我如何才能向后迭代？ [英] How can I iterate over a backwards range?

问题描述

我正在尝试创建pub fn sing(start: i32, end: i32) -> String，它返回对start和end之间的每个数字重复调用pub fn verse(num: i32) -> String的结果的串联字符串。

我已经在谷歌上搜索了答案，似乎Rust String concatenation回答了我的问题，如果我甚至用playground编写代码，它都可以工作，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.
Go to the store and buy some more, 99 bottles of beer on the wall.
".to_string(),
        1 => "1 bottle of beer on the wall, 1 bottle of beer.
Take it down and pass it around, no more bottles of beer on the wall.
".to_string(),
        2 => "2 bottles of beer on the wall, 2 bottles of beer.
Take one down and pass it around, 1 bottle of beer on the wall.
".to_string(),
        num => format!("{0} bottles of beer on the wall, {0} bottles of beer.
Take one down and pass it around, {1} bottles of beer on the wall.
",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.
Take one down and pass it around, 7 bottles of beer on the wall.

7 bottles of beer on the wall, 7 bottles of beer.
Take one down and pass it around, 6 bottles of beer on the wall.

6 bottles of beer on the wall, 6 bottles of beer.
Take one down and pass it around, 5 bottles of beer on the wall.
");
}

失败，beer::sing(8,6)返回""。

推荐答案

您的问题与字符串连接无关。这与8..6是一个空迭代器的事实有关，因为范围只向前迭代。由于8 >= 6，迭代器在第一次调用next时生成None。

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

可以通过交换start和end并调用rev()向后迭代来修复此问题。

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

然而，还有一个问题。在范围start..end中，start是包含的，但end是排除的。例如，上面的代码打印7和6；8不打印。请参见How do I include the end value in a range?

将所有这些放在一起，sing应该如下所示：

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，因为它预期每行之间有两个换行符，但您的代码只生成一个换行符。我会让你来解决这件事的。🙂

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