Rust Tips 比较数值

寻月隐君
更新于 2024-07-07 22:50
阅读 1014

RustTips比较数值内容比较与类型转换浮点类型比较可以用这些运算符比较数值><==!=>=<=无法比较不同类型的值fnmain(){leta:i32=10;letb:u16=100;ifa<b{//报错

# Rust Tips 比较数值

### 内容

- 比较与类型转换
- 浮点类型比较

### 可以用这些运算符比较数值

`>  <  ==  !=  >=  <=`

### 无法比较不同类型的值

```rust
fn main() {
  let a: i32 = 10;
  let b: u16 = 100;
  if a < b { // 报错 mismatched types
    println!("Ten is less than one hundred.");
  }
}
```

### 解决办法 1：使用 as 进行类型转换

```rust
fn main() {
  let a: i32 = 10;
  let b: u16 = 100;
  if a < (b as i32) {
    println!("Ten is less than one hundred.");
  }
}
```

注意：从比较小的类型转成比较大的类型通常是比较安全的

```rust
fn main() {
    let a: i32 = 10;
    let b: u16 = 100;

if a < (b as i32) {
        println!("10 is less than 100.")
    }

let c : i32 = 1203414;
    println!("{}", c as i8);
}

```

运行

```bash
rust_compare_numerical_values on  master [?] is 📦 0.1.0 via 🦀 1.67.1 via 🅒 base 
➜ cargo run                              
   Compiling rust_compare_numerical_values v0.1.0 (/Users/qiaopengjun/rust/rust_compare_numerical_values)
    Finished dev [unoptimized + debuginfo] target(s) in 0.22s
     Running `target/debug/rust_compare_numerical_values`
10 is less than 100.
-42

rust_compare_numerical_values on  master [?] is 📦 0.1.0 via 🦀 1.67.1 via 🅒 base 
➜ 
```

### 解决办法 2：使用 try_into() 进行类型转换

```rust
use std::convert::TryInto;

fn main() {
    let a: i32 = 10;
    let b: u16 = 100;

let b_ = b.try_into().unwrap();

if a < b_ {
        println!("10 is less than 100.")
    }
}

```

运行

rust_compare_numerical_values on  master [?] is 📦 0.1.0 via 🦀 1.67.1 via 🅒 base 
➜ 
```

### try_into() 方法

- 导入 `std::convert::TryInto Trait`
- 返回 Result 类型

### 浮点类型（例如 f32、f64）有坑

- 浮点类型所代表的数字仅是近似值
  - 浮点类型是基于 2 进制实现的，但我们通常用 10 进制来计算数值
- 浮点类型的某些值不能很好的结合在一起
  - 例如 f32 和 f64 只实现了 `std::cmp::PartialEq`，而其它数值类型还实现了`std::cmp::Eq`

### 针对浮点类型需遵循的指导方针

- 避免测试浮点类型的相等性
- 如果结果在数学上属于未定义的，这时候要小心

```rust
fn main() {
    assert!(0.1 + 0.2 == 0.3);
}

```

运行

```bash
rust_compare_numerical_values on  master [?] is 📦 0.1.0 via 🦀 1.67.1 via 🅒 base 
➜ cargo run
   Compiling rust_compare_numerical_values v0.1.0 (/Users/qiaopengjun/rust/rust_compare_numerical_values)
    Finished dev [unoptimized + debuginfo] target(s) in 0.15s
     Running `target/debug/rust_compare_numerical_values`
thread 'main' panicked at 'assertion failed: 0.1 + 0.2 == 0.3', src/main.rs:2:5
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

rust_compare_numerical_values on  master [?] is 📦 0.1.0 via 🦀 1.67.1 via 🅒 base 
➜ 
```

例子

```rust
fn main() {
    let abc: (f32, f32, f32) = (0.1, 0.2, 0.3);
    let xyz: (f64, f64, f64) = (0.1, 0.2, 0.3);

println!("abc (f32)");
    println!(" 0.1 + 0.2: {:x}", (abc.0 + abc.1).to_bits());
    println!("       0.3: {:x}", (abc.2).to_bits());
    println!();

println!("xyz (f64)");
    println!(" 0.1 + 0.2: {:x}", (xyz.0 + xyz.1).to_bits());
    println!("       0.3: {:x}", (xyz.2).to_bits());
    println!();

assert!(abc.0 + abc.1 == abc.2);
    assert!(xyz.0 + xyz.1 == xyz.2);
}

```

运行

```bash
rust_compare_numerical_values on  master [?] is 📦 0.1.0 via 🦀 1.67.1 via 🅒 base 
➜ cargo run
   Compiling rust_compare_numerical_values v0.1.0 (/Users/qiaopengjun/rust/rust_compare_numerical_values)
    Finished dev [unoptimized + debuginfo] target(s) in 0.89s
     Running `target/debug/rust_compare_numerical_values`
abc (f32)
 0.1 + 0.2: 3e99999a
       0.3: 3e99999a

xyz (f64)
 0.1 + 0.2: 3fd3333333333334
       0.3: 3fd3333333333333

thread 'main' panicked at 'assertion failed: xyz.0 + xyz.1 == xyz.2', src/main.rs:16:5
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

rust_compare_numerical_values on  master [?] is 📦 0.1.0 via 🦀 1.67.1 via 🅒 base 
➜ 
```

### 怎么比较浮点类型？

- 一般来说，测试数学运算是否在其真实数学结果的可接受范围内更安全。这个边界通常被称为ε。
- Rust 提供了一些可容忍的误差值：f32::EPSILON 和 f64::EPSILON

```rust
fn main() {
    let result: f32 = 0.1 + 0.2;
    let desired: f32 = 0.3;

let abs_diff = (desired - result).abs();

assert!(abs_diff <= f32::EPSILON)
}

```

运行

rust_compare_numerical_values on  master [?] is 📦 0.1.0 via 🦀 1.67.1 via 🅒 base 
➜ 
```

例子

```rust
fn main() {
    let result: f64 = 0.1 + 0.2;
    let desired: f64 = 0.3;

print!("{}\n", desired - result);

let abs_diff = (desired - result).abs();

assert!(abs_diff <= f64::EPSILON)
}

```

运行

```bash
rust_compare_numerical_values on  master [?] is 📦 0.1.0 via 🦀 1.67.1 via 🅒 base 
➜ cargo run
   Compiling rust_compare_numerical_values v0.1.0 (/Users/qiaopengjun/rust/rust_compare_numerical_values)
    Finished dev [unoptimized + debuginfo] target(s) in 0.13s
     Running `target/debug/rust_compare_numerical_values`
-0.00000000000000005551115123125783

rust_compare_numerical_values on  master [?] is 📦 0.1.0 via 🦀 1.67.1 via 🅒 base 
➜

```

- Rust 编译器实际上将比较的工作交给了 CPU，浮点运算是使用芯片内的定制硬件实现的

### NAN

- 表示”不是一个数“，例如负数的平方根就是 NAN
- NAN 会影响其它数值：
  - 几乎所有与 NAN 交互的操作都返回 NAN
  - NAN 值永远不相等

```rust
fn main() {
    let x = (-42.0_f32).sqrt();

assert_eq!(x, x);
}

```

运行

```bash
rust_compare_numerical_values on  master [?] is 📦 0.1.0 via 🦀 1.67.1 via 🅒 base 
➜ cargo run
   Compiling rust_compare_numerical_values v0.1.0 (/Users/qiaopengjun/rust/rust_compare_numerical_values)
    Finished dev [unoptimized + debuginfo] target(s) in 0.38s
     Running `target/debug/rust_compare_numerical_values`
thread 'main' panicked at 'assertion failed: `(left == right)`
  left: `NaN`,
 right: `NaN`', src/main.rs:4:5
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

rust_compare_numerical_values on  master [?] is 📦 0.1.0 via 🦀 1.67.1 via 🅒 base 
➜ 
```

- Is_nan() 和 is_finite() 方法

```rust
fn main() {
    let x: f32 = 1.0 / 0.0;

println!("{}", x);
    
    assert!(x.is_finite());
}

```

运行

```bash
rust_compare_numerical_values on  master [?] is 📦 0.1.0 via 🦀 1.67.1 via 🅒 base 
➜ cargo run
   Compiling rust_compare_numerical_values v0.1.0 (/Users/qiaopengjun/rust/rust_compare_numerical_values)
    Finished dev [unoptimized + debuginfo] target(s) in 0.08s
     Running `target/debug/rust_compare_numerical_values`
inf
thread 'main' panicked at 'assertion failed: x.is_finite()', src/main.rs:6:5
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

rust_compare_numerical_values on  master [?] is 📦 0.1.0 via 🦀 1.67.1 via 🅒 base 
➜ 
```

Rust Tips 比较数值

内容

比较与类型转换
浮点类型比较

可以用这些运算符比较数值

> < == != >= <=

无法比较不同类型的值

fn main() {
  let a: i32 = 10;
  let b: u16 = 100;
  if a &lt; b { // 报错 mismatched types
    println!("Ten is less than one hundred.");
  }
}

解决办法 1：使用 as 进行类型转换

fn main() {
  let a: i32 = 10;
  let b: u16 = 100;
  if a &lt; (b as i32) {
    println!("Ten is less than one hundred.");
  }
}

注意：从比较小的类型转成比较大的类型通常是比较安全的

fn main() {
    let a: i32 = 10;
    let b: u16 = 100;

    if a &lt; (b as i32) {
        println!("10 is less than 100.")
    }

    let c : i32 = 1203414;
    println!("{}", c as i8);
}

运行

rust_compare_numerical_values on  master [?] is 📦 0.1.0 via 🦀 1.67.1 via 🅒 base 
➜ cargo run                              
   Compiling rust_compare_numerical_values v0.1.0 (/Users/qiaopengjun/rust/rust_compare_numerical_values)
    Finished dev [unoptimized + debuginfo] target(s) in 0.22s
     Running `target/debug/rust_compare_numerical_values`
10 is less than 100.
-42

rust_compare_numerical_values on  master [?] is 📦 0.1.0 via 🦀 1.67.1 via 🅒 base 
➜

解决办法 2：使用 try_into() 进行类型转换

use std::convert::TryInto;

fn main() {
    let a: i32 = 10;
    let b: u16 = 100;

    let b_ = b.try_into().unwrap();

    if a &lt; b_ {
        println!("10 is less than 100.")
    }
}

运行

rust_compare_numerical_values on  master [?] is 📦 0.1.0 via 🦀 1.67.1 via 🅒 base 
➜ cargo run
   Compiling rust_compare_numerical_values v0.1.0 (/Users/qiaopengjun/rust/rust_compare_numerical_values)
    Finished dev [unoptimized + debuginfo] target(s) in 0.20s
     Running `target/debug/rust_compare_numerical_values`
10 is less than 100.

rust_compare_numerical_values on  master [?] is 📦 0.1.0 via 🦀 1.67.1 via 🅒 base 
➜

try_into() 方法

导入 std::convert::TryInto Trait
返回 Result 类型

浮点类型（例如 f32、f64）有坑

浮点类型所代表的数字仅是近似值
- 浮点类型是基于 2 进制实现的，但我们通常用 10 进制来计算数值
浮点类型的某些值不能很好的结合在一起
- 例如 f32 和 f64 只实现了 std::cmp::PartialEq，而其它数值类型还实现了std::cmp::Eq

针对浮点类型需遵循的指导方针

避免测试浮点类型的相等性
如果结果在数学上属于未定义的，这时候要小心

fn main() {
    assert!(0.1 + 0.2 == 0.3);
}

运行

rust_compare_numerical_values on  master [?] is 📦 0.1.0 via 🦀 1.67.1 via 🅒 base 
➜ cargo run
   Compiling rust_compare_numerical_values v0.1.0 (/Users/qiaopengjun/rust/rust_compare_numerical_values)
    Finished dev [unoptimized + debuginfo] target(s) in 0.15s
     Running `target/debug/rust_compare_numerical_values`
thread 'main' panicked at 'assertion failed: 0.1 + 0.2 == 0.3', src/main.rs:2:5
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

rust_compare_numerical_values on  master [?] is 📦 0.1.0 via 🦀 1.67.1 via 🅒 base 
➜

例子

fn main() {
    let abc: (f32, f32, f32) = (0.1, 0.2, 0.3);
    let xyz: (f64, f64, f64) = (0.1, 0.2, 0.3);

    println!("abc (f32)");
    println!(" 0.1 + 0.2: {:x}", (abc.0 + abc.1).to_bits());
    println!("       0.3: {:x}", (abc.2).to_bits());
    println!();

    println!("xyz (f64)");
    println!(" 0.1 + 0.2: {:x}", (xyz.0 + xyz.1).to_bits());
    println!("       0.3: {:x}", (xyz.2).to_bits());
    println!();

    assert!(abc.0 + abc.1 == abc.2);
    assert!(xyz.0 + xyz.1 == xyz.2);
}

运行

rust_compare_numerical_values on  master [?] is 📦 0.1.0 via 🦀 1.67.1 via 🅒 base 
➜ cargo run
   Compiling rust_compare_numerical_values v0.1.0 (/Users/qiaopengjun/rust/rust_compare_numerical_values)
    Finished dev [unoptimized + debuginfo] target(s) in 0.89s
     Running `target/debug/rust_compare_numerical_values`
abc (f32)
 0.1 + 0.2: 3e99999a
       0.3: 3e99999a

xyz (f64)
 0.1 + 0.2: 3fd3333333333334
       0.3: 3fd3333333333333

thread 'main' panicked at 'assertion failed: xyz.0 + xyz.1 == xyz.2', src/main.rs:16:5
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

rust_compare_numerical_values on  master [?] is 📦 0.1.0 via 🦀 1.67.1 via 🅒 base 
➜

怎么比较浮点类型？

一般来说，测试数学运算是否在其真实数学结果的可接受范围内更安全。这个边界通常被称为ε。
Rust 提供了一些可容忍的误差值：f32::EPSILON 和 f64::EPSILON

fn main() {
    let result: f32 = 0.1 + 0.2;
    let desired: f32 = 0.3;

    let abs_diff = (desired - result).abs();

    assert!(abs_diff &lt;= f32::EPSILON)
}

运行

rust_compare_numerical_values on  master [?] is 📦 0.1.0 via 🦀 1.67.1 via 🅒 base 
➜ cargo run
   Compiling rust_compare_numerical_values v0.1.0 (/Users/qiaopengjun/rust/rust_compare_numerical_values)
    Finished dev [unoptimized + debuginfo] target(s) in 0.14s
     Running `target/debug/rust_compare_numerical_values`

rust_compare_numerical_values on  master [?] is 📦 0.1.0 via 🦀 1.67.1 via 🅒 base 
➜

例子

fn main() {
    let result: f64 = 0.1 + 0.2;
    let desired: f64 = 0.3;

    print!("{}\n", desired - result);

    let abs_diff = (desired - result).abs();

    assert!(abs_diff &lt;= f64::EPSILON)
}

运行

rust_compare_numerical_values on  master [?] is 📦 0.1.0 via 🦀 1.67.1 via 🅒 base 
➜ cargo run
   Compiling rust_compare_numerical_values v0.1.0 (/Users/qiaopengjun/rust/rust_compare_numerical_values)
    Finished dev [unoptimized + debuginfo] target(s) in 0.13s
     Running `target/debug/rust_compare_numerical_values`
-0.00000000000000005551115123125783

rust_compare_numerical_values on  master [?] is 📦 0.1.0 via 🦀 1.67.1 via 🅒 base 
➜

Rust 编译器实际上将比较的工作交给了 CPU，浮点运算是使用芯片内的定制硬件实现的

NAN

表示”不是一个数“，例如负数的平方根就是 NAN
NAN 会影响其它数值：
- 几乎所有与 NAN 交互的操作都返回 NAN
- NAN 值永远不相等

fn main() {
    let x = (-42.0_f32).sqrt();

    assert_eq!(x, x);
}

运行

rust_compare_numerical_values on  master [?] is 📦 0.1.0 via 🦀 1.67.1 via 🅒 base 
➜ cargo run
   Compiling rust_compare_numerical_values v0.1.0 (/Users/qiaopengjun/rust/rust_compare_numerical_values)
    Finished dev [unoptimized + debuginfo] target(s) in 0.38s
     Running `target/debug/rust_compare_numerical_values`
thread 'main' panicked at 'assertion failed: `(left == right)`
  left: `NaN`,
 right: `NaN`', src/main.rs:4:5
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

rust_compare_numerical_values on  master [?] is 📦 0.1.0 via 🦀 1.67.1 via 🅒 base 
➜

Is_nan() 和 is_finite() 方法

fn main() {
    let x: f32 = 1.0 / 0.0;

    println!("{}", x);

    assert!(x.is_finite());
}

运行

rust_compare_numerical_values on  master [?] is 📦 0.1.0 via 🦀 1.67.1 via 🅒 base 
➜ cargo run
   Compiling rust_compare_numerical_values v0.1.0 (/Users/qiaopengjun/rust/rust_compare_numerical_values)
    Finished dev [unoptimized + debuginfo] target(s) in 0.08s
     Running `target/debug/rust_compare_numerical_values`
inf
thread 'main' panicked at 'assertion failed: x.is_finite()', src/main.rs:6:5
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

rust_compare_numerical_values on  master [?] is 📦 0.1.0 via 🦀 1.67.1 via 🅒 base 
➜

原创
学分: 16
分类: Rust
标签: Rust 编程