Why is this code exiting at this point?

Question

I'm new to C++. I stumbled upon one tutorial problem, and I thought I'd use the few things I have learnt to solve it. I have written the code to an extent but the code exits at a point, and I really can't figure out why. I do not want to go into details about the tutorial question because I actually wish to continue with it based on how I understood it from the start, and I know prospective answerers might want to change that. The code is explanatory, I have just written few lines.

Here comes the code:

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

using namespace std;

double average_each_student() {

    cout << "\n>Enter your scores seperated by spaces and terminated with a period\n"
         << ">Note: scores after total number of six will be truncated.\n"
         << endl;

    double sum = 0, average = 0;
    int user_input, counter = 0;
    const double no_of_exams = 6;

    while(cin >> user_input) {
        ++counter;
        if(counter < 5) sum += 0.15 * user_input;
        else if(counter > 4 && counter < 7) sum += 0.20 * user_input;

    }

    return sum / no_of_exams;
}

int reg_number() {

    cout << "Enter your registration number: " << endl;
    int reg_numb;
    cin >> reg_numb;
    return reg_numb;

}

int main() {

    vector<int> reg_num_list;
    vector<double> student_average;

    reg_num_list.push_back(reg_number());
    student_average.push_back(average_each_student());

    string answer;
    cout << "\n\nIs that all??" << endl;
    //everything ends at this point.
    //process returns 0

    cin >> answer;

    cout << answer;


}

The code exits at cout << "\n\nIs that all??" << endl;. The rest part after that is not what I intend doing, but I'm just using that part to understand what's happening around there.

PS: I know there are other ways to improve the whole thing, but I'm writing the code based on my present knowledge and I wish to maintain the idea I'm currently implementing. I would appreciate if that doesn't change for now. I only need to know what I'm not doing right that is making the code end at that point.

Answer 1

The loop inside average_each_student() runs until further input for data fails and std::cin gets into failure state (i.e., it gets std::ios_base::failbit set).

As a result, input in main() immediately fails and the output of what was input just prints the unchanged string. That is, your perception of the program existing prior to the input is actually wrong: it just doesn't wait for input on a stream in fail state. Since your output doesn't add anything recognizable the output appears to do nothing although it actually prints an empty string. You can easily verify this claim by adding something, e.g.

std::cout << "read '" << answer << "'\n";

Whether it is possible to recover from the fail state on the input stream depends on how it failed. If you enter number until you indicate stream termination (using Ctrl-D or Ctrl-Z on the terminal depending on what kind of system you are using), there isn't any way to recover. If you terminate the input entering a non-number, you can use

std::cin.clear();

To clear the stream's failure stated. You might want to ignore entered characters using

std::cin.ignore(); // ignore the next character

or

std::cin.ignore(std::numeric_limits<std::streamsize>::max(), '\n');
    // ignore everything up to the end of the line