Determining the unique rows of a 2D array (vector >)

Question

I am using a datatype of std::vector > to store a 2D matrix/array. I would like to determine the unique rows of this matrix. I am l

Answer 1

EDIT: I forgot std::vector already defines operator< and operator== so you need not even use that:

template 
std::vector > GetUniqueRows(std::vector > input)
{
    std::sort(input.begin(), input.end());
    input.erase(std::unique(input.begin(), input.end()), input.end());
    return input;
}

Use std::unique in concert with a custom functor which calls std::equal on the two vectors.

std::unique requires that the input be sorted first. Use a custom functor calling std::lexicographical_compare on the two vectors input. If you need to recover the unreordered output, you'll need to store the existing order somehow. This will achieve M*n log n complexity for the sort operation (where M is the length of the inner vectors, n is the number of inner vectors), while the std::unique call will take m*n time.

For comparison, both your existing approaches are m*n^2 time.

EDIT: Example:

template 
struct VectorEqual : std::binary_function&, const std::vector&, bool>
{
    bool operator()(const std::vector& lhs, const std::vector& rhs)
    {
        if (lhs.size() != rhs.size()) return false;
        return std::equal(lhs.first(), lhs.second(), rhs.first());
    }
};

template 
struct VectorLess : std::binary_function&, const std::vector&, bool>
{
    bool operator()(const std::vector& lhs, const std::vector& rhs)
    {
        return std::lexicographical_compare(lhs.first(), lhs.second(), rhs.first(), rhs.second());
    }
};

template 
std::vector > GetUniqueRows(std::vector > input)
{
    std::sort(input.begin(), input.end(), VectorLess());
    input.erase(std::unique(input.begin(), input.end(), VectorEqual()), input.end());
    return input;
}