题解 | 简单计算器

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

using namespace std;

// 用&传值可在函数内改变函数外的值
double GetNumber(string str,int &index){
	double number=0;
	while(str[index]>='0' && str[index]<='9'){
		number=number*10+(str[index]-'0');
		index++;
	}
	return number;
}

int Priority(char c){
    int priority;
    
	if(c=='#')  priority=0;
	else if(c=='$') priority=1;
	else if(c=='+' || c=='-') priority=2;
	else if(c=='*' || c=='/') priority=3;

    return priority;
}

double Calculate(double a,double b,char operation){
    double result;

	if(operation=='+')  result=a+b;
	else if(operation=='-') result=a-b;
	else if(operation=='*') result=a*b;
	else if(operation=='/') result=a/b;

    return result;
}

int main(){
    string str;
    while(getline(cin,str)){
    	if(str=="0")    break;

    	stack<double> number;

    	stack<char> operation;
    	operation.push('#');
    	str+='$';

    	int index=0;
    	while(index<str.size()){
    		if(str[index]==' ')  index++;
    		else if(str[index]>='0' && str[index]<='9')  number.push(GetNumber(str,index));
    		else{
				if(Priority(str[index])>Priority(operation.top())){
					operation.push(str[index]);
					index++;
				}
				else{
					double b=number.top();
					number.pop();
					double a=number.top();
					number.pop();
					number.push(Calculate(a,b,operation.top()));
					operation.pop();
				}
			}
		}
		printf("%.2f\n",number.top());
	}

	return 0;
}
//http://t.cn/AiKoGS94