Add：四则运算

#include <iostream>
#include<math.h>
#include<cstdio>
#include <algorithm>
using namespace std;
double ex();  //表达式
double term();//项
double sub();  //因子

int main()
{
    cout<<ex();
}

double ex()
{
    double result=term();
    bool more =true;
    while(more)
    {
        char op=cin.peek();
        if(op==‘+‘)
        {
            cin.get();
            result=result+term();
        }
        else if(op==‘-‘)
        {
            cin.get();
            result=result-term();
        }
        else
            more = false;
    }
    return result;
}

double term()
{
    double result=sub();
    bool more =true;
    while(more)
    {
        char op=cin.peek();
        if(op==‘*‘)
        {
            cin.get();
            result=result*sub();
        }

        else if(op==‘/‘)
        {
            cin.get();
            result=result/sub();
        }
        else
            more = false;
    }
    return result;
}
double sub()
{
    double result=0;
    char op=cin.peek();
    if (op==‘(‘)
    {
        cin.get();
        result=ex();
        cin.get();
    }
    else
    {
        while(isdigit(op))
        {
            result=result*10+op-‘0‘;
            cin.get();
            op=cin.peek();
        }
    }
   return result;
}