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Reverse printing on a stack

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Join Date: Aug 2008

Posts: 55

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RayvenHawk RayvenHawk is offline

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Reverse printing on a stack

Aug 30th, 2008

I've finished writing the program for my class and everything actually does what it's suppose to (well 99% anyway).

I'm going to add snippets of the code that should help figure out the small glitch. What's happening is I can add x amount of elements to my stack and display them, but when they are displayed they are in reverse order instead of from bottom to top (the order they were input into the stack).

 Help with Code Tags 
 C++ Syntax (Toggle Plain Text) 
template<class Type>
Type mystack<Type>::top() const
{
	assert(stackTop != 0);
	return list[stackTop - 1];
}
template<class Type>
Type mystack<Type>::top() const
{
	assert(stackTop != 0);
	return list[stackTop - 1];
}

the above is the command to display the elements

 Help with Code Tags 
 C++ Syntax (Toggle Plain Text) 
template<class Type>
void mystack<Type>::push(const Type& newItem)
{
	if(!isFullStack())
	{
		list[stackTop] = newItem;
		stackTop++;
	}
	else
		cout <<"Cannot add to a full stack."<<endl;
}
template<class Type>
void mystack<Type>::push(const Type& newItem)
{
	if(!isFullStack())
	{
		list[stackTop] = newItem;
		stackTop++;
	}
	else
		cout <<"Cannot add to a full stack."<<endl;
}

the above is the code to push the new element into the stack

 Help with Code Tags 
 C++ Syntax (Toggle Plain Text) 
if (choice == 1)
		{
			do
			{
				cin >> num;
				intStack.push(num);
				j++;
			}
			while(j < amount);
			workStack = intStack;
		}
if (choice == 1)
		{
			do
			{
				cin >> num;
				intStack.push(num);
				j++;
			}
			while(j < amount);
			workStack = intStack;
		}

the above is the main function to call the push command. (int j is initialized to 0)

Any reason why my stack is displaying backwards??

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Join Date: May 2008

Posts: 351

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Radical Edward Radical Edward is offline

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Re: Reverse printing on a stack

Aug 30th, 2008

•

Originally Posted by RayvenHawk

Any reason why my stack is displaying backwards??

That's how a stack works, it's a "last-in, first-out" structure. The item at the top will always be the last item that was pushed:

 Help with Code Tags 
 C++ Syntax (Toggle Plain Text) 
#include <iostream>
#include <stack>
 
int main()
{
  using namespace std;
 
  stack<int> st;
 
  for (int i = 0; i < 5; ++i)
    st.push(i);
 
  // Display backward
  while (!st.empty()) {
    cout << st.top() << '\n';
    st.pop();
  }
}
#include <iostream>
#include <stack>

int main()
{
  using namespace std;

  stack<int> st;

  for (int i = 0; i < 5; ++i)
    st.push(i);

  // Display backward
  while (!st.empty()) {
    cout << st.top() << '\n';
    st.pop();
  }
}

To get the items in the order they were pushed you need a queue:

 Help with Code Tags 
 C++ Syntax (Toggle Plain Text) 
#include <iostream>
#include <queue>
 
int main()
{
  using namespace std;
 
  queue<int> q;
 
  for (int i = 0; i < 5; ++i)
    q.push(i);
 
  // Display forward
  while (!q.empty()) {
    cout << q.front() << '\n';
    q.pop();
  }
}
#include <iostream>
#include <queue>

int main()
{
  using namespace std;

  queue<int> q;

  for (int i = 0; i < 5; ++i)
    q.push(i);

  // Display forward
  while (!q.empty()) {
    cout << q.front() << '\n';
    q.pop();
  }
}

Or you can figure out a way to reverse the stack. One way is to use two stacks:

 Help with Code Tags 
 C++ Syntax (Toggle Plain Text) 
#include <iostream>
#include <stack>
 
int main()
{
  using namespace std;
 
  stack<int> st;
 
  for (int i = 0; i < 5; ++i)
    st.push(i);
 
  stack<int> temp;
 
  // Display forward
  while (!st.empty()) {
    temp.push(st.top());
    st.pop();
  }
 
  while (!temp.empty()) {
    cout << temp.top() << '\n';
    temp.pop();
  }
}
#include <iostream>
#include <stack>

int main()
{
  using namespace std;

  stack<int> st;

  for (int i = 0; i < 5; ++i)
    st.push(i);

  stack<int> temp;

  // Display forward
  while (!st.empty()) {
    temp.push(st.top());
    st.pop();
  }

  while (!temp.empty()) {
    cout << temp.top() << '\n';
    temp.pop();
  }
}

Each item is popped from the reversed stack into the temporary stack. Reversing a reversed list gives you the forward list:

 Help with Code Tags 
 C++ Syntax (Toggle Plain Text) 
st:
 
|4|
|3| |3|
|2| |2| |2| 
|1| |1| |1| |1|
|0| |0| |0| |0| |0|
 
temp:
 
                |0|
            |1| |1|
        |2| |2| |2|
    |3| |3| |3| |3|
|4| |4| |4| |4| |4|
st:

|4|
|3| |3|
|2| |2| |2| 
|1| |1| |1| |1|
|0| |0| |0| |0| |0|

temp:

                |0|
            |1| |1|
        |2| |2| |2|
    |3| |3| |3| |3|
|4| |4| |4| |4| |4|

Another way is to display the stack using recursion:

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 C++ Syntax (Toggle Plain Text) 
#include <iostream>
#include <stack>
 
namespace EdRules {
  using namespace std;
 
  void DisplayForward(stack<int> st)
  {
    if (st.empty())
      return;
 
    int temp = st.top();
 
    st.pop();
    DisplayForward(st);
 
    cout << temp << '\n';
  }
}
 
int main()
{
  using namespace std;
 
  stack<int> st;
 
  for (int i = 0; i < 5; ++i)
    st.push(i);
 
  // Display forward
  EdRules::DisplayForward(st);
}
#include <iostream>
#include <stack>

namespace EdRules {
  using namespace std;

  void DisplayForward(stack<int> st)
  {
    if (st.empty())
      return;

    int temp = st.top();

    st.pop();
    DisplayForward(st);

    cout << temp << '\n';
  }
}

int main()
{
  using namespace std;

  stack<int> st;

  for (int i = 0; i < 5; ++i)
    st.push(i);

  // Display forward
  EdRules::DisplayForward(st);
}

This is exactly the same thing as using two stacks because recursion takes the place of the temporary stack.