为什么修改了变量的类型，程序就运行不了；我把num和add的类型改为char，就得不到想要的结果

Question

#include <iostream>

int main()

{

using namespace std;

double num = 0;

double add = 0;

cout << "input the number: ";

cin >> num;

while (num!=0)

{

add += num;

cout << "input the number again: ";

cin >> num;

}

cout << "the total of these number is " << add << endl;

return 0;

}