How to Efficiently Append a Vector to Itself Without a Loop?

Elegant Solution for Appending a Vector to Itself

Duplicating the contents of a vector and appending them to the original vector is a common operation. However, implementing this efficiently without a loop can be challenging.

Challenge Discussion

The question highlights the limitations of std::vector::insert and std::copy for this specific task. std::vector::insert prohibits using an iterator to refer to the current vector, and std::copy triggers a segmentation fault when used in this context.

Optimal Solution

The optimal solution involves combining std::vector::resize (or std::vector::reserve) with std::copy_n. The following code demonstrates this approach:

auto old_count = xx.size();
xx.resize(2 * old_count);
std::copy_n(xx.begin(), old_count, xx.begin() + old_count);

Alternatively, you can use std::vector::reserve and std::back_inserter:

auto old_count = xx.size();
xx.reserve(2 * old_count);
std::copy_n(xx.begin(), old_count, std::back_inserter(xx));

Explanation

std::vector::resize reallocates the vector if its new size exceeds its old capacity. std::copy_n copies the specified number of elements from the beginning of the vector to a new location.

When using std::vector::reserve, std::copy_n is necessary because the end() iterator points one element past the end of the vector. This makes it invalid for insertions.

Conclusion

This solution offers a concise and efficient way to append the contents of a vector to itself without resorting to a loop-based implementation. By utilizing the resize and copy_n operations, you can achieve the desired functionality with minimal code overhead.

The above is the detailed content of How to Efficiently Append a Vector to Itself Without a Loop?. For more information, please follow other related articles on the PHP Chinese website!