What is the best way to be aware of the loss of precision when converting a usize to a f64 in Rust? -

this code:

use std::usize;  fn main() {     fn main() {     let = usize::max;     println!("{:?}", );     println!("{:?}", f64 ); } 

returns

18446744073709551615 18446744073709552000 

what best way aware of loss of precision?

the conv crate designed handle this. valuefrom/valueinto traits perform value-preserving conversions return error if input value cannot represented in output type. example:

/*! add `cargo.toml`:  ```cargo [dependencies] conv = "0.3.2" ``` */ extern crate conv;  use conv::prelude::*;  fn main() {     let = std::u32::max;     println!("u32::max -> f32: {:?}", a.value_as::<f32>());     println!("u32::max -> f64: {:?}", a.value_as::<f64>()); } 

outputs:

u32::max -> f32: err(posoverflow(..)) u32::max -> f64: ok(4294967295) 

alternately, can replicate checking conv hand: check input integer not fall outside range of integers floating point type can represent. specifically:

• f32: ±16,777,215
• f64: ±9,007,199,254,740,991