[英]How can I convert a f64 to f32 and get the closest approximation and the next greater or smaller value?
該操作可能的偽代碼可能是:
fn f32_greater(x: f64) -> f32 {
let mut y = x as f32; //I get closest
while f64::from(y) < x {
y = nextafter(y, f32::INFINITY);
}
y
}
fn f32_smaller(x: f64) -> f32 {
let mut y = x as f32; //I get closest
while f64::from(y) > x {
y = nextafter(y, f32::NEG_INFINITY);
}
y
}
我在libc crate或f64
上的方法中找不到C11的nextafter
函數的等價物
對於上下文,我有一個使用f32
的R樹索引。 我想搜索帶有作為f64
提供的坐標的區域,所以我需要f32
中包含f64
值的最小可能區域。
此功能已從標准庫中刪除 。 一個解決方案可能是使用float_extras包 ,但我真的不喜歡這個包的方式所以我的解決方案:
mod float {
use libc::{c_double, c_float};
use std::{f32, f64};
#[link_name = "m"]
extern "C" {
pub fn nextafter(x: c_double, y: c_double) -> c_double;
pub fn nextafterf(x: c_float, y: c_float) -> c_float;
// long double nextafterl(long double x, long double y);
// double nexttoward(double x, long double y);
// float nexttowardf(float x, long double y);
// long double nexttowardl(long double x, long double y);
}
pub trait NextAfter {
fn next_after(self, y: Self) -> Self;
}
impl NextAfter for f32 {
fn next_after(self, y: Self) -> Self {
unsafe { nextafterf(self, y) }
}
}
impl NextAfter for f64 {
fn next_after(self, y: Self) -> Self {
unsafe { nextafter(self, y) }
}
}
pub trait Succ {
fn succ(self) -> Self;
}
impl Succ for f32 {
fn succ(self) -> Self {
self.next_after(f32::INFINITY)
}
}
impl Succ for f64 {
fn succ(self) -> Self {
self.next_after(f64::INFINITY)
}
}
pub trait Pred {
fn pred(self) -> Self;
}
impl Pred for f32 {
fn pred(self) -> Self {
self.next_after(f32::NEG_INFINITY)
}
}
impl Pred for f64 {
fn pred(self) -> Self {
self.next_after(f64::NEG_INFINITY)
}
}
}
use crate::float::{Pred, Succ};
use num_traits::cast::{FromPrimitive, ToPrimitive};
fn f32_greater<T>(x: T) -> Option<f32>
where
T: ToPrimitive + FromPrimitive + std::cmp::PartialOrd,
{
let mut y = x.to_f32()?;
while T::from_f32(y)? < x {
y = y.succ();
}
Some(y)
}
fn f32_smaller<T>(x: T) -> Option<f32>
where
T: ToPrimitive + FromPrimitive + std::cmp::PartialOrd,
{
let mut y = x.to_f32()?;
while T::from_f32(y)? > x {
y = y.pred();
}
Some(y)
}
fn main() {
let a = 42.4242424242424242;
println!(
"{:.16?} < {:.16} < {:.16?}",
f32_smaller(a),
a,
f32_greater(a)
);
}
我不明白為什么他們不把它包含在數字板中 。
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