反向迭代向量

2022-01-07 00:00:00 iteration c++ stl

我需要从结尾到开头遍历一个向量.正确"的方式是

I need to iterate over a vector from the end to the beginnig. The "correct" way is

for(std::vector<SomeT>::reverse_iterator rit = v.rbegin(); rit != v.rend(); ++rit)
{
    //do Something
}

当doSomething涉及知道实际的索引时，那么需要用rit做一些计算才能得到它，比如index = v.size() - 1 - (rit - v.rbegin)

When doSomething involves knowing the actual index, then some calculations need to be done with rit to obtain it, like index = v.size() - 1 - (rit - v.rbegin)

如果无论如何都需要索引，那么我坚信最好使用该索引进行迭代

If the index is needed anyway, then I strongly believe it is better to iterate using that index

for(int i = v.size() - 1; i >= 0; --i)
{
    //do something with v[i] and i; 
}

这会警告 i 已签名且 v.size() 未签名.更改为

This gives a warning that i is signed and v.size() is unsigned. Changing to

for(unsigned i = v.size() - 1; i >= 0; --i) 只是功能错误，因为这本质上是一个无限循环 :)

for(unsigned i = v.size() - 1; i >= 0; --i) is just functionally wrong, because this is essentially an endless loop :)

做我想做的事情的美学上的好方法是什么

What is an aesthetically good way to do what I want to do which

  • 无警告
  • 不涉及演员表
  • 不过分冗长

我希望我不是在寻找不存在的东西:)

I hope I am not looking for something that doesn't exist :)

推荐答案

如您所见，i >= 0 条件在未签名时的问题在于条件总是真的.不是在初始化 i 时减 1，然后在每次迭代后再次减 1，而是在检查循环条件后减 1:

As you've noted, the problem with a condition of i >= 0 when it's unsigned is that the condition is always true. Instead of subtracting 1 when you initialize i and then again after each iteration, subtract 1 after checking the loop condition:

for (unsigned i = v.size(); i-- > 0; )

我喜欢这种风格有几个原因:

I like this style for several reasons:

  • 尽管 i 会在循环结束时返回到 UINT_MAX，但它并不依赖这种行为――它会起作用如果类型已签名，则相同.依赖未签名的环绕对我来说有点像黑客.
  • 它只调用一次 size().
  • 它不使用 >=.每当我在 for 循环中看到该运算符时，我都必须重新阅读它以确保没有逐一错误.
  • 如果您更改条件中的间距，则可以使用 转到"运算符.
  • Although i will wrap around to UINT_MAX at the end of the loop, it doesn't rely on that behavior ― it would work the same if the types were signed. Relying on unsigned wraparound feels like a bit of a hack to me.
  • It calls size() exactly once.
  • It doesn't use >=. Whenever I see that operator in a for loop, I have to re-read it to make sure there isn't an off-by-one error.
  • If you change the spacing in the conditional, you can make it use the "goes to" operator.

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