使用std :: reverse反转向量前几个元素的顺序

Question

If I have a

vector<int> vec { 1, 2, 3, 4}

How to use

std::reverse

to turn it into

vec {2, 1, 3, 4}

Answer 1

You can use reverse with iterators as arguments like this:

vector<int> vec { 1, 2, 3, 4};
reverse(vec.begin(), vec.begin()+2);

You may take a look here

Answer 2

You don't need to use std::reverse for what is essentially a much simpler operation. If you want to just swap the first two elements, use:

std::swap (vec[0], vec[1]);

If your desire is to swap elements in groups of two (leaving any odd one at the end alone), you can use something like:

for (int i = 0, lim = vec.size() - vec.size() % 2; i < lim; i += 2)
    std::swap (vec[i], vec[i+1]);

If you want to reverse a section of the vector that's more than two elements, that's where I'd be contemplating the use of std::reverse . You could reverse sections of the vector vec containing {1, 2, 3, 4, 5} with calls like:

std::reverse (vec.begin(),     vec.end()      ); // -> {5, 4, 3, 2, 1}
std::reverse (vec.begin(),     vec.begin() + 3); // -> {3, 2, 1, 4, 5}
std::reverse (vec.begin() + 1, vec.begin() + 4); // -> {1, 4, 3, 2, 5}