# Stack in C++

## What is Stack?

Stack is a data structure that follows LIFO(last in first out) principle, Stack can be visualized as a pile of books, when we pile a stack of books in a box we only have access to the topmost book if we have to remove a book or pick a book we will only be able to access the topmost book.

### Why is Stack useful?

Stack has quite a few advantages as a data structure some of them are the following:-

* **Arithmetic operations:** Stack is very useful in performing arithmetic operations such as in evaluating mathematical expressions and postfix operations.
    
* **LIFO operation:** Stack is especially helpful when we need to process items in reverse or while using a backtracking algorithm or undoing any task.
    
* **Simplicity:** Stack is a fairly simple data structure with few operations like push, pop, size, and top which makes it easy to use.
    
* **Memory management:** Stacks use a contiguous block of memory, which allows for efficient memory management. The allocation and deallocation of memory for stack elements follow a simple and predictable pattern, resulting in minimal overhead.
    
* **Recursion:** Stacks are essential for handling recursive function calls. Recursive algorithms rely on the stack to keep track of function calls and their associated data. As each recursive call is made, a new stack frame is created, and when the recursion reaches the base case, the stack frames are gradually popped, allowing the algorithm to return results in the correct order.
    

### Syntax of Stack functions:

1. **Push:** push method is used to add elements in the stack, elements are also added in the LIFO manner.
    
    **Syntax:** stack name.push(item);
    
2. **Pop:** pop method removes the element from the top of the stack if present.
    
    **Syntax:** stackName.pop();
    
3. **Top:** This method returns the top element from the stack if present.
    
    **Syntax:** [stackName.top](http://stackName.top)();
    
4. Empty: This method tells us whether the stack is empty or not.
    
    **Syntax:** stackEmpty();
    
5. Size: This method is used to find the number of elements present in the stack.
    
    **Syntax:** stackName.size();
    

### Example:

```cpp
#include "iostream"
#include "stack"

using namespace std;

int main() {

    stack<char>st;

    // PUSH
    cout << "Pushing a and b into the stack" << endl;
    st.push('a');
    st.push('b');

    // TOP
    cout << "The top element is: " << st.top() << endl;

    // POP
    cout << "Popping an element" << endl;
    st.pop();
    cout << "The top element is: " << st.top() << endl;

    // EMPTY
    cout << "Checking if the stack is empty: ";
    cout << st.empty() << endl;

    // SIZE
    cout << "Size of the stack: " << st.size() << endl;

    cout << "Popping another element" << endl;
    st.pop();

    cout << "Size of the stack: " << st.size() << endl;
    cout << "Checking if the stack is empty: ";
    cout << st.empty() << endl;
    return 0;
}

```

### Output:

```cpp
Pushing a and b into the stack
The top element is: b
Popping an element
The top element is: a
Checking if the stack is empty: 0
Size of the stack: 1
Popping another element
Size of the stack: 0
Checking if the stack is empty: 1
```