以双精度数除以双精度数来获得循环中的余数和char参数不起作用

我遇到了一个问题，我似乎不知道我做错了什么。我必须写这个程序，由于某些原因它不允许我在循环中比较一个字符变量和一个常量字符变量它不允许我%两个双变量?有人能帮忙吗?

    // Nathan Brown
// Nathan Owen Brown's Space Travle Company
// CSCI 1010 PASS9
#include <iostream>
#include <cmath>
#include <iomanip>
using namespace std;
void mostPowerful (double &power1, double &power2, double &power3)
{
    if (power1 > power2 && power1 > power3)
        cout << "The largest power output is " << power1 << " and is Jetpack number 1" << endl;
    else if (power2 > power1 && power2 > power3)
        cout << "The largest power output is " << power2 << " and is Jetpack number 2" << endl;
    else if (power3 > power1 && power3 > power2)
        cout << "The largest power output is " << power3 << " and is Jetpack number 3" << endl;
}
void discountResults (double &price, double &disAmount) 
{
    disAmount = price * disAmount;
    price = price - disAmount;
}
void howMany (double &moneyAvail, double &cost)
{
    int howMany;
    double leftOver;
    howMany = moneyAvail / cost;
    leftOver = moneyAvail % cost;
    moneyAvail = howMany;
    cost = leftOver;
}
char menu ()
{
    char choice;
    cout << "Welcom to Nathan Owen Brown's Space Travel Company" << endl;
    cout << "(M)ost Powerful Calculation" << endl;
    cout << "(D)iscount Calculation" << endl;
    cout << "(H)ow Many Calculation" << endl;
    cout << "(Q)uit" << endl;
    cout << endl;
    cout << endl;
    cout << "Please enter the option (M, D, H, or Q)    ";
    cin >> choice;
    return choice;
}
int main ()
{
    double power1, power2, power3, price, disAmount, moneyAvail, cost;
    char choice;
    menu ();
    while (choice != "Q "|| choice != "q")
    {
        if (choice == "M" || choice == "m" )
        { 
            cout << "Please enter 3 power output measurements in MW: " << endl;
            cin >> power1 >> power2 >> power3;
            mostPowerful (power1, power2, power3);
        }
        else if (choice == "D" || choice == "d")
        {
            cout << "Please enter a price and a discount amount: " << endl;
            cin >> price >> disAmount;
            discountResults (price, disAmount);
            cout << "The discount amount is " << disAmount << " and the dicounted price is " << price << "." << endl;
        }
        else if (choice == "H" || choice == "h")
        {
            cout << "Please enter amount available and cost of each: " << endl;
            cin >> moneyAvail >> cost;
            howMany (moneyAvail, cost);
            cout << "You can buy " << moneyAvail << " and have " << cost << " left over." << endl;
        }
    menu ();
    }
    system ("PAUSE");
        return 0;
}