题解 | #简单计算器#

#include<iostream>
#include<string>
#include<stack>
#include<cctype>

using namespace std;

//优先级顺序#,$,+-,*/
int priority(char c) {
    if(c=='#') {
        return 0;
    } else if(c=='$') {
        return 1;
    } else if(c=='+'||c=='-') {
        return 2;
    } else {
        return 3;
    }
}

//获得数字
double getNumber(string str,int& index) {
    double number=0;
    while(isdigit(str[index])) {
        number=number*10+str[index]-'0';
        index++;
    }
    return number;
}

double Calculate(double x,double y,char op) {
    double ans=0;
    if(op=='+') {
        ans=x+y;
    } else if(op=='-') {
        ans=x-y;
    } else if(op=='*') {
        ans=x*y;
    } else if(op=='/')  {
        ans=x/y;
    }
    return ans;
}

int main() {
    string str;
    while(getline(cin,str)) {
        if(str=="0") 
        {
            break;
        }
        int index=0;
        stack<char> operation;
        stack<double> number;
        str+='$';
        operation.push('#');
        while(index<str.size())
        {
            if(str[index]==' ') 
            {
                index++;   //向后继续扫描
            } 
            else if(isdigit(str[index])) 
            {
                double tt=getNumber(str,index);  //将运算数压入栈中
                number.push(tt);
            } 
            else 
            {
                if(priority(operation.top())<priority(str[index])) //比较栈顶运算符与当前运算符
                {
                    operation.push(str[index]);   //将当前运算符压入栈中
                    index++;
                } 
                else 
                {
                    double y=number.top();   //可运算的局部区域，依次弹出数字栈顶与次栈顶进行运算
                    number.pop();
                    double x=number.top();
                    number.pop();
                    number.push(Calculate(x,y,operation.top()));  //将计算结果放入数字栈中
                    operation.pop();
                }
            }
        }
        printf("%0.2f\n",number.top());   //输出数字栈栈顶
    }
    return 0;
}