题解 | #简单计算器#

#include <cctype>
#include <iostream>
#include <unordered_map>
#include <stack>
#include <algorithm>
using namespace std;

stack<char> op;
stack<float> num;
unordered_map<char, int> prime{{'+', 1}, {'-', 1}, {'*', 2},{'/', 2}};

void eval(){
    float b = num.top(); num.pop();//第2个操作数
    float a = num.top(); num.pop();//第1个操作数
    char c = op.top(); op.pop();
    float x;
    if(c == '+') x = a + b;
    else if(c == '-') x = a - b;
    else if(c == '*') x = a * b;
    else if(c == '/') x = a / b;
    num.push(x);
}

int main() {
    string str;

    while(getline(cin, str) && str != "0"){

        for(int i = 0; i < str.size(); i ++){
            if(str[i] == ' ')
                continue;
            else if (isdigit(str[i])){//数字处理
                float x = 0;
                for(; i < str.size() && isdigit(str[i]); i ++)
                    x = x * 10 + str[i] - '0';
                num.push(x);
                i --;
            }
            else {
                while (!op.empty() && prime[op.top()] >= prime[str[i]])
                    eval();
                op.push(str[i]);
            }
        }
        //处理栈中剩余的运算符
        while(!op.empty())
            eval();
        printf("%.2f\n", num.top());
        num.pop();//清空
    }

    return 0;

}