如何在每次循环迭代后向字符串中添加一个字符?

我试图创建一个罗马计算器，从文件读取。我正在努力弄清楚如何添加字符到字符串。我希望在每次循环迭代后添加一个没有空格的新字符，这将在程序编写答案时使用。

我试过了。

#include <iostream>
#include <iomanip>
#include <string>
#include <fstream>
using namespace std;

string convert_to_Roman(int num)
{
string c;
while (num>0)
{
    string c;
    if (num >= 1000)
    {
        num = num - 1000;
        return c='M';
    }
    else if (num >= 500 && num<1000)
    {
        num = num -500;
        return c = 'D';
    }
    else if (num >= 100 && num<500)
    {
        num = num -100;
        return c= 'C';
    }
    else if (num >= 50 && num<100)
    {
        num = num - 50;
        return c = 'L';
    }
    else if (num >= 10 && num<50)
    {
        num = num - 10;
        return c = 'X';
    }
    else if (num >= 5 && num<10)
    {
        num = num - 5;
        return c = 'V';
    }
    else if (num<5)
    {
        num = num - 1;
        return c = 'I';
    }
    c +=c;
    //cout<<"answerswer= "<< + answer<<endl;
}
cout << c;
}
int convert_from_Roman(string & s)
{
    int num=0;
    int length; //length of string
    length = s.length();
    for (int i = 0; i < length; i++)
    {
        char c = s[i];
        int digit;
        if (c == 'M')
        {
            return num = 1000;
        }
        else if (c == 'D')
        {
            return num = 500;
        }
        else if (c == 'C')
        {
            return num = 100;
        }
        else if (c == 'L')
        {
            return num = 50;
        }
        else if (c == 'X')
        {
            return num = 10;
        }
        else if (c == 'V')
        {
            return num = 5;
        }
        else if (c == 'I')
        {
            return num = 1;
        }
        else
        {
            cout << "invalid entry" << endl;
            continue;
        }
        num += num;
    }
    cout<<num<<endl;
}

void print_Result(/* figure out the calling sequence */)
{
    // fill in your code
}
// Note the call by reference parameters:
string finalAnswer()
{
    string operand1, operand2;
    char oper;
    cout << "enter operation: " << endl;
    cin >> operand1 >> operand2 >> oper;
    int value1, value2, answer;
    value1 = convert_from_Roman(operand1);
    value2 = convert_from_Roman(operand2);
    switch (oper)
    {
    case '+':
        {
        answer = value1 + value2;
        break;
        }
    case '-':
        {   
        answer = value1 - value2;
        break;
        }
    case '*':
        {
        answer = value1*value2;
        break;
        }
    case '/':
        {
        answer = value1 / value2;
        break;
        }
    default:
    {
        cout << "bad operator : " << oper << endl;
        return;
    }
        string answerInRoman = convert_to_Roman(answer);
        return answerInRoman;
        cout << "answerswer= " << answerInRoman << " (" << answer << ") " << endl;
    }