Question

Say you have two integer vectors:

I would like to define a function that allows me to swap a range of elements among the two vectors passing start index and lenght of the two sequences as arguments.

For instance: where and are vectors and the numbers passed as arguments represents starting index and lenght of the sequences.

In this case I should get as autput

v1 = 1,2, 13,14,15 ,5,6,7,8,9

v2 = 10,11,12, 3,4 ,16,17,18

the signature of the function I defined as an example is not a constraint, if you think there is a better way it is ok

Answer 1

It appears that all the regular STL algorithms fall short of what you want to do exactly:

std::swap_ranges would be almost there, but it requires that you swap equally long ranges
std::rotate would also not be bad, but it requires that the end point of one range equals the begin point of the second range.

// pseudo-splice on vector
v1.insert(v1.begin() + 2 + 2, v2.begin() + 3, v2.begin() + 3 + 3);
v2.erase(v2.begin() + 3, v2.begin() + 3 + 3);

// pseudo-splice on vector
v2.insert(v2.begin() + 3, v1.begin() + 2, v1.begin() + 2 + 2);
v1.erase(v1.begin() + 2, v1.begin() + 2 + 2);

You can of course easily abstract this into a function template that takes arbitrary iterator boundaries for your two ranges.

Edit based on David’s comment, you can do some optimization to avoid needless resizing

// compute smallest range here, in this case it's the v1 part
std::swap_ranges(v1.begin() + 2, v1.begin() + 2 + 2, v2.begin() + 3);

// now handle the remaining part of the longest range, in this case it's element v2 + 3 + 2
std::insert(v1.begin() + 2 + 2, v2.begin() + 3 + 2);
std::erase(v2.begin() + 3 + 2);

Update: it would be easier if you used std::list since then you could use splice (I rearranged the insert / erase part there to mimic the code below)

v1.splice(v1.begin() + 2 + 2, v2, v2.begin() + 3, v2.begin() + 3 + 3);
v2.splice(v2.begin() + 3, v1, v1.begin() + 2, v1.begin() + 2 + 2);