rust - 在循环中使用read_line时出现奇怪的行为

Question

我在Rust中的第一个程序应该以字符形式(C或F)从用户那里接受输入:

use std::io;

fn main() {
    let mut srcunit = String::new();

    let mut switch = true;
    while switch {
        println!("source unit? F or C?");
        io::stdin().read_line(&mut srcunit).expect(
            "failed to read src unit",
        );

        if srcunit.trim() == "F" || srcunit.trim() == "C" {
            println!("doing things right with {}", srcunit);
            switch = false;
        } else {
            println!("either F or C, not {}", srcunit);
        }
    }

    println!("you pressed {}", srcunit);
}

当我启动程序并按F或C时，它可以正常工作，因此我在这里跳过了这一点。当我按下另一个字符时，出现了一个奇怪的部分。我希望程序再次询问F或C，直到我按这些字符之一为止。当我这样做时，它应该离开while循环，并告诉我所按下的内容。

source unit? F or C?
G
either F or C, not G //so far so good

source unit? F or C?
F
either F or C, not G //why is F not assigned to the srcunit variable? It's supposed to leave the loop now.
F                    //why does it print this line? I didn't press a key or have a println function/macro that does this

source unit? F or C?
V
either F or C, not G //it's still G, wtf
F                    //again, why are those two lines printed? Where does it store, that I pressed F previously?
V

Answer 1

从documentation for read_line :

Read all bytes until a newline (the 0xA byte) is reached, and append them to the provided buffer.

(强调一下。)您需要在读取下一行之前清除字符串，例如，通过在字符串上调用 clear() 方法，否则答案将累积在变量中。

或者，您可以在循环中定义变量(但是效率稍低，因为这种方式 String将无法为下一个答案重用已分配的存储，因此必须重新分配并重新分配)。

另请参见 this question，最近问过。看起来这是一个常见的陷阱。