Thread: Getting garbage...[Queue question]

Input does exactly what it looks like... Some user enters a string of characters and that is then appended char by char to preQ....

This is code from my main, I bolded that cout because it doesn't even get there... It just prints garbage, and it doesn't iterate at all... So there's a problem with retrieve/append I believe... But I'm not sure what.

Code:

void input (Queue preQ)
{
	string expression;
	int i;

	cout<<"Enter a prefix expression: ";
	cin>>expression;

	for (i=0; i<=expression.length(); ++i)
	{
		preQ.append(expression[i]);
	}
}

void pre_to_post (Queue preQ, Queue postQ)
{
	Stack opStack;
	Stack_entry qItem; //queue item
	Stack_entry sItem; //stack item

	do
	{
		preQ.retrieve(qItem.element);
		preQ.serve();
		cout<<qItem.element;
		if(is_operator(qItem.element)==1)
			opStack.push(qItem);
		else
		{
			postQ.append(qItem.element);
			opStack.top(sItem);
			opStack.pop();
			if(sItem.flag==0)
			{
				sItem.flag=1;
				opStack.push(sItem);
			}
			else
			{
				postQ.append(sItem.element);
			}
		}
	}while (!(preQ.empty()));

}

Stack Class:

Code:

// Stack.cpp

#include "Stack.h"

Error_code Stack::push(const Stack_entry &item)
/*Pre: None
  Post: If the Stack is not full, item is added to the top of the Stack.
  If the Stack is empty, an Error_code of underflow is returned.*/
{
	Error_code outcome = success;
	if (myTop >= MAXSTACK-1)
		outcome = overflow;
	else
		entry[myTop++] = item;
	return outcome;
}

Error_code Stack::pop()
/*Pre: None
  Post: If the Stack is not empty, the top of the Stack is removed.
  If the stack is empty, an Error_code of underflow is returned.*/
{
	Error_code outcome = success;
	if (myTop == -1)
		outcome = underflow;
	else --myTop;
	return outcome;
}

Error_code Stack::top(Stack_entry &item) const
/*Pre: None
  Post: If the Stack is not empty, the top of the Stack is returned
  in item. If the Stack is empty an Error_code of underflow is
  returned.*/
{
	Error_code outcome = success;
	if (myTop == -1)
		outcome = underflow;
	else
		item = entry[myTop];
	return outcome;
}

bool Stack::empty() const
/*Pre: None
  Post: If the Stack is empty, true is returned. Otherwise false
  is returned.*/
{
	bool outcome = true;
	if (myTop > -1) outcome = false;
	return outcome;
}

Stack::Stack()
/*Pre: None
  Post: The stack is initialized to be empty.*/
{
	myTop = -1;
}

Queue class:

Code:

// Queue.cpp

#include "Queue.h"

Queue::Queue()
/*Post: The Queue is initialized to be empty. */
{
	count = 0;
	rear = maxqueue -1;
	front = 0;
}

bool Queue::empty() const
/*Post: Return true if the Queue is empty, otherwise
  return false.*/
{
	return count == 0;
}

Error_code Queue::append(const Queue_entry &item)
/*Post: item is added to the rear of the Queue. If
  the Queue is full return an Error_code of overflow 
  and leave the Queue unchanged. */
{
	if (count>=maxqueue) return overflow;
	count++;
	rear = ((rear + 1)== maxqueue)?0:(rear+1);
	entry[rear] = item;
	return success;
}

Error_code Queue::serve()
/*Post: The front of the Queue is removed. If
  the Queue is empty return an Error_code of
  underflow. */
{
	if (count <= 0) return underflow;
	count--;
	front = ((front+1) == maxqueue)?0:(front+1);
	return success;
}

Error_code Queue::retrieve(Queue_entry &item) const
/*Post: The front of the Queue retrieved to
  the output parameter item. If the Queue is
  empty return an Error_code of underflow. */
{
	if (count <= 0) return underflow;
	item = entry[front];
	return success;
}

[edit] - Agh! forgot the header files, sorry!!

Stack.h

Code:

// Stack.h

#ifndef STACK_H
#define STACK_H
#include "Utility.h"

const int MAXSTACK = 10;

struct Stack_entry
{	char element;
	bool flag;
};

class Stack {
public:
	Stack();
	bool empty() const;
	Error_code pop();
	Error_code top(Stack_entry &item) const;
	Error_code push(const Stack_entry &item);

private:
	int myTop; //myTop is -1 when stack is empty
	Stack_entry entry[MAXSTACK];
};
#endif

Queue.h

Code:

// Queue.h

#ifndef QUEUE_H
#define QUEUE_H

#include "Utility.h"

const int maxqueue = 10; // small value for testing

typedef char Queue_entry;

class Queue {
public:
	Queue();
	bool empty() const;
	Error_code serve();
	Error_code append(const Queue_entry &item);
	Error_code retrieve(Queue_entry &item) const;
private:
	int count;
	int front, rear;
	Queue_entry entry[maxqueue];
};
#endif

I've looked it over and over but I can't seem to spot the problem.

You probably want to pass preQ as a reference.

Code:

void input (Queue preQ)
{
	string expression;
	int i;

	cout<<"Enter a prefix expression: ";
	cin>>expression;

	for (i=0; i<=expression.length(); ++i)
	{
		preQ.append(expression[i]);
	}
}

Code:

void input (Queue &preQ)
{
	string expression;
	int i;

	cout<<"Enter a prefix expression: ";
	cin>>expression;

	for (i=0; i<=expression.length(); ++i)
	{
		preQ.append(expression[i]);
	}
}

Originally Posted by Sparrowhawk

Apparently VC++ doesn't like trying to increment myTop while also using that value for its entry. Or something.

You are using post-increment, so it results in entry[-1] = item when myTop == -1. You could use pre-increment if you really wanted, i.e., entry[++myTop] = item.