iterator - 如何遍历向后范围？

Question

我正在尝试创建 pub fn sing(start: i32, end: i32) -> String 返回调用 pub fn verse(num: i32) -> String 在 start 和 end 之间的每个数字上重复。

我用谷歌搜索了答案，似乎是 Rust String concatenation回答我的问题，如果我什至在 playground 中编写代码它有效，但是:

我的代码:

pub fn verse(num: i32) -> String {
    match num {
        0 => "No more bottles of beer on the wall, no more bottles of beer.\nGo to the store and buy some more, 99 bottles of beer on the wall.\n".to_string(),
        1 => "1 bottle of beer on the wall, 1 bottle of beer.\nTake it down and pass it around, no more bottles of beer on the wall.\n".to_string(),
        2 => "2 bottles of beer on the wall, 2 bottles of beer.\nTake one down and pass it around, 1 bottle of beer on the wall.\n".to_string(),
        num => format!("{0} bottles of beer on the wall, {0} bottles of beer.\nTake one down and pass it around, {1} bottles of beer on the wall.\n",num,(num-1)),
    }
}

pub fn sing(start: i32, end: i32) -> String {
    (start..end).fold(String::new(), |ans, x| ans+&verse(x))
}

问题是

#[test]
fn test_song_8_6() {
    assert_eq!(beer::sing(8, 6), "8 bottles of beer on the wall, 8 bottles of beer.\nTake one down and pass it around, 7 bottles of beer on the wall.\n\n7 bottles of beer on the wall, 7 bottles of beer.\nTake one down and pass it around, 6 bottles of beer on the wall.\n\n6 bottles of beer on the wall, 6 bottles of beer.\nTake one down and pass it around, 5 bottles of beer on the wall.\n");
}

失败，beer::sing(8,6) 返回 ""。

Answer 1

您的问题与字符串连接无关。它与 8..6 是一个空迭代器这一事实有关，因为范围只能向前迭代。因为 8 >= 6，迭代器在第一次调用 next 时产生 None。

fn main() {
    for i in 8..6 {
        println!("{}", i); // never reached
    }
}

这可以通过交换 start 和 end 并调用 rev() 向后迭代来解决。

fn main() {
    for i in (6..8).rev() {
        println!("{}", i);
    }
}

但是，还有一个问题。在 start..end 范围内，start 包含在内，但 end 不包含在内。例如，上面的代码打印出 7 和 6； 8 未打印。参见 How do I include the end value in a range?

将它们放在一起，sing 应该是这样的:

pub fn sing(start: i32, end: i32) -> String {
    (end..=start)
        .rev()
        .fold(String::new(), |ans, x| ans + &verse(x))
}

注意:您的测试仍然失败，因为它期望每节之间有两个换行符，但您的代码只生成一个。我会把这个留给你来解决。 🙂