51.使用?当作错误处理符
?
是 Rust 中的错误处理操作符。通常用于尝试解析或执行可能失败的操作,并在出现错误时提前返回错误,以避免程序崩溃或出现未处理的错误。
具体来说,?
用于处理 Result
或 Option
类型的返回值。
rust
// errors2.rs
// Say we're writing a game where you can buy items with tokens. All items cost
// 5 tokens, and whenever you purchase items there is a processing fee of 1
// token. A player of the game will type in how many items they want to buy,
// and the `total_cost` function will calculate the total number of tokens.
// Since the player typed in the quantity, though, we get it as a string-- and
// they might have typed anything, not just numbers!
// Right now, this function isn't handling the error case at all (and isn't
// handling the success case properly either). What we want to do is:
// if we call the `parse` function on a string that is not a number, that
// function will return a `ParseIntError`, and in that case, we want to
// immediately return that error from our function and not try to multiply
// and add.
// There are at least two ways to implement this that are both correct-- but
// one is a lot shorter!
// Execute `rustlings hint errors2` or use the `hint` watch subcommand for a hint.
// I AM NOT DONE
use std::num::ParseIntError;
pub fn total_cost(item_quantity: &str) -> Result<i32, ParseIntError> {
let processing_fee = 1;
let cost_per_item = 5;
let qty = item_quantity.parse::<i32>()?;
Ok(qty * cost_per_item + processing_fee)
}
#[cfg(test)]
mod tests {
use super::*;
#[test]
fn item_quantity_is_a_valid_number() {
assert_eq!(total_cost("34"), Ok(171));
}
#[test]
fn item_quantity_is_an_invalid_number() {
assert_eq!(
total_cost("beep boop").unwrap_err().to_string(),
"invalid digit found in string"
);
}
}
52.在main函数中需要对ParseIntError进行Err匹配
这题只能这么理解,具体原理没翻到
rust
// errors3.rs
// This is a program that is trying to use a completed version of the
// `total_cost` function from the previous exercise. It's not working though!
// Why not? What should we do to fix it?
// Execute `rustlings hint errors3` or use the `hint` watch subcommand for a hint.
// I AM NOT DONE
use std::num::ParseIntError;
fn main() {
let mut tokens = 100;
let pretend_user_input = "8";
// let cost = total_cost(pretend_user_input)?;
// if cost > tokens {
// println!("You can't afford that many!");
// } else {
// tokens -= cost;
// println!("You now have {} tokens.", tokens);
// }
match total_cost(pretend_user_input) {
Ok(cost) =>{
if cost > tokens {
println!("You can't afford that many!");
} else {
tokens -= cost;
println!("You now have {} tokens.", tokens);
}
}
Err(message) => {
}
}
}
pub fn total_cost(item_quantity: &str) -> Result<i32, ParseIntError> {
let processing_fee = 1;
let cost_per_item = 5;
let qty = item_quantity.parse::<i32>()?;
Ok(qty * cost_per_item + processing_fee)
}