RustDay05------Exercise[51-60]

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。

// 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 {
    let processing_fee = 1;
    let cost_per_item = 5;
    let qty = item_quantity.parse::()?;

    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

// 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 {
    let processing_fee = 1;
    let cost_per_item = 5;
    let qty = item_quantity.parse::()?;

    Ok(qty * cost_per_item + processing_fee)
}