如何将向量的后半部分移动到另一个向量? [英] How to move the later half of a vector into another vector?

问题描述

我有一个向量，我想使用STL算法将向量的后半部分有效地分解为另一个向量.这是我看到的一种方法，但是希望有更有效和简洁的答案，或者至少使用stl算法:

I have a vector and I would like to efficiently break out the second half of the vector into another vector using STL algorithms. Here is one way I see to do this, but expect there are more efficient and succinct answers, or at the least, one that uses the stl algorithms:

std::vector<Entry> &entries = someFunction();
int numEntries = entries.size();

// Assume numEntries is greater than or equal to 2.

std::vector<Entry> secondEntries;
std::vector<Entry>::iterator halfway = entries.begin() + numEntries / 2;
std::vector<Entry>::iterator endItr  = entries.end() 

// Copy the second half of the first vector in the second vector:
secondEntries.insert(secondEntries.end(), halfway, endItr);

// Remove the copied entries from the first vector:
entries.erase(halfway, endItr);

推荐答案

退后一步，请记住要确保您使用自己的算法而不是(必要的)容器来处理迭代器.所以，如果你有这个:

Taking a step back, keep in mind to make sure that you're working with iterators with your own algorithms, and not (necessarily) containers. So if you have this:

void foo(const std::vector<Entry>& v) { /* ... */ }

现在您陷入了这种情况:

And now you're stuck in this situation:

std::vector<Entry> entries = someFunction();

// have to split entries! make more containers? :(
foo(first_half(entries));
foo(second_half(entries));

考虑改用迭代器:

// or a template, if it doesn't hurt
void foo(std::vector<Entry>::const_iterator first, 
         std::vector<Entry>::const_iterator second) { /* ... */ }

所以现在您表示范围而不是容器:

So now you denote ranges and not containers:

std::vector<Entry> entries = someFunction();

// easy to split entries! :)
auto middle = entries.begin() + entries.size() / 2;
foo(entries.begin(), middle);
foo(middle + 1, entries.end());

这限制了您不必要的容器和分配的数量.

This limits the number of unnecessary containers and allocations you make.

通过这种方式，在C ++ 11中，您可以做到这一点(其余部分相同):

With that out of the way, in C++11 you can do this (rest is the same):

// *Move* the second half of the first vector in the second vector:           
secondEntries.insert(secondEntries.end(),
                        std::make_move_iterator(halfway),
                        std::make_move_iterator(endItr));

如果 Entry 具有move构造函数，则 move_iterator 适配器将确保在插入过程中使用它(如果未制作普通副本).在C ++ 03中，您拥有的可能是最好的.

If Entry has a move constructor, the move_iterator adapter will ensure that it is used during the insertion (if it doesn't a normal copy is made). In C++03, what you have is probably best.

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