在基于“索引”的范围内跳过？ [英] Skipping in Range-based for based on 'index'?

问题描述

有一种方法可以访问迭代器（假设在基于C ++ 11范围的for循环中没有循环索引 ..？）。

Is there a way to access the iterator (suppose there's no loop index..?) in a C++11 range-based for loop?

通常，我们需要对容器的第一个元素做一些特殊的事情，并对其余的元素进行迭代。

我正在寻找类似于 c>< / c>< / c>语句中的 语句：

Often we need to do something special with the first element of a container and iterate over the remaining elements.
I'm looking for something like the c++11_get_index_of statement in this pseudo-code:

for (auto& elem: container) 
{
  if (c++11_get_index_of(elem) == 0)
     continue;

  // do something with remaining elements
}

'd真的很想避免回到该场景中的旧式手动迭代器处理代码。 / p>

I'd really like to avoid going back to old-style manual iterator handling code in that scenario..

推荐答案

通常我们需要对
容器的第一个元素

Often we need to do something special with the first element of a container and iterate over the remaining elements.

我惊讶地发现迄今为止没有人提出这个解决方案：

I am surprised to see that nobody has proposed this solution so far:

  auto it = std::begin(container);

  // do your special stuff here with the first element

  ++it;

  for (auto end=std::end(container); it!=end; ++it) {

      // Note that there is no branch inside the loop!

      // iterate over the rest of the container
  }

它的最大优点是分支被移出循环。它使循环更简单，也许编译器也可以更好地优化它。

It has the big advantage that the branch is moved out of the loop. It makes the loop much simpler and perhaps the compiler can also optimize it better.

如果你坚持基于范围的for循环，也许最简单的方法是这样做（还有其他更丑陋的方法）：

If you insist on the range-based for loop, maybe the simplest way to do it is this (there are other, uglier ways):

std::size_t index = 0;

for (auto& elem : container) {

  // skip the first element
  if (index++ == 0) {
     continue;
  }

  // iterate over the rest of the container
}

但是，如果你需要的是跳过第一个元素，我会认真地将分支移出循环。

However, I would seriously move the branch out of the loop if all you need is to skip the first element.

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