用于生成和复制矢量的STL算法

Question

Can you please tell me how to do the following using STL algorithms?

// Create a vector of 50 elements, and assign elem value same as index value
std::vector<int> a(50);
for (int i = 0; i < a.size(); i++)
{
    a[i] = i;
}

// Create another vector by copying a section of vector a
std::vector<int> b;
size_t ind = 20;
b.resize(a.size() - ind);
for (int i = 0; i < b.size(); i++)
{
    b[i] = a[i+ind];
}

Essentially, I am trying to create a new vector b, from vector a, by skipping the first 'ind' elements of a.

Answer 1

I'd probably do it something like this:

std::vector<int> a(50);

// fill a with 0..N
std::iota(a.begin(), a.end(), 0);

size_t ind = 20;

// initialize `b` from elements of `a`:    
std::vector<int> b{a.begin()+ind, a.end()};

You could use std::copy for the second part, but for the case at hand I'd prefer to initialize b from the iterators as I've done above.

Answer 2

With boost you could do the first part with initialization as well (see Jerry's answer).

auto r = boost::irange(0,50);
auto a = std::vector<int>(std::begin(r), std::end(r));

Eric Neibler's range library I think includes this type of thing and I fully expect it'll make it into C++17. Until then you have to use his or boost's as a third-party lib.

Answer 3

Use the

template <class InputIterator>
vector (InputIterator first, InputIterator last,
    const allocator_type& alloc = allocator_type());

constructor as follows.

auto start = std::next(a.begin(), 20);
std::vector<int> b(start, a.end());