如何用边界值做一个'for'循环，并作为浮点值来执行？

Z时代
2024-01-10
分类：问答

我需要实现一个for循环，该循环从一个浮点数转到另一个浮点数。如何用边界值做一个'for'循环，并作为浮点值来执行？

我知道如何实现，在C类语言：

for (float i = -1.0; i < 1.0; i += 0.01) { /* ... */ }

我也知道，在鲁斯特我可以指定使用step_by循环一步，这给了我我想要的，如果我有边界价值观和步为整数：

#![feature(iterator_step_by)] 
fn main() { 
    for i in (0..30).step_by(3) { 
     println!("Index {}", i); 
    } 
}

当我这样做，浮点数，它会导致编译错误：

#![feature(iterator_step_by)] 
fn main() { 
    for i in (-1.0..1.0).step_by(0.01) { 
     println!("Index {}", i); 
    } 
}

这里是编译输出：

error[E0599]: no method named `step_by` found for type `std::ops::Range<{float}>` in the current scope 
--> src/main.rs:4:26 
    | 
4 |  for i in (-1.0..1.0).step_by(0.01) { 
    |       ^^^^^^^ 
    | 
    = note: the method `step_by` exists but the following trait bounds were not satisfied: 
      `std::ops::Range<{float}> : std::iter::Iterator` 
      `&mut std::ops::Range<{float}> : std::iter::Iterator`

我如何能实现这个循环中，锈？

回答：

如果你还没有，我请你读戈德堡的What Every Computer Scientist Should Know About Floating-Point Arithmetic。

与浮点的问题，是你的代码可以做200或201次迭代，这取决于循环的最后一个步骤是否结束是i = 0.99或i = 0.999999（这仍然是< 1即使真的很近）。

为了避免这个脚枪，Rust不允许迭代f32或f64的范围。相反，它强迫你使用积分步骤：

for i in -100..100 { 
    let i = i as f32 * 0.01; 
    // ... 
}

回答：

作为一个真正的迭代器：

Playground

/// produces: [ linear_interpol(start, end, i/steps) | i <- 0..steps ] 
/// (does NOT include "end") 
/// 
/// linear_interpol(a, b, p) = (1 - p) * a + p * b 
pub struct FloatIterator { 
    current: u64, 
    current_back: u64, 
    steps: u64, 
    start: f64, 
    end: f64, 
} 
impl FloatIterator { 
    pub fn new(start: f64, end: f64, steps: u64) -> Self { 
     FloatIterator { 
      current: 0, 
      current_back: steps, 
      steps: steps, 
      start: start, 
      end: end, 
     } 
    } 
    /// calculates number of steps from (end - start)/step 
    pub fn new_with_step(start: f64, end: f64, step: f64) -> Self { 
     let steps = ((end - start)/step).abs().round() as u64; 
     Self::new(start, end, steps) 
    } 
    pub fn length(&self) -> u64 { 
     self.current_back - self.current 
    } 
    fn at(&self, pos: u64) -> f64 { 
     let f_pos = pos as f64/self.steps as f64; 
     (1. - f_pos) * self.start + f_pos * self.end 
    } 
    /// panics (in debug) when len doesn't fit in usize 
    fn usize_len(&self) -> usize { 
     let l = self.length(); 
     debug_assert!(l <= ::std::usize::MAX as u64); 
     l as usize 
    } 
} 
impl Iterator for FloatIterator { 
    type Item = f64; 
    fn next(&mut self) -> Option<Self::Item> { 
     if self.current >= self.current_back { 
      return None; 
     } 
     let result = self.at(self.current); 
     self.current += 1; 
     Some(result) 
    } 
    fn size_hint(&self) -> (usize, Option<usize>) { 
     let l = self.usize_len(); 
     (l, Some(l)) 
    } 
    fn count(self) -> usize { 
     self.usize_len() 
    } 
} 
impl DoubleEndedIterator for FloatIterator { 
    fn next_back(&mut self) -> Option<Self::Item> { 
     if self.current >= self.current_back { 
      return None; 
     } 
     self.current_back -= 1; 
     let result = self.at(self.current_back); 
     Some(result) 
    } 
} 
impl ExactSizeIterator for FloatIterator { 
    fn len(&self) -> usize { 
     self.usize_len() 
    } 
    //fn is_empty(&self) -> bool { 
    // self.length() == 0u64 
    //} 
} 
pub fn main() { 
    println!(
     "count: {}", 
     FloatIterator::new_with_step(-1.0, 1.0, 0.01).count() 
    ); 
    for f in FloatIterator::new_with_step(-1.0, 1.0, 0.01) { 
     println!("{}", f); 
    } 
}

回答：

另一个答案使用迭代器，但在一个稍微不同的方式playground

extern crate num; 
use num::{Float, FromPrimitive}; 
fn linspace<T>(start: T, stop: T, nstep: u32) -> Vec<T> 
where 
    T: Float + FromPrimitive, 
{ 
    let delta: T = (stop - start)/T::from_u32(nstep - 1).expect("out of range"); 
    return (0..(nstep)) 
     .map(|i| start + T::from_u32(i).expect("out of range") * delta) 
     .collect(); 
} 
fn main() { 
    for f in linspace(-1f32, 1f32, 3) { 
     println!("{}", f); 
    } 
}

在夜间，您可以使用conservative impl trait功能，以避免Vec分配playground

#![feature(conservative_impl_trait)] 
extern crate num; 
use num::{Float, FromPrimitive}; 
fn linspace<T>(start: T, stop: T, nstep: u32) -> impl Iterator<Item = T> 
where 
    T: Float + FromPrimitive, 
{ 
    let delta: T = (stop - start)/T::from_u32(nstep - 1).expect("out of range"); 
    return (0..(nstep)) 
     .map(move |i| start + T::from_u32(i).expect("out of range") * delta); 
} 
fn main() { 
    for f in linspace(-1f32, 1f32, 3) { 
     println!("{}", f); 
    } 
}