题解 | #简单计算器#

//stack应用：表达式求值

/*
//不知道错哪了：一直说我有段错误：您的程序发生段错误，可能是数组越界，堆栈溢出（比如，递归调用层数太多）等情况引起
//此方法没有考虑整数是多位数！！

#include <iostream>
#include <stack>
using namespace std;


int main()
{
	stack<double> num;
	stack<char> fu;
	string ans;
	fu.push('#');

	while(cin>>ans)
	{
		if(ans=="0")break;
		for(int i=0;i<ans.size();i++)
		{
			if(ans[i]!=' ')
			{
				if((ans[i]-'0')>=0&&(ans[i]-'0')<=9)
			{
				num.push(ans[i]-'0');
		//		cout<<num.top()<<" ";//
			}
				else 
			{
				fu.push(ans[i]);
		//		cout<<fu.top()<<" ";//

			}
			}
			
		}


		while(fu.top()!='#')/////判断条件
		{
			double number=num.top();//暂存数字
			num.pop();
			
			if(fu.top()=='/'||fu.top()=='*')
			{
				char fuhao=fu.top();//暂存符号
				fu.pop();

				double n2;
				if(fuhao=='/')
				{
					n2=num.top();
					num.pop();
					double n3=(n2*1.0)/(number);
					num.push(n3);
				}
				else if(fuhao=='*')
				{
					n2=num.top();
					num.pop();
					double n3=(n2*1.0)*(number);
					num.push(n3);

				}
			}
			
			else if(fu.top()=='+'||fu.top()=='-')

			{
				char fu1;//暂存栈顶符号
				fu1=fu.top();
				fu.pop();
				if(fu.top()=='+'||fu.top()=='-'||fu.empty())
				{
					double shu1;
					shu1=num.top();
					num.pop();
					if(fu1=='+')
					{
						shu1=shu1+number;
					}
					else 
					{
						shu1=shu1-number;
					}
					num.push(shu1);
				}
				else if(fu.top()=='/'||fu.top()=='*')
				{
					double shuh;//5
					double shuq;//2
					shuh=num.top();
					num.pop();
					shuq=num.top();
					num.pop();
					if(fu.top()=='/')
					{
						shuq=shuq*1.0/shuh;
						num.push(shuq);
					}
					else{
						shuq=shuq*1.0*shuh;
						num.push(shuq);

					}
					fu.pop();
					num.push(number);
					fu.push(fu1);

				}
			}
		}
		
		printf("%.2f\n",num.top());

		
	}
	
	
	return 0; 
}


*/


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

using namespace std;

int Prior(char x)
{
	if(x=='#')return 1;
	else if(x=='$')return 2;
	else if(x=='+'||x=='-')return 3;
	else return 4; //else if(x=='*'||x=='/')return 4; //写成这样会报错，说函数没有返回值

}

double GetNumber(string str,int& index)//传引用，形参变化，实参也变
{
	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
	result=x/y;

	return result;
}
int main()
{
	string str;
	while(getline(cin,str))
	{
		if(str=="0")break;
		int index=0;
		stack<double> data;
		stack<char> oper;
		str+='$';
		oper.push('#');
		while(index<str.size())
		{
			if(str[index]==' ')index++;
			else if(isdigit(str[index]))
				data.push(GetNumber(str,index));
			else if(Prior(oper.top())<Prior(str[index]))//不能写成<=   因为优先级相同时，从左往右运算
			{
				oper.push(str[index]);
				index++;
			}
			else {
				double y=data.top();
				data.pop();
				double x=data.top();
				data.pop();
				data.push(Calculate(x,y,oper.top()));
				oper.pop();
			}
		}
		printf("%.2f\n",data.top());
	}


	return 0;
}