Copy index of vector Elements with condition

Question

I want to save the indices of my bool vector where the vector element is false.

I have the following code:

vector<bool> incumbent_solution; // (0,0,0,1,1,0,0)...
vector<int> I_minus_S(incumbent_solution.size());

auto it = copy_if(incumbent_solution.begin(), incumbent_solution.end(),
        I_minus_S.begin(), [&incumbent_solution](auto i) {if (incumbent_solution[i] == 0] return i; });
I_minus_S.erase(it, I_minus_S.end());

But it only stores True in my Vector and not the indices. What is my lambda doing wrong?

Answer 1

The std::copy_if works differently than you expected, it passes the actual element to the predicate and copies it into the second container if the predicate returns true.

If you want indexes, use a simple for loop:

std::vector<bool> incumbent_solution { 0, 0, 0, 1, 1, 0, 0, 1, 1 };
std::vector<int> I_minus_S(incumbent_solution.size());

std::size_t last = 0;

for(std::size_t index = 0; index < incumbent_solution.size(); ++index) {
    if(incumbent_solution[index] == false)
        I_minus_S[last++] = index;
}

I_minus_S.erase(I_minus_S.begin() + last, I_minus_S.end());