我该如何应对懒惰的迭代器? [英] How do I cope with lazy iterators?

问题描述

我正在尝试在迭代器上使用map()对数组进行排序.

I'm trying to sort an array with a map() over an iterator.

struct A {
    b: Vec<B>,
}

#[derive(PartialEq, Eq, PartialOrd, Ord)]
struct B {
    c: Vec<i32>,
}

fn main() {
    let mut a = A { b: Vec::new() };

    let b = B { c: vec![5, 2, 3] };
    a.b.push(b);

    a.b.iter_mut().map(|b| b.c.sort());
}

给出警告:

warning: unused `std::iter::Map` that must be used
  --> src/main.rs:16:5
   |
16 |     a.b.iter_mut().map(|b| b.c.sort());
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = note: #[warn(unused_must_use)] on by default
   = note: iterators are lazy and do nothing unless consumed

这是正确的，sort()实际上未在此处调用. 书中对此警告进行了说明，但我不明白为什么iter_mut()的这种变体效果很好:

Which is true, sort() isn't actually called here. This warning is described in the book, but I don't understand why this variation with iter_mut() works fine:

a.b.iter_mut().find(|b| b == b).map(|b| b.c.sort());

推荐答案

所链接的书说:

如果您尝试在迭代器上执行闭包以解决其副作用，请改用for.

这样，它就可以工作，而且对于阅读代码的人来说，这一点要清晰得多.要将向量转换为其他向量时，应使用map.

That way it works, and it's much clearer to anyone reading the code. You should use map when you want to transform a vector to a different one.

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