C++ extend a vector with another vector

问题:

I'm a C/Python programmer in C++ land working with the STL for the first time.

In Python, extending a list with another list uses the .extend method:

>>> v = [1, 2, 3]
>>> v_prime = [4, 5, 6]
>>> v.extend(v_prime)
>>> print(v)
[1, 2, 3, 4, 5, 6]

I currently use this algorithmic approach to extend vectors in C++:

v.resize(v.size() + v_prime.size());
copy(v_prime.begin(), v_prime.end(), v.rbegin());

Is this the canonical way of extending vectors, or if there is a simpler way that I'm missing?

回答1:

From here

// reserve() is optional - just to improve performance
v.reserve(v.size() + distance(v_prime.begin(),v_prime.end()));
v.insert(v.end(),v_prime.begin(),v_prime.end());

回答2:

copy(v_prime.begin(), v_prime.end(), back_inserter(v));

回答3:

There are multiple ways to achieve your target.

std::vector::insert

The vector can be extended by inserting new elements before the element at the specified position, effectively increasing the container size by the number of elements inserted. You can follow one of below approaches. The second version uses C++11 and it can be considered as a more generic answer, as b could also be an array.

a.insert(a.end(), b.begin(), b.end());
a.insert(std::end(a), std::begin(b), std::end(b));

Sometimes in usage it is a best practice to use reserve function before using std::vector::insert. std::vector::reserve function increases the capacity of the container to a value that's greater or equal to new_cap. If new_cap is greater than the current capacity(), new storage is allocated, otherwise the method does nothing.

a.reserve(a.size() + distance(b.begin(), b.end()));

Use of reserve function is not required but may be advisable. And it is best practive to use reserve if you are repeatedly inserting into a vector for which you know the final size, and that size is large. Otherwise, it is better to let the STL grow your vector as needed.

std::copy

std::copy is the second option that you can consider to achieve your target. This function copies the elements in the range (first,last) into the range beginning at result.

std::copy (b.begin(), b.end(), std::back_inserter(a));

However use of std::copy is slower than use of std::vector::insert(), because std::copy() can't reserve enough space before-hand (it doesn't have access to the vector itself, only to an iterator which has), while std::vector::insert(), being a member function, can. Due to that std::copy is indeed slower than using std::vector::insert. Most of the people, over use std::copy without knowing this scenario.

boost::push_back

The third option that you can consider is the use of boost's push_back function.

boost::push_back(a, b);

回答4:

I needed two different variants of the extend function in C++14, where one supported move semantics for each element of the vector to be appended.

vec is your v, and ext is your v_prime.

/**
 * Extend a vector with elements, without destroying source one.
 */
template<typename T>
void vector_extend(std::vector<T> &vec, const std::vector<T> &ext) {
    vec.reserve(vec.size() + ext.size());
    vec.insert(std::end(vec), std::begin(ext), std::end(ext));
}

/**
 * Extend a vector with elements with move semantics.
 */
template<typename T>
void vector_extend(std::vector<T> &vec, std::vector<T> &&ext) {
    if (vec.empty()) {
        vec = std::move(ext);
    }
    else {
        vec.reserve(vec.size() + ext.size());
        std::move(std::begin(ext), std::end(ext), std::back_inserter(vec));
        ext.clear();
    }
}

回答5:

Using std::vector::insert;

A.reserve(A.size() + B.size());
A.insert(A.end(), B.begin(), B.end());

reserve() is optional, but using it helps to improve performance.

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