std::vector<std::pair<std::string, double>> payments = { {"8/18", 0.0}, {"7/18", 771.98}, {"6/18", 0.0}, {"5/18", 771.98},
                                                    {"4/18", 771.98}, {"3/18", 771.98}, {"2/18", 0.0}, {"1/18", 3859.90},
                                                    {"12/17", 771.98}, {"11/17", 0.0}, {"10/17", 1543.96}, {"9/17", 771.98} };

payment_months = [8,7,6,5,4,3,2,1,12,11,10,9]

std::vector<int> paymentMonths;
for (auto it : payments)
{
    paymentMonths.push_back(it.first[0] - '0');
}