从头到尾迭代C++向量

2021-12-21 00:00:00 iterator vector c++

是否可以从尾到头迭代一个向量?

Is it possible to iterate a vector from the end to the beginning?

for (vector<my_class>::iterator i = my_vector.end();
        i != my_vector.begin(); /* ?! */ ) {
}

或者只有这样的事情才有可能:

Or is that only possible with something like that:

for (int i = my_vector.size() - 1; i >= 0; --i) {
}

推荐答案

一种方法是:

for (vector<my_class>::reverse_iterator i = my_vector.rbegin(); 
        i != my_vector.rend(); ++i ) { 
}

rbegin()/rend() 专为此目的而设计.(是的，增加 reverse_interator 会将其向后移动.)

rbegin()/rend() were especially designed for that purpose. (And yes, incrementing a reverse_interator moves it backward.)

现在，理论上，您的方法(使用 begin()/end() & --i)会起作用，std::vector 的迭代器是双向的，但请记住，end() 不是最后一个元素 - 它是最后一个元素之后的一个，因此您必须递减首先，当你到达 begin() 时你就完成了――但你仍然需要做你的处理.

Now, in theory, your method (using begin()/end() & --i) would work, std::vector's iterator being bidirectional, but remember, end() isn't the last element ― it's one beyond the last element, so you'd have to decrement first, and you are done when you reach begin() ― but you still have to do your processing.

vector<my_class>::iterator i = my_vector.end();
while (i != my_vector.begin())
{
     --i;
    /*do stuff */

}

更新:在将 for() 循环重写为 while() 循环时，我显然过于激进.(重要的部分是 --i 在开头.)

UPDATE: I was apparently too aggressive in re-writing the for() loop into a while() loop. (The important part is that the --i is at the beginning.)

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