Question

I have this vector

vector <string> data

data = ["this is", "data that", "is in", "this is", "vector", "vector", "vector"]

how do I get a vector (or 2D array) that removes duplicates and instead has the counts for each ith entry?

i.e.

 results = [("this is", 2), ("data that", 1), ("is in", 1), ("vector", 3)]

Answer 1

The straightforward solution would be to accumulate the unique values and their counts into a map:

std::map<std::string, std::size_t> results;
std::for_each(begin(data), end(data), [&](std::string const& s)
{
    ++results[s];
});

This has linearithmic (n lg n) time complexity, though because it must make a copy of each distinct string value, it may be rather expensive. You could also sort the list in-place, then count the number of each value, which would likely perform better if you have a move-aware implementation of std::string.