RPN

#include <iostream>
#include <stack>
#include <string>
#include <algorithm>
#include <math.h>
#include <cctype>
using namespace std;

int lower_case ( int c )
{
  return tolower ( c );
}

int main()
{
stack <double> RPN;
string buff = NULL;
double N1 = 0;
double N2 = 0;
double answer = 0;
getline(cin, buff, ' ');

transform (buff.begin(),buff.end(), buff.begin(), lower_case);	//Convewrt to lower case

while (buff!="q")
{
	if (isdigit((int)buff[0]))
	{
		RPN.push(atof(buff.c_str()));
	}
	else if (buff == "sqrt")
	{
		if(!RPN.top()<0)
		{
			answer = sqrt(RPN.top());
			RPN.pop();
			RPN.push(answer);
		}
		else
		{
			cout<<"<ERROR"<<endl;
		}
	}
	else if (buff == "sum")
	{
		while( !RPN.empty())
		{
			answer += RPN.top(); 
			RPN.pop();
		}
		RPN.push(answer);
	}
	
	else
	{
		switch ((int)buff[0])
		{
			case '+':		//Addition
				
				N1 = RPN.top();
				RPN.pop();
				N2 = RPN.top();
				RPN.pop();
				answer = (N2+N1);
				break;
			
			case '-':		//Subtraction
				
				N1 = RPN.top();
				RPN.pop();
				N2 = RPN.top();
				RPN.pop();
				answer = (N2-N1);
				break;
			
			case '*':		//Multiplication
				
				N1 = RPN.top();
				RPN.pop();
				N2 = RPN.top();
				RPN.pop();
				answer = (N2*N1);
				break;
			
			case '/':		//Division with test for zero in denominator
				
				if(!RPN.top()==0)
				{
					N1 = RPN.top();
					RPN.pop();
					N2 = RPN.top();
					RPN.pop();
					answer = (N2/N1);
				}
				else
				{
					cout<<"ERROR"<<endl;
				}
				break;
			
			case '=':		//write top number and toss it
			
				cout<<RPN.top()<<endl;
				RPN.pop();
				break;
			
			case '.':		//write top number and leave it
			
				cout<<RPN.top()<<endl;
				break;

			default:		
			
				cout<<"ERROR"<<endl;
				break;
			
		}
	}
}
if (buff=="q")
{
	return 0;
}
};