c++ Program stops mid loop with no errors

Question

I'm working on a hospital simulation program where patients are sorted by severity level and placed in corresponding queues. Everything seems to work fine when I'm debugging it, but for some reason, it stops after a random amount of loops. I don't get any errors--my program just quits mid loop. What could cause that to happen?

priority_queue<Hospital_Visit> queue1, queue2;
vector<int> nurse_time_vector, doctor_time_vector;
vector<Hospital_Visit> current_hospital_visit;

for (int i = 0; i < master_hospital_visit.size(); i++){ //make a copy of the master_hospital_visit to manipulate
    current_hospital_visit.push_back(master_hospital_visit[i]);
}



if (num_nurses>0)
    nurse->set_minute(0);
if (num_doctors>0)
    doctor->set_minute(0);

int treatment;

map<string, vector<Hospital_Visit>>::iterator it;

srand(time(NULL));

bool hour_changed = true;

do{

    if (hour_changed){ //update queues with new hour's patients added to end
        for (int i = 0; i < current_hospital_visit.size(); i++){
            if (current_hospital_visit[i].get_hour() > current_hour){
                break;
            }
            if (current_hospital_visit[i].get_hour() <= current_hour)
            {
                if (current_hospital_visit[i].get_severity() <= 10) // For nurses
                {
                    queue1.push(current_hospital_visit[i]);
                    current_hospital_visit.erase(current_hospital_visit.begin() + i);
                }
                else // for doctors
                {
                    queue2.push(current_hospital_visit[i]);
                    current_hospital_visit.erase(current_hospital_visit.begin() + i);
                }
            }
        }
    }

    hour_changed = false;


        if (nurse->get_minute() != nurse->get_max_time() && queue1.size() != 0 && queue1.top().get_hour() <= current_hour && num_nurses > 0)
        {
            treatment = (rand() % 11) + 1;

            queue1.top().set_treatment_time(treatment);

            queue1.top().set_day(current_hour / 24);

            queue1.top().set_hour(current_hour);

            nurse->set_minute(treatment);

            queue1.top().set_wait_time(current_hour - queue1.top().get_hour() + treatment);

            queue1.top().set_medic(false);


            queue1.top().set_severity(queue1.top().get_severity() - (current_hour - queue1.top().get_hour()));


            it = patients_map.find(queue1.top().get_name());
            if (it == patients_map.end())
            {
                vector<Hospital_Visit> patient_visits;

                patient_visits.push_back(queue1.top());

                patients_map.insert(make_pair(queue1.top().get_name(), patient_visits));
            }
            else
            {
                it->second.push_back(queue1.top());
            }
            queue1.pop();
        }
        else if (queue1.size() == 0 && num_nurses > 0) //was !=
        {
            nurse_time_vector.push_back(nurse->get_max_time() - nurse->get_medic_hour());
        }

        if (doctor->get_medic_hour() != doctor->get_max_time() && queue2.size() != 0 &&
            queue2.top().get_hour() <= current_hour && num_doctors > 0)
        {
            treatment = (rand() % 21) + 1;

            queue2.top().set_treatment_time(treatment);

            queue2.top().set_day(current_hour / 24);

            queue2.top().set_hour(current_hour);

            doctor->set_minute(treatment);

            queue2.top().set_wait_time(current_hour - queue2.top().get_hour() + treatment);

            queue2.top().set_medic(true);

            queue2.top().set_severity(queue2.top().get_severity() - (current_hour - queue2.top().get_hour()));

            it = patients_map.find(queue2.top().get_name());
            if (it == patients_map.end())
            {

                vector<Hospital_Visit> patient_visits;

                patient_visits.push_back(queue2.top());

                patients_map.insert(make_pair(queue2.top().get_name(), patient_visits));
            }
            else
            {

                it->second.push_back(queue2.top());
            }
            queue2.pop();
        }

        else if (doctor->get_minute() != doctor->get_max_time() && queue1.size() != 0 && queue2.size() == 0
            && queue1.top().get_hour() <= current_hour && num_doctors > 0){

            treatment = (rand() % 21) + 1;

            queue1.top().set_treatment_time(treatment);

            queue1.top().set_day(current_hour / 24);

            queue1.top().set_hour(current_hour);

            doctor->set_minute(treatment);

            queue1.top().set_wait_time(current_hour - queue1.top().get_hour() + treatment);

            queue1.top().set_medic(true);

            queue1.top().set_severity(queue1.top().get_severity() - (current_hour - queue1.top().get_hour()));

            it = patients_map.find(queue1.top().get_name());
            if (it == patients_map.end())
            {
                vector<Hospital_Visit> patient_visits;
                patient_visits.push_back(queue1.top());
                patients_map.insert(make_pair(queue1.top().get_name(), patient_visits));
            }
            else
            {
                it->second.push_back(queue1.top());
            }
            queue1.pop();

        }

        if ((doctor->get_minute() <= doctor->get_max_time() + 20 && doctor->get_minute() >= doctor->get_max_time()-5) 
            || (queue1.size() == 0 && queue2.size() == 0))
            { // if the max time is reached or the queues are empty, then change the hour

            current_hour++;
            hour_changed = true;

            if (num_nurses>0)
                nurse->set_minute(0);
            if (num_doctors>0)
                doctor->set_minute(0);

            }

} while (current_hour != 169);

Answer 1

Take a look at this loop:

for (int i = 0; i < current_hospital_visit.size(); i++){
    ...
    if (...)
    {
        current_hospital_visit.erase(current_hospital_visit.begin() + i);
    }
    ...
}

it will not check every element. Suppose i is 3, and the if condition is true. Element 3 is erased, making element 4 the new element 3. But in the next iteration, i is incremented and becomes 4, so this new element 3 will never be tested.

A possible solution is to decrement i when you erase something:

for (int i = 0; i < current_hospital_visit.size(); i++){
    ...
    if (...)
    {
        current_hospital_visit.erase(current_hospital_visit.begin() + i);
        i--;
    }
    ...
}