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The Stack Data Structure
The Stack Data Structure
Classic structure
What is a Stack?
An abstract data type in which accesses are made at only one end
Last In First Out (LIFO)Typical Functions
◦ Constructor: set data to valid state◦ Push: add data to TOP of stack◦ Pop: delete data at TOP of stack◦ Peek: view or return data at TOP of stack
Typical Data◦ Size: total size of stack◦ IsEmpty: is there any data?◦ Top: where is the top of the stack?
Linear collection
Why Use a Stack?
Usage◦ Constructor creates an empty stack◦ Call push function to add objects, pop function to remove
Limited-access container◦ Can only add/remove from top of stack◦ Why???
Useful for◦ Reversing a sequence◦ Managing a series of undoable actions◦ Tracking history (web browsing, undo operations)◦ Prevents making mistakes to protected data
The client doesn’t have to remember last push to get it back or delete it.
Animations
Push and Pophttp://www.youtube.com/watch?v=ggogs3P73Ok
http://www.youtube.com/watch?v=A_SobdSCY4Y
Stack Underlying Structure
ArrayLinked List
Stack Interface Using an Array
#include vectortemplate <class Item>class MyStack{public:MyStack();bool isEmpty(); //can use vector’s empty()int size(); //can use vector’s size()void push(Item e);void pop();Item peek();private:vector<Item> elems;
};
Which End of the Array is Top?
Push operations: Beginning of the Array?
Which End of the Array is Top?
Push operations: Beginning of the Array?Possible, but must move any existing data over to
make room for new entries—HARD Push operations: End of the Array?
Which End of the Array is Top?
Push operations: Beginning of the Array?◦Possible, but must move any existing data over
to make room for new entries—HARD Push operations: End of the Array?
Possible and when space is available no shuffling needed—EASY
Pop operations: Beginning of the Array?
Which End of the Array is Top?
Push operations: Beginning of the Array?◦Possible, but must move any existing data over
to make room for new entries—HARD Push operations: End of the Array?
Possible and when space is available no shuffling needed—EASY
Pop operations: Beginning of the Array?◦Possible, but must move any existing data up
to the top—HARDPop operations: End of the Array?
Which End of the Array is Top?
Push operations: Beginning of the Array?◦Possible, but must move any existing data over to make
room for new entries—HARD Push operations: End of the Array?
◦Possible and when space is available no shuffling needed—EASY
Pop operations: Beginning of the Array?◦Possible, but must move any existing data up to the top—
HARDPop operations: End of the Array?
◦Possible and no shuffling is needed when numUsed is tracked—EASY
Which End of the Array is Top?
Push operations: End of the Array!◦Possible and when space is available no
shuffling needed—EASY Pop operations: End of the Array!
◦Possible and no shuffling is needed when numUsed is tracked—EASY
Stack Interface Using a Linked List
template <class Item>class MyStack{
public:MyStack();bool isEmpty();void push(Item e);void pop();Item peek();
private:struct cellT{
Item val;cellT *next;
};cellT *head;
};
Which End of the List is Top?
Push operations: Beginning of the List?
Which End of the List is Top?
Push operations: Beginning of the List?◦We know where the head pointer is—EASY
Push operations: End of the List?
Which End of the List is Top?
Push operations: Beginning of the List?◦We know where the head pointer is—EASY
Push operations: End of the List?◦Possible, but would require traversing the list—
HARD ◦With a tail pointer—Easy
Pop operations: Beginning of the List?
Which End of the List is Top?
Push operations: Beginning of the List?◦We know where the head pointer is—EASY
Push operations: End of the List?◦Possible, but would require traversing the list—
HARD ◦With a tail pointer—Easy
Pop operations: Beginning of the List?◦We know where the head pointer is—Easy
Pop operations: End of the List?
Which End of the List is Top?
Push operations: Beginning of the List?◦We know where the head pointer is—EASY
Push operations: End of the List?◦Possible, but would require traversing the list—HARD ◦With a tail pointer—Easy
Pop operations: Beginning of the List?◦We know where the head pointer is—Easy
Pop operations: End of the List?◦Possible, but would require traversing the list with a
trailing cursor—HARD◦Not made easier with a tail pointer (where is the last
node?) Must traverse the list--HARD
Which End of the List is Top?
Push operations: Beginning of the List!◦We know where the head pointer is—EASY
Pop operations: Beginning of the List!◦We know where the head pointer is—Easy
Client Use of Stack
using namespace std;int main(){MyStack<int> s;s.push(1);s.push(2);s.push(3);while (!isEmpty())cout << s.pop() << endl;return 0;
}
