void addNumbers(vector<double> &vec) {
    double add_num {};
    cout << "Enter an integer to add: ";
    cin >> add_num;
    vec.push_back(add_num);
    cout << add_num << " added" << endl;
}

向量是空的，我只希望人们能够向其中添加数字，每当他们尝试其他任何东西时它都会说“无效数字”。
完整的代码如下，如果我在lol中输入了数字以外的内容，它就会一遍又一遍地循环说“0.00 added

#include <iostream>
#include <vector>
#include <bits/stdc++.h>
#include <iomanip>
#include <cctype>
using namespace std;

char choice {};

char menu();
void print(vector<double>);
void mean(vector<double>);
void addNumbers(vector<double> &vec);
void smallest(vector<double>);
void largest(vector<double>);

char menu() {
    cout << "\nP - Print numbers" << endl;
    cout << "A - Add a number" << endl;
    cout << "M - Display mean of the numbers" << endl;
    cout << "S - Display the smallest number" << endl;
    cout << "L - Display the largest number" << endl;
    cout << "Q - Quit" << endl;
    cout << "\nEnter your choice: ";
    cin >> choice;
    choice = toupper(choice);
    return choice;
}

void print(vector<double> vec) {
    if (vec.size() != 0) {
    cout << "[ ";
    for (auto i : vec) {
        cout << i << " ";
    }
    cout << "]";
    }
    else {
        cout << "[] - the list is empty" << endl;
    }
}

void addNumbers(vector<double> &vec) {
    double add_num {};
    cout << "Enter an integer to add: ";
    cin >> add_num;
    vec.push_back(add_num);
    cout << add_num << " added" << endl;
}

void mean(vector<double> vec) {
    if (vec.size() != 0) {
    double result {};
    for (auto i : vec) {
        result += i;
    }
    cout << "The mean is " << result / vec.size() << endl;
    }
    else {
        cout << "Unable to calculate the mean - no data" << endl;
    }
}

void smallest(vector<double> vec) {
    if (vec.size() != 0) {
        cout << "The smallest number is " << *min_element(vec.begin(), vec.end()) << endl;
    }
    else {
        cout << "Unable to determine the smallest number - list is empty" << endl;
    }
}

void largest(vector<double> vec) {
    if (vec.size() != 0) {
        cout << "The largest number is " << *max_element(vec.begin(), vec.end()) << endl;
    }
    else {
        cout << "Unable to determine the largest number - list is empty" << endl;
    }
}

int main() {
    vector<double> vec {};
    bool done {true};
    cout << fixed << setprecision(2);

    do {
        menu();
        switch (choice) {
            case 'P':
                print(vec);
                break;
            case 'A': {
                addNumbers(vec);
                break;
            }
            case 'M': {
                mean(vec);
                break;
            }
            case 'S': {
                smallest(vec);
                break;
            }
            case 'L':
                largest(vec);
                break;
            case 'Q':
                cout << "Goodbye" << endl;
                done = false;
                break;
            default:
                cout << "Unknown selection, please try again" << endl;
        }
    } while (done == true);
    return 0;   
}