题解 | 简单计算器

#include <iostream>
#include <stack>
#include <cctype>
#include <string>
#include <cstdio>
using namespace std;
int Priority(char op){
    if(op=='$') return 0;
    else if(op=='#') return 1;
    else if(op=='+'||op=='-') return 2;
    else return 3;
}

double getNum(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 result=0;
        if(op=='+')  result = x+y;
        else if(op=='-') result = x-y;
        else if(op=='*') result = x*y;
        else if(op=='/') result = x/y;
        return result;
}

int main() {
    string str;
    while(getline(cin,str)){
        if(str=="0") break;
        int index = 0;
        stack<char> oper;
        stack<double> nums;
        oper.push('#');
        str+='$';
        while(index<str.size()){
            if(str[index]==' ') index++;
            else if(isdigit(str[index])){
                nums.push(getNum(str, index));
            }
            else{
                if(Priority(str[index])>Priority(oper.top())){
                    oper.push(str[index]);
                    index++;
                }
                else{
                    if(oper.top() == '#' && str[index] == '$') {
                        index++; 
                        break;
                    }
                    double b,a,result;
                    char op;
                    op=oper.top();
                    oper.pop();
                    b=nums.top();
                    nums.pop();
                    a=nums.top();
                    nums.pop();
                    result=Calculate(a, b, op);
                    nums.push(result);
                }
            }
        }
        printf("%.2f\n", nums.top());
    }
    return 0;
}