Thread: Calculate an expression with +/ - using stack

Hi , i need to calculate an expression with +/- only , it will get result once it see '=' sign using stack linked list (for exp: 200-100=) ,result 100. (multi digit needed)



My code is not working , can someone enlighten me ? Thanks a lot

Code:

#include <iostream>
#include <cstdlib>
#include <cstring>
#include <iomanip>
#include <ctime>
using namespace std;


const int MAX =100;
int getResult(char *);


class Stack
{
    public:
        Stack();
        ~Stack();
        void push(int);
        int pop();
		int top ();
		
        bool isEmpty () const;
        


    private:


        struct Node;
        typedef Node* NodePtr ;


        struct Node
        {
            int data;
            NodePtr next;


        };
		
		NodePtr head;


};


Stack::Stack()
{
	head =NULL;	   
}
Stack::~Stack()
{
	//Let the compiler to do it
}
void Stack::push (int item)
{
	NodePtr pNew= new Node;
	pNew -> data = item;
	pNew -> next = head;
	head = pNew;
}


int Stack::pop()
{
	int item = NULL;
	try
	{
		if ( head == NULL)
			throw exception ();
		
		
		NodePtr temp = head;
		head = head ->next;
		
		item = temp ->data;
		
		delete temp;
	}
	catch (exception e)
	{
		cout << " Empty exception caught : Remove head failed "<< endl;
	}
	return item;
}
	


bool Stack::isEmpty () const
{
	return head == NULL;
}


int Stack::top ()
{
	if (!isEmpty())
		return head-> data;
}






int main ()
{
	char * exp = new char[MAX];
	cout <<"Enter an expression" <<endl;
   cin.getline(exp,MAX);
	int result = getResult (exp);
	
	cout <<result <<endl;


}




int getResult(char *exp)
{
	
	int i =0;
	int k,k1,k2;
	int result = 0;
	char *p;
	Stack s;
	p = &exp[0];
	
	// Calculate the result if last element is '=' sign
	while (*p!='=')
	{
		/* To remove spaces and tabs*/
		while (*p ==' ' || *p =='\t')
		{
			p++;
		}
		
		// If is operand
		if (isdigit(*p))
		{
			s.push(*p);
			
		}
		else
		{
		//If is operator +,-
				k1 = s.pop();
				k2 = s.pop();
				
			switch (*p)
			{
				case '+' : 	result =k1 + k2;
							break;
							
				case '-' :  result =k1 - k2;
							break
				
				default : return result;
						  
				
			// push back to stack
			s.push(result);
			}
					
		
		}
		
		p++; // Go to next element
		
	}
	
	
	result= s.pop();
	
	


	return result;
}