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deque iterator not dereferencable

Amarki (2)
The program is supposed to evaluate a line of mathematical expression, then push and pop operands and operators respectively into two different stacks.

Problem is, that although the code looks fine to me as I wrote it, and even though it compiles just fine, as soon as I enter in an equation and hit enter, I get a nice fancy "Expression: deque iterator not dereferencable" error. 

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  int evaluate(string expression)
{
	istringstream in;
	in.str(expression);
	stack <int> Operands;
	stack <char> Operators;


	char operators;	
	int operands;
	int num1;
	int num2;
	int result;
	/*int eqtop = 0;*/
	/*int top;*/
	int tempNum;
	char tempChar;
	//char peek = in.peek();

	while ( in.good() )
	{
		
		//read operand
		in >> operands;
		if(in.good())
		{
			Operands.push(operands);
		
		}

	//read in operator
			in >> operators;
			if (Operators.empty())
			{
				
				if (in.good())
					Operators.push(operators);
			}
			else if (!Operators.empty())
			{
				//in >> operators;
				if (Operators.top() != '*' && Operators.top() != '/')
				{
					if (in.good())
						Operators.push(operators);
				}
				if (Operators.top() == '*' || Operators.top() == '/')
				{
					tempChar = Operators.top();
					Operators.pop();
					num1 = Operands.top();
					Operands.pop();
					num2 = Operands.top();
					if (tempChar == '*')
					{
						result = num1 * num2;
						Operands.pop();
						Operands.push(result);
					}
					if (tempChar == '/')
					{
						result = num1/num2;
						Operands.pop();
						Operands.push(result);
					}
					Operators.push(operators);
				}
							
					/*tempChar = operators;*/
			}

	}
	while (!Operators.empty())
	{
		tempChar = Operators.top();
		if (tempChar == '+')
		{
			num1 = Operands.top();
			Operands.pop();
			num2 = Operands.top();
			result = num1 + num2;
			Operands.pop();
			Operands.push(result);
		}
		else if (tempChar == '-')
		{
			num1 = Operands.top();
			Operands.pop();
			num2 = Operands.top();
			result = num1 - num2;
			Operands.pop();
			Operands.push(result);
		}
		Operators.pop();
	}

	result = Operands.top();
	return result;


}


I have a hunch it has to do with the program trying to access a stack that is already empty somewhere. Nothing seems to be accessing any stack before it attempts to access its top or tries to pop anything empty though, at least nothing I see.

I've also read somewhere on the General C++ Programming forum about someone with a similar problem, and mentioned something about using pointers? Problem is, although I'm experienced with vectors, classes, and so on, I'm not a genius with pointers.

What do you guys think? Am I overlooking something?
Last edited on
JLBorges (13770)
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int evaluate(string expression)
{
	istringstream in;
	in.str(expression);
	stack <int> Operands;
	stack <char> Operators;
// *********************************************************************
	string this_is_terrible = "trying to access top() of an empty stack\n" ;
// *********************************************************************

	char operators;
	int operands;
	int num1;
	int num2;
	int result;
	/*int eqtop = 0;*/
	/*int top;*/
	int tempNum;
	char tempChar;
	//char peek = in.peek();

	while ( in.good() )
	{

		//read operand
		in >> operands;
		if(in.good())
		{
			Operands.push(operands);

		}

	//read in operator
			in >> operators;
			if (Operators.empty())
			{

				if (in.good())
					Operators.push(operators);
			}
			else if (!Operators.empty())
			{
				//in >> operators;
				if (Operators.top() != '*' && Operators.top() != '/')
				{
					if (in.good())
						Operators.push(operators);
				}
				if (Operators.top() == '*' || Operators.top() == '/')
				{
					tempChar = Operators.top();
					Operators.pop();
// *********************************************************************
		            if( Operands.empty() ) std::cerr << this_is_terrible ;
					num1 = Operands.top();
					Operands.pop();
		            if( Operands.empty() ) std::cerr << this_is_terrible ;
					num2 = Operands.top();
// *********************************************************************
					if (tempChar == '*')
					{
						result = num1 * num2;
						Operands.pop();
						Operands.push(result);
					}
					if (tempChar == '/')
					{
						result = num1/num2;
						Operands.pop();
						Operands.push(result);
					}
					Operators.push(operators);
				}

					/*tempChar = operators;*/
			}

	}
	while (!Operators.empty())
	{
		tempChar = Operators.top();
		if (tempChar == '+')
		{
// *********************************************************************
		    if( Operands.empty() ) std::cerr << this_is_terrible ;
			num1 = Operands.top();
			Operands.pop();
		    if( Operands.empty() ) std::cerr << this_is_terrible ;
			num2 = Operands.top();
// *********************************************************************
			result = num1 + num2;
			Operands.pop();
			Operands.push(result);
		}
		else if (tempChar == '-')
		{
			num1 = Operands.top();
			Operands.pop();
			num2 = Operands.top();
			result = num1 - num2;
			Operands.pop();
			Operands.push(result);
		}
		Operators.pop();
	}

	result = Operands.top();
	return result;
}

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