How to get the sorted index of a vector?

Question

I have a vector. It is not sorted. Now I want to get its indices which will sort the vector. For example vector<int> v{1, 3, 2}, sorted indices are {0, 2, 1} because v[0] <= v[2] <= v[1]. If two equal, doesn't matter which one go first.

Answer 1

What you're looking for is called tag sorting (or index sorting). Here is a minimal example using lambdas in C++11:

#include <algorithm>
#include <numeric>
#include <iostream>
#include <vector>

template<typename T>
std::vector<std::size_t> tag_sort(const std::vector<T>& v)
{
    std::vector<std::size_t> result(v.size());
    std::iota(std::begin(result), std::end(result), 0);
    std::sort(std::begin(result), std::end(result),
            [&v](const auto & lhs, const auto & rhs)
            {
                return v[lhs] < v[rhs];
            }
    );
    return result;
}

int main()
{
    std::vector<char> v{'a', 'd', 'b', 'c'};
    auto idxs = tag_sort(v);
    for (auto && elem : idxs)
        std::cout << elem << " : " << v[elem] << std::endl;
}

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