Improving code to find intersection of n vectors

Asked Sep 07 '17 at 15:15

Active Sep 07 '17 at 15:15

Viewed 35 times

My code right now:

#include <algorithm>
#include <iterator>
#include <string>
#include <vector>
using namespace std;

void FindIntersection(vector<vector<string>>& data)
{
    while (data.size() > 1)
    {
        auto lastVec = data.size() - 1;
        vector<string> result;
        std::set_intersection(data[0].begin(), data[0].end(),
                              data[lastVec].begin(), data[lastVec].end(),
                              std::back_inserter(result));
        data[0] = result;
        data.resize(lastVec);
    }
}

This narrows down the input to a single vector, which I then access using data[0]. The code works, but I don't like the assignment data[0] = result;, since vector copying, especially of string is not the most efficient. Any ideas how to improve this code?

P.S. I know that this optimization will have no real world benefit right now, but I am just curious to see how much better this can get.

asked Sep 07 '17 at 15:15

Eyal K.

1,042
8
23

1

Requests for feedback on working code should be made at https://codereview.stackexchange.com/ – François Andrieux Sep 07 '17 at 15:16
`data[0] = std::move(result);`. – Jarod42 Sep 07 '17 at 15:22
Who says copying blocks of contiguous data is slow? – Richard Hodges Sep 07 '17 at 15:23
`std::vector::pop_back` can be used to clearly reduce the size of a vector by 1, rather than resizing to one less than it's current size. – François Andrieux Sep 07 '17 at 15:23
@RichardHodges Copying a vector of `std::string` is likely to imply copying non-contiguous data. – François Andrieux Sep 07 '17 at 15:25
@FrançoisAndrieux that depends on the lengths of the strings. But in any case, the complexity in this algorithm is the set_intersection, not the copying. Sure, if the strings are long we could benefit a little from std::move, but not much. – Richard Hodges Sep 07 '17 at 15:27
and You might also use `move_iterator` for `data.back()`. – Jarod42 Sep 07 '17 at 15:28

Improving code to find intersection of n vectors

0 Answers0