Problem with postfix expressions evalutation (stacks)

[galiwocky]

I am solving this problem on evaluating reverse polish notation expressions or postfix expressions involving integers between -2000000 and 2000000. The procedure is pretty straightforward:

 if x is a number, put the number in stack
  if x is the operator +, pop two values b and a, and push a+b in stack
  if x is the operator -, pop two values b and a, and push a-b in stack
  if x is the operator *, pop two values b and a, and push a*b in stack
  if x is the operator /, pop two values b and a, and push a/b in stack

My program is able to print correct results for the given sample inputs. However, when I submitted it to an online judge, it was judged as Incorrect. Here's the code:

#include <iostream>
#include <stack>
#include <cstdlib>
#include <string>
#include <sstream>
#include <cstring>
#include <cctype>

using namespace std;

int main()
{
	
	int N, i = 0;
	bool undefined;
	string dummy;
	
	cin >> N;
	getline(cin, dummy);
	
	long long a, b;
	string sequence;
	
	while (i < N)
	{
		stack<long long> s;
		getline(cin, sequence);
		stringstream sin(sequence);
		undefined = false;
		string tokens;
		while (sin >> tokens)
		{
			if (isdigit(tokens[0]) || isdigit(tokens[1]))
			{
				s.push(atoi(tokens.c_str()));
			}
			
			else 
			{
				switch(tokens[0])
				{
					case '+':
						b = s.top();
						s.pop();
						a = s.top();
						s.pop();
						s.push(a + b);
						break;
					
					case '-':
						b = s.top();
						s.pop();
						a = s.top();
						s.pop();
						s.push(a - b);
						break;
					
					case '*':
						b = s.top();
						s.pop();
						a = s.top();
						s.push(a * b);
						break;
				
					case '/':
						b = s.top();
						s.pop();
						if (b == 0)
							undefined = true;
						else
						{
							a = s.top();
							s.pop();
							s.push(a / b);
						}
						break;
				}
			}
		}
		if (undefined)
			cout << "UNDEFINED" << endl;
		else
		{
			cout << s.top() << endl;
			s.pop();
		}
		
		i++;

	}
	return 0;
}

Here's the sample input

5
4 5 +
3 2 / 4 3 - +
2 0 /
2000000 -2000000 *
2000000 1000000 43000 + *

And here is the ouput

5
4 5 +
3 2 / 4 3 - +
2 0 /
2000000 -2000000 *
2000000 1000000 43000 + *

I really feel my algorithm is right. But what could be wrong in my code? Can anyone point the flaws and how I can fix it? Providing critical test input can also help. Thanks in advance!

[TheIdeasMan]

I think it would be really educational to learn how to use the debugger. If you are using an IDE there should be a built in one that is easy to use. If not, then it is a bit trickier, but not impossible to use one from a command line. For example gdb, can be used from the cmd line, or incorporated into an IDE.

You can set breakpoints, have a watchlist of variable values, and step through the code 1 line at a time.

The whole process can be quicker and less boring than what one might expect initially.

Hope all goes well.

Edit:

Here is a link to K&R C Programming - it has a RPN program.

http://zanasi.chem.unisa.it/download/C.pdf

[Fovv]

stupid question but on line 21 you have
long long a, b;

would this not work?
long a, b;

[TheIdeasMan]

long long is a type in C++.

You can also have unsigned long long

Sometimes, it is not necessary - on my system a long is 64bits.

I tend to use the stdint.h header, it has types like uint64_t which is an unsigned 64 bit quantity. It has others like int8_t which is a signed 8 bit - I find this less confusing than using a char as a small int.