C++:递归求阶乘

本来想添加一个输入是否为数字的判断，没成功，以后再改进！

#include <iostream>

using namespace std;

double f1( double a, double b=1){
    b*=a;
    if(a==1) return b;
    else f1(a-1,b);
}

void f2(int& c){
    double a,b;
    cout << "Enter the number: ";
    cin >> a;
    b=f1(a);
    cout << a << "!= "<< b <<endl;
    cout <<'\n';
}

int main (){
    int a(1);
    while(a==1){
        f2(p);
        cout<<"Do you want to do another calculation? y/n \n";
        char str;
        cin>>str;
        if(str=='Y'||str=='y')continue;
        if(str=='N'||str=='n')a=0;
    }
   
return 0;

}