How to idiomatically convert between u32 and usize

Question

This code works and prints "b":

fn main() {
    let s = "abc";
    let ch = s.chars().nth(1).unwrap();
    println!("{}", ch);
}

On the other hand, this code results in a mismatch type error.

fn main() {
    let s = "abc";
    let n: u32 = 1;
    let ch = s.chars().nth(n).unwrap();
    println!("{}", ch);
}

error[E0308]: mismatched types
 --> src/main.rs:5:28
  |
5 |     let ch = s.chars().nth(n).unwrap();
  |                            ^ expected usize, found u32

For some external reason, I have to use the u32 type for variable n. How can I convert u32 to usize and use it in nth()?

Answer 1

The as operator works for all number types:

let ch = s.chars().nth(n as usize).unwrap();

Rust forces you to cast integers to make sure you're aware of signedness or overflows.

Integer constants can have a type suffix:

let n = 1u32;

However, note that negative constants, such as -1i32 is internally - 1i32.

Integer variables declared without an explicit type specification are shown as {integer} and will be properly inferred from one of the method calls.

Answer 2

We now have a pretty different answer when we try to compile your code, replacing the number 1 with a variable of type i32:

error[E0308]: mismatched types
 --> src/main.rs:5:28
  |
5 |     let ch = s.chars().nth(n).unwrap();
  |                            ^ expected usize, found i32
help: you can convert an `i32` to `usize` and panic if the converted value wouldn't fit
  |
5 |     let ch = s.chars().nth(n.try_into().unwrap()).unwrap();
  |    

It means that now the compiler recommends you to use n.try_into().unwrap() that makes use of the trait TryInto which in turn relies on TryFrom and returns a Result<T, T::Error>. That's why we need to extract the result with a .unwrap()

TryInto documentation

Answer 3

The most cautious thing you can do is to use TryFrom and panic when the value cannot fit within a usize:

use std::convert::TryFrom;

fn main() {
    let s = "abc";
    let n: u32 = 1;
    let n_us = usize::try_from(n).unwrap();
    let ch = s.chars().nth(n_us).unwrap();
    println!("{}", ch);
}

By blindly using as, your code will fail in mysterious ways when run on a platform where usize is smaller than 32-bits. For example, some microcontrollers use 16-bit integers as the native size:

fn main() {
    let n: u32 = 0x1_FF_FF;
    // Pretend that `usize` is 16-bit
    let n_us: u16 = n as u16;

    println!("{}, {}", n, n_us); // 131071, 65535
}