在#[no_std]下执行数学运算

如果你想要执行数学相关的函数，像是计算平方根或者一个数的指数且你有完整的标准库支持，你的代码可能看起来像这个:

  1. //! 可用一些标准支持的数学函数
  3. fn main() {
  4.     let float: f32 = 4.82832;
  5.     let floored_float = float.floor();
  7.     let sqrt_of_four = floored_float.sqrt();
  9.     let sinus_of_four = floored_float.sin();
  11.     let exponential_of_four = floored_float.exp();
  12.     println!("Floored test float {} to {}", float, floored_float);
  13.     println!("The square root of {} is {}", floored_float, sqrt_of_four);
  14.     println!("The sinus of four is {}", sinus_of_four);
  15.     println!(
  16.         "The exponential of four to the base e is {}",
  17.         exponential_of_four
  18.     )
  19. }

没有标准库支持的时候，这些函数不可用。反而可以使用像是libm这样一个外部库。示例的代码将会看起来像这样:

  1. #![no_main]
  2. #![no_std]
  4. use panic_halt as _;
  6. use cortex_m_rt::entry;
  7. use cortex_m_semihosting::{debug, hprintln};
  8. use libm::{exp, floorf, sin, sqrtf};
  10. #[entry]
  11. fn main() -> ! {
  12.     let float = 4.82832;
  13.     let floored_float = floorf(float);
  15.     let sqrt_of_four = sqrtf(floored_float);
  17.     let sinus_of_four = sin(floored_float.into());
  19.     let exponential_of_four = exp(floored_float.into());
  20.     hprintln!("Floored test float {} to {}", float, floored_float).unwrap();
  21.     hprintln!("The square root of {} is {}", floored_float, sqrt_of_four).unwrap();
  22.     hprintln!("The sinus of four is {}", sinus_of_four).unwrap();
  23.     hprintln!(
  24.         "The exponential of four to the base e is {}",
  25.         exponential_of_four
  26.     )
  27.     .unwrap();
  28.     // 退出QEMU
  29.     // 注意不要在硬件上允许这个; 它能破坏OpenOCD的状态
  30.     // debug::exit(debug::EXIT_SUCCESS);
  32.     loop {}
  33. }

如果你需要在你的MCU上执行更复杂的操作，像是DSP信号处理或者更高级的线性代数，下列的crates可能可以帮到你