Sorting a vector using values in another vector

Question

I have a std::vector called foo_vec containing objects of class Foo. Suppose that Foo has a member variable int x, and I also implemented a function CompareInts(int a, int b) which returns the minimum of a and b. Then, I could do an std::sort the vector in terms of the object's x values.

However, what if these x values are not member variables of Foo, but are in another std::vector called x_vec. Here, the first element of x_vec corresponds to the first element of foo_vec, and so on. How can I perform an std::sort on foo_vec based on the corresponding values in x_vec?

Answer 1

You can make a third vector of indexes and sort that indirectly. After it's sorted, you can access the original vector through the sorted indexes:

std::vector<Foo> foo_vec = /* ... */;
std::vector<int> x_vec = /* ... */;
std::vector<std::size_t> index_vec;

assert(foo_vec.size() == x_vec.size());
for (std::size_t i = 0; i != foo_vec.size(); ++i) { index_vec.push_back(i); }

std::sort(
    index_vec.begin(), index_vec.end(),
    [&](std::size_t a, std::size_t b) { return x_vec[a] < x_vec[b]; });

for (std::size_t i = 0; i != index_vec.size(); ++i)
{
    std::cout << "Sorted element " << i << " is "
              << foo_vec[index_vec[i]] << "\n";
}

Note that this operation is entirely non-invasive, since everything happens indirectly.