A stack is a last-in, first-out (LIFO) data structure where elements can only be added or removed from one end of the structure, called the top. The std::stack class in C++ uses an underlying container like vector or deque to store elements. It provides functions like push() to add elements, pop() to remove elements, and top() to access the top element. Example code shows how to use a stack to store integers, display the size, insert and remove elements, and check the top element.

1. STL : Stack
Prepared by
Dr.S.Raja Ratna
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STL Stack
● A stack is a data structure that operates based on LIFO (Last In First
Out) technique. The std::stack allows elements to be added and
removed from one end only.
● The std::stack class is a container adapter. Container objects hold data
of a similar data type. You can create a stack from various sequence
containers.
● Stacks are a type of container adaptors with LIFO type of working..
● Stack uses an encapsulated object of either vector or deque as its
underlying container, providing a specific set of member functions to
access its elements.

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Stack Syntax:-
template <class Type, class Container = deque<Type> class stack;
For creating a stack, we must include the <stack> header file in our code.
We then use this syntax to define the std::stack:
Type – is the Type of element contained in the std::stack. It can be any valid
C++ type or even a user-defined type.
Container – is the Type of underlying container object.
Member Types:-
value_type- The first template parameter, T. It denotes the element types.
container_type- The second template parameter, Container. It denotes the
underlying container type.
size_type- Unsigned integral type.

4. The functions associated with stack are:
Functions associated with stack
1. empty() – Returns whether the stack is empty – Time Complexity : O(1)
2. size() – Returns the size of the stack – Time Complexity : O(1)
3. top() – Returns a reference to the top most element of the stack – Time
Complexity : O(1)
4. push(g) – Adds the element ‘g’ at the top of the stack – Time
Complexity : O(1)
5. pop() – Deletes the top most element of the stack – Time Complexity :
O(1)
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5. Operations in Stack
A C++ stack supports the following basic operations:
● push – It adds/pushes an item into the stack.
● pop – It removes/pops an item from the stack.
● peek – Returns the top item of the stack without removing it.
● isFull – Checks whether a stack is full.
● isEmpty – Checks whether a stack is empty.
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6. #include <iostream>
#include <stack>
using namespace std;
int main() {
stack<int> stack;
stack.push(21);
stack.push(22);
stack.push(24);
stack.push(25);
stack.pop();
stack.pop();
while (!stack.empty()) {
cout << stack.top() <<" ";
stack.pop();
} 6

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● Algorithm
● Begin
● Declare stack vector.
● Take the input as per choice.
● Call the functions within switch operation:
● s.size() = Returns the size of stack.
● s.push() = It is used to insert elements to the stack.
● s.pop() = To pop out the value from the stack.
● s.top() = Returns a reference to the top most element of stack.
● End.

8. Example Code
#include <iostream>
#include <stack>
#include <string>
#include <cstdlib>
using namespace std;
int main() {
stack<int> s;
int c, i;
while (1) {
cout<<"1.Size of the Stack"<<endl;
cout<<"2.Insert Element into the Stack"<<endl;
cout<<"3.Delete Element from the Stack"<<endl;
cout<<"4.Top Element of the Stack"<<endl;
cout<<"5.Exit"<<endl;
cout<<"Enter your Choice: ";
cin>>c;
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9. switch(c) {
case 1:
cout<<"Size of the stack: ";
cout<<s.size()<<endl;
break;
case 2:
cout<<"Enter value to be inserted: ";
cin>>i;
s.push(i);
break;
case 3:
i = s.top();
if (!s.empty()) {
s.pop();
cout<<i<<" Deleted"<<endl;
}else {
cout<<"Stack is Empty"<<endl;
}
break;
case 4:
cout<<"Top Element of the Stack: ";
cout<<s.top()<<endl;
break;
case 5:
exit(1);
break;
default:
cout<<"Wrong Choice"<<endl;
}
}
return 0;
}
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10. ● Output
● 1.Size of the Stack
● 2.Insert Element into the Stack
● 3.Delete Element from the Stack
● 4.Top Element of the Stack
● 5.Exit
● Enter your Choice: 1
● Size of the stack: 0
● 1.Size of the Stack
● 2.Insert Element into the Stack
● 3.Delete Element from the Stack
● 4.Top Element of the Stack
● 5.Exit
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11. Enter your Choice: 2
Enter value to be inserted: 1
1.Size of the Stack
2.Insert Element into the Stack
3.Delete Element from the Stack
4.Top Element of the Stack
5.Exit
Enter your Choice: 2
Enter value to be inserted: 7
1.Size of the Stack
2.Insert Element into the Stack
3.Delete Element from the Stack
4.Top Element of the Stack
5.Exit
Enter your Choice: 2
Enter value to be inserted: 6
1.Size of the Stack
2.Insert Element into the Stack
3.Delete Element from the Stack
4.Top Element of the Stack
5.Exit
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