如何乘/除/加/减不同类型的数字? [英] How to multiply/divide/add/subtract numbers of different types?

问题描述

我正在研究Rust手册的第二版，并决定尝试制作经典的摄氏转华氏转换器:

I'm working through the second edition of the Rust handbook, and decided to try and make the classic Celsius-to-Fahrenheit converter:

fn c_to_f(c: f32) -> f32 {
    return ( c * ( 9/5 ) ) + 32;
}

使用cargo build进行编译将产生编译时错误:

Compiling this with cargo build will yield the compile-time error:

error[E0277]: the trait bound `f32: std::ops::Mul<{integer}>` is not satisfied
 --> src/main.rs:2:12
  |
2 |     return (c * (9 / 5)) + 32;
  |            ^^^^^^^^^^^^^ the trait `std::ops::Mul<{integer}>` is not implemented for `f32`
  |
  = note: no implementation for `f32 * {integer}`

作为新的Rust程序员，我的解释是我不能将float和integer类型相乘.我通过使所有常量成为浮点数来解决了这个问题:

As a new Rust programmer, my interpretation is that I cannot multiply float and integer types together. I solved this by making all of my constants floating points:

fn c_to_f(c: f32) -> f32 {
    return ( c * ( 9.0/5.0 ) ) + 32.0;
}

这让我有所保留.来自C/C ++/Java/Python，令人惊讶的是得知您不能简单地对不同类型的数字执行算术运算.像我在这里所做的那样，将它们简单地转换为相同类型是正确的选择吗?

This leaves me with reservations. Coming from C/C++/Java/Python, it was surprising to learn that you cannot simply perform arithmetic on numbers of different types. Is the right thing to simply convert them to the same type, as I did here?

推荐答案

TL; DR :as是在原始数字类型之间进行转换的最常见方法，但是使用它需要思考. /p>

TL;DR: as is the most common way to convert between the primitive numeric types but using it requires thinking.

fn c_to_f(c: f32) -> f32 {
    (c * (9 as f32 / 5 as f32)) + 32 as f32
}

不过在此示例中，使用浮点字面量以以下内容开头更合理:

In this example though, it's more reasonable to just use floating point literals to start with:

fn c_to_f(c: f32) -> f32 {
    (c * (9. / 5.)) + 32.
}

---

真实问题是，进行混合类型算术有点复杂.

---

The real problem is that doing mixed type arithmetic is a bit complicated.

如果将 ¹ a T乘以T，则通常希望至少使用基本类型得到类型为T的结果.

If you are multiplying¹ a T by a T, you generally expect to get a result of type T, at least with the basic types.

但是，在混合类型时，存在一些困难:

When mixing types, however, there are some difficulties:

• 混合签名，
• 混合精度.

例如，i8 * u32的理想结果是什么?可以包含所有i8和u32值的完整集合的最小类型是i64.那应该是结果吗?

So, for example, what is the ideal result of i8 * u32? The smallest type that can encompass the full set of all i8 and u32 values is a i64. Should that be the result?

作为另一个示例，f32 * i32的理想结果是什么?可以包含所有f32和i32值的完整集合的最小类型是f64.那应该是结果吗?

As another example, what is the ideal result of f32 * i32? The smallest type that can encompass the full set of all f32 and i32 values is a f64. Should that be the result?

我发现进行这样的扩大的想法相当混乱.它也会对性能产生影响(一旦向量化，对f32的操作可能比对f64的操作要快得多).

I find the idea of having a such widening rather confusing. It also has performance impacts (operations on f32 can be much speedier than operations on f64, once vectorized).

由于这些问题，Rust现在要求您明确:您希望将计算携带到哪种类型?哪种类型适合您的特定情况?

Due to those issues, Rust for now requires you to be explicit: which type do you want the computation to be carried in? Which type makes sense for your particular situation?

然后使用as进行适当的转换，并考虑要应用哪种舍入模式(当从浮点变为整数时，是.round()，.ceil()，.floor()或.trunc()).

And then cast appropriately, using as, and do think about which rounding mode to apply (.round(), .ceil(), .floor() or .trunc() when going from floating point to integral).

¹ 以类似方式进行加，减和除工作.

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