C++ Reverse a smaller range in a vector - c++

Question

What would be the best way to do the following?
I would like to reverse a smaller range in a vector and haven't found any better solution than this: I extract the smaller range to a newly created vector, reverse it and then add the newly created vector at the former position in the original vector.
Another way to explain what I want to do is something like this:
Original vector: 1 2 3 4 5 6 7 8 9 10 11.
Wanted result:   1 2 3 4 5 6 7 10 9 8 11.
Copy 10, 9, 8 in that order into a new vector with three element or copy element 8, 9, 10 into a new vector an reverse it. The original vector consists now of nine elements because the elements 8, 9, 10 were erased in the procedure.
2.The new vector with the 3 elements 10, 9, 8 is then copied/appended into the original vector at position 8 as a vector or element by element at position 8, 9, 10 respectively.
I am sure there are better solutions then the method mentioned above.

Answer 1

You could in fact write an in-place swap,
that gets the last and the first index to swap,
swap these,
decreases the last index and increases the first index,
and repeats until last_index - 1 <= first_index.
Now, that sounds like less copying to me, but as Stroustrup himself once said:
I don't really understand your data structure, but I'm pretty sure that on real hardware, std::vector will kick the shit out of it.
I.e. accessing memory linearly is almost always faster, so the cost of copying a few numbers over to a new vector really isn't that bad, compared to having to jump back and forth, possibly thrashing your CPU cache if the jumps are larger than a cache line size.
Hence, I think for all practical reasons, your implementation is optimal, unless you run out of RAM.

Answer 2

I am sorry I was not clear enough. What I was asking for was something better than this:
cout<<"vpc contains:"<
//Create a sub-vector - new_vpc.
vector<PathCoordinates>::const_iterator begin=vpc.begin();
typedef PathCoordinates type;
int iFirst=problemsStartAt;//first index to copy
int iLast=problemsEndAt-1;//last index -1, 11th stays
int iLen=iLast-iFirst;//10-8=2
vector<PathCoordinates> new_vpc;
//Pre-allocate the space needed to write the data directly.
new_vpc.resize(iLen);
memcpy(&new_vpc[0],&vpc[iFirst],iLen*sizeof(PathCoordinates));
cout<<"new_vpc.size():"<<new_vpc.size()<<endl;
for(int i=0;i<new_vpc.size();i++)
{
cout<<"new_vpc[i]:"<<new_vpc[i].strt_col<<", "<<new_vpc[i].strt_row<<", "<<new_vpc[i].end_col<<", "<<new_vpc[i].end_row<<endl;
}
reverse(new_vpc.begin(),new_vpc.end());
for(int i=0;i<new_vpc.size();i++)
{
cout<<"new_vpc[i]:"<<new_vpc[i].strt_col<<", "<<new_vpc[i].strt_row<<", "<<new_vpc[i].end_col<<", "<<new_vpc[i].end_row<<endl;
}
//Add sub-vector - new_vpc to main vector - vpc.
copy_n(new_vpc.begin(),new_vpc.size(),&vpc[problemsStartAt]);
//Output
for(int i=0;i<vpc.size();i++)
{
cout<<"vpc[i]:"<<vpc[i].strt_col<<", "<<vpc[i].strt_row<<", "<<vpc[i].end_col<<", "<<vpc[i].end_row<<endl;
}
/*
Output:
Inside backTrack()8,11
vpc contains:11
vpc contains:11
new_vpc.size():2
new_vpc[i]:265, 185, 100, 105
new_vpc[i]:240, 185, 121, 125
new_vpc[i]:240, 185, 121, 125
new_vpc[i]:265, 185, 100, 105
vpc[i]:440, 288, 460, 303
vpc[i]:440, 263, 460, 225
vpc[i]:440, 238, 498, 210
vpc[i]:388, 185, 459, 155
vpc[i]:363, 185, 823, 171
vpc[i]:338, 185, 823, 425
vpc[i]:308, 185, 308, 144
vpc[i]:290, 185, 65, 193
vpc[i]:240, 185, 121, 125
vpc[i]:265, 185, 100, 105
vpc[i]:228, 700, 80, 750
*/