Loops 无法创建函数以确保为int，发生无限循环，平均函数保持加法

所以我的问题是，我试图使用一个函数来确保int加上_值，但它不起作用。如果我输入一个字母，它就会开始一个无限循环。而且，如果我再次按下M，我的平均函数会不断增加，我不知道为什么。对不起，如果这是一个非常简单的问题，我真的是编程新手。我已经搜索了一些方法来确保它是一个int，但是它没有向向量添加值

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


using namespace std;

int main()
{
    //variables must stay outside the loop otherwise
    //every iteration it will replaced by a new value
    bool control_end = false;
    vector<int> vec{};
    int mean{0};
    int running_sum{0};
    bool is_numb{0};
    //int min_value{0};

    do
    {
    cout << "Please enter your desired function within the menu: " << endl;
    cout << endl;
    cout << "P--Print list of numbers" << endl;
    cout << "A--Add a number"<< endl;
    cout << "M--Display mean of the vector"<< endl;
    cout << "S--Display the smallest number"<< endl;
    cout << "L--Display the largest number"<< endl;
    cout << "Q--Quit "<< endl;
    cout << endl;
    cout << "Enter your choice: ";
    cout << endl;

    char letra{};
    cin >> letra;

    switch(letra)
    {
      case 'A':
      case 'a':
       {
        cout << "Please enter the value (integer) to be added to vector: ";
        int value_add;
        cin >> value_add;
        cout << endl;
        vec.push_back(value_add);
        cout << "Value added: " << value_add << endl;
        // create a function to ensure that the value is an integer

       if(isdigit(value_add))
            {
             vec.push_back(value_add);
             cout << "Value added: " << value_add << endl;
            }
       else
            {
               cout << "Invalid value."<< endl;
               cout << endl;
            } 
       break;
       }
      case 'p':
      case 'P':
       {
        if (vec.size()>0)
                {
                cout << "This is your vector: \n";
                //for (unsigned int i=0 ;i < vec.size(); i++)
                //cout << "{"<< vec[i] << "}" << " ";
                //cout << endl;
                for (auto print: vec)
                cout << print << " ";
                cout << endl;
                }
        else
                {
                cout << "The vector is empty." << endl;
                cout << endl;
                }
        break;
       }
      case 'm':
      case 'M':
       {
        for (auto mean_func:vec)
        {   // it continues to keep adding and calculating the mean
            // cout << mean_func << endl;
             running_sum += mean_func;
            // cout << running_sum << endl;
             mean = running_sum/vec.size();
             //cout << vec.size() << endl;
        }
             cout << "The mean is: " << mean << " ";
             cout << endl;
             //cout << mean << endl;

        break;
       }

      case 's':
      case 'S':
        cout << endl;
        cout << "Min Element = " << *min_element(vec.begin(), vec.end());
        cout << endl;
        break;

      case 'L':
      case 'l':
        cout << endl;
        cout << "Max Element = " << *max_element(vec.begin(), vec.end());
        cout << endl;
        break;

      case 'q':
      case 'Q':
        cout << "Thank you for using the program" << endl;
        control_end = true;
        break;

      default:
        cout << "Invalid char. " << endl;
        cout << endl;
        break;
    }

    }



      while (control_end !=true);



      return 0;
}

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使用名称空间std；
int main（）
{
//否则变量必须留在循环之外
//每次迭代它都将被一个新值替换
bool control_end=假；
向量向量{}；
int平均值{0}；
int运行_sum{0}；
布尔是{0}；
//int min_值{0}；
做
{
库特