Copy another vector for a smaller size

Question

I have 2 vectors. I want the second vector to copy the first vector for the size of n which is less than the length of the first vector. (the second vector length should be n too)

I tried doing this by a loop:

for (int i = 0; i < n; ++i)
{
    //secVector[i] will equal firstVector[i] and n is less than fristVector length
}

but the time complexity of this is O(n) and it takes a lot of time in large lengths, I wonder if there is any function could do this faster.

Answer 1

This cannot be done with std vector.

There are immutable vectors where this can be done in logarithmic time, such as https://sinusoid.es/immer/ - this uses wide B trees and copy on write to give near-vector performance with O(1) copy and O(lg n) slice.

Such structures are considered exotic.