使用for循环体内仅有条件的空格连接字符串

Question

我正在做Rust Koans，我坚持这个问题：

#[test]
fn for_loops_two() {
    let words: [&'static str; 3] = ["I", "love", "Rust"];
    let space: &str = " ";
    let mut sentence: String = String::new();
    for word in words.iter() {
        // __
    }

    println!("{:?}", sentence);
    assert!(sentence == "I love Rust".to_string());
}

我知道我需要连接字符串，但这会失败：

#[test]
fn for_loops_two() {
    let words: [&'static str; 3] = ["I", "love", "Rust"];
    let mut sentence: String = String::new();
    for word in words.iter() {
        sentence.push_str(word);
    }

    println!("{:?}", sentence); // "ILoveRust"
    assert!(sentence == "I love Rust".to_string());
}

我可以在每次迭代后添加一个空格：

#[test]
fn for_loops_two() {
    let words: [&'static str; 3] = ["I", "love", "Rust"];
    let space: &str = " ";
    let mut sentence: String = String::new();
    for word in words.iter() {
        sentence.push_str(word);
        sentence.push_str(space);
    }

    println!("{:?}", sentence); // "I Love Rust "
    assert!(sentence == "I love Rust".to_string());
}

这也将失败，因为最后的迭代将添加一个空格。

如果我们在最后一次迭代中，我想我可以编写一个条件，但我正在努力使语法正确。此外，我觉得有一个更好的解决方案，所有这一切，我只是无法弄清楚语法。

如何使上面的断言在循环中通过条件传递，以便不在最后一次迭代中添加空格？

Answer 1

你可以使用slice::join：

#[test]
fn for_loops_two() {
    let words: [&'static str; 3] = ["I", "love", "Rust"];

    let sentence = words.join(" ");

    assert!(sentence == "I love Rust".to_string());
}

关于链接的SliceConcatExt特性的注释：它在文档中列为不稳定，但方法是稳定的 - 上面的编译在当前稳定版的Rust下正常。

如果你宁愿坚持公案的约束并使用for循环，你可以按照你的建议使用if（弄清楚你是否最后使用enumerate），或者pop结束时的最后一个空格的字符串：

#[test]
fn for_loops_two_with_len_check() {
    let words: [&'static str; 3] = ["I", "love", "Rust"];
    const SPACE: char = ' ';
    let number_of_words = words.len();
    let mut sentence = String::new();

    for (i, word) in words.iter().enumerate() {
        sentence.push_str(word);
        if i < number_of_words-1 {
            sentence.push(SPACE);
        }
    }

    assert!(sentence == "I love Rust".to_string());
}


#[test]
fn for_loops_two_with_pop() {
    let words: [&'static str; 3] = ["I", "love", "Rust"];
    const SPACE: char = ' ';
    let mut sentence = String::new();

    for word in words.iter() {
        sentence.push_str(word);
        sentence.push(SPACE);
    }
    let _ = sentence.pop();

    assert!(sentence == "I love Rust".to_string());
}