.data.len()
   |               ^^^^^^^^^^^^^^^ expected type parameter, found usize
   |
   = note: expected type `T`
              found type `usize`

我知道我可以将签名更改为:

I know I could change the signature to:

impl<T: Copy + Zero + Add<T, Output = T> + Div<usize, Output = T>> Vector<T>

它将编译-但这并未为Rust原语类型实现.我该怎么办?

It would compile - but this isn't implemented for the Rust primitive types. How should I go about this?

答

原始类型实现特征 "rel =" nofollow noreferrer> num板条箱，以允许在原始类型(包括usize)之间进行转换.我们可以在函数上添加FromPrimitive绑定，然后将usize转换为T:

The primitive types implement the FromPrimitive trait, defined in the num crate, to allow conversions between primitive types, including usize. We can add a FromPrimitive bound on the function, and then we can convert the usize to a T:

extern crate num; // 0.2.0

use num::{FromPrimitive, Zero};
use std::ops::{Add, Div};

impl<T> Vector<T>
where
    T: Copy + Zero + Add<T, Output = T> + Div<T, Output = T> + FromPrimitive,
{
    pub fn mean(&self) -> T {
        let sum = self.sum();
        sum / FromPrimitive::from_usize(self.data.len()).unwrap()
    }
}

实现泛型类型的均值函数

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