postfix calculator

hello .. am still a bit new to c++ and am teaching myself
i was trying to do a postfix calculator using a strings stack
the stack alone works just fine !
this is my code for calculating the post fix formula

#include "PostFixCalculator.h"

double PostFixCalculator::calculate(string &expression)
{
	double op1,op2;//first and second operands
	double result;//to store result of operation
	int index;//index of space to split formula string
	string temp,t;//to store part of the formula temporarly
	index=expression.find(" ");
	ostringstream sstream;//to convert from int to string

	while(index!=string::npos)
	{
		temp=expression.substr(0,index);
		if(temp=="=")
			break;
		else
		{
			if(temp=="+")
			{
				op1=atof(stack.top().c_str());//convert from string to double
				stack.pop();//remove first element
				op2=atof(stack.top().c_str());//convert from string to double
				stack.pop();//remove first element
				result=op1+op2;//perform operation
				sstream<<result;
				t=sstream.str();//convert from double to string
				stack.push(t);
			}
			else
			{
				if(temp=="-")
				{
				//	string x=stack.top();
					op1=atof(stack.top().c_str());//convert from string to double
					stack.pop();//remove first element
//					x=stack.top();
					op2=atof(stack.top().c_str());//convert from string to double
					stack.pop();//remove first element
					result=op1-op2;//perform operation
					//cout<<result<<endl<<"r"<<endl;
					sstream<<result;
					t=sstream.str();//convert from double to string
					stack.push(t);
				}
				else
				{
					if(temp=="*")
					{
						op1=atof(stack.top().c_str());//convert from string to double
						stack.pop();//remove first element
						op2=atof(stack.top().c_str());//convert from string to double
						stack.pop();//remove first element
						result=op1*op2;//perform operation
						sstream<<result;
						t=sstream.str();//convert from double to string
						stack.push(t);
					}
					else
					{
						if(temp=="/")
						{
							op1=atof(stack.top().c_str());//convert from string to double
							stack.pop();//remove first element
							op2=atof(stack.top().c_str());//convert from string to double
							stack.pop();//remove first element
							result=op1/op2;//perform operation
							sstream<<result;
							t=sstream.str();//convert from double to string
							stack.push(t);
						}
						else
						{
							stack.push(temp);
						}
					}
				}
			}
		}
		expression=expression.substr(index+1,expression.length());
		index=expression.find(" ");
	}
	return atof(stack.top().c_str());//convert and return result
}

now when i enter a formula of any 2 numbers say like 1 2 + =
the output is 3 which is correct (goes for anhy other formula)
but when its more than 2 numbers the result is wrong
i just cant figure out why its doing that!!
any help is really appreciated
thank you in advace