重复向量中的元素 [英] Repeat elements in a vector

问题描述

我有一个向量

vector<int>v = {1,2,3,4,5};

我想重复向量中的元素，比如说 3 次，这样向量就变成了

I'd like to repeat the elements in the vector for, say, 3 times, such that the vector becoms

v = {1,2,3,4,5, 1,2,3,4,5, 1,2,3,4,5};

编辑:事实上，如果我需要多次重复元素，比如 1000 次，显然我必须快速而轻松地来?

EDIT: In fact, if I need to repeat the elements for many times, say 1000, obviously I have to come with something quick and light?

我该怎么做?

推荐答案

这可能很棘手.如果您想避免创建临时工作对象，您必须小心避免在执行过程中使迭代器失效.应该这样做:

This can be tricky. If you want to avoid creating a temporary working object you have to be careful to avoid invalidating iterators as you go. This should do it:

std::vector<int> v = {1, 2, 3, 4, 5};

// to avoid invalidating iterators, preallocate the memory
v.reserve(v.size() * 3);

// remember the end of the range to be duplicated
// (this is the iterator we don't want to invalidate)
auto end = std::end(v);

// insert two duplicates
v.insert(std::end(v), std::begin(v), end);
v.insert(std::end(v), std::begin(v), end);

for(auto i: v)
    std::cout << i << '\n';

更一般地，您可以修改它以添加多个重复项，如下所示:

More generally you could modify this to add multiple duplicates like this:

std::vector<int> v = {1, 2, 3, 4, 5};

std::size_t const no_of_duplicates = 1000;

// to avoid invalidating iterators, preallocate the memory
v.reserve(v.size() * no_of_duplicates);

// remember the end of the range to be duplicated
// (this is the iterator we don't want to invalidate)
auto end = std::end(v);

// insert duplicates (start from one because already have the first)
for(std::size_t i = 1; i < no_of_duplicates; ++i)
    v.insert(std::end(v), std::begin(v), end);

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