# What Is Stack Data Structure?

//

Scott Campbell

A stack is a fundamental data structure in computer science that follows the Last-In-First-Out (LIFO) principle. It is a collection of elements where the addition and removal of items occur only at one end, known as the top of the stack. This makes it a highly efficient and easily manageable data structure for certain types of operations.

## Understanding the Stack Data Structure

In a stack, elements are added and removed from the same end, commonly referred to as the top. It resembles a vertical stack of objects, such as plates in a cafeteria or books on a shelf.

Key features of a stack:

• Last-In-First-Out (LIFO) principle: The last element inserted into the stack is the first one to be removed.
• Operations: The two primary operations performed on a stack are push, which adds an element to the top, and pop, which removes and returns the topmost element.
• No random access: Unlike arrays or lists, stacks do not allow direct access to elements at arbitrary positions. Only the top element can be accessed or modified.

## Implementing a Stack

A stack can be implemented using various programming languages. One common approach involves using an array or linked list as the underlying data structure.

### The Array Implementation

An array implementation of a stack allocates a fixed-size array where elements are pushed and popped by manipulating an index variable representing the top position. Here’s an example in C++:

``````#include <iostream>
using namespace std;

const int MAX_SIZE = 100;
int stack[MAX_SIZE];
int top = -1;

void push(int element) {
if (top >= MAX_SIZE - 1) {
cout << "Stack Overflow" << endl;
return;
}
stack[++top] = element;
}

int pop() {
if (top < 0) {
cout << "Stack Underflow" << endl;
return -1;
}
return stack[top--];
}

int main() {
push(10);
push(20);
push(30);

cout << "Popped: " << pop() << endl; // Outputs: Popped: 30
cout << "Popped: " << pop() << endl; // Outputs: Popped: 20

return 0;
}
```
```

A linked list implementation of a stack involves creating a node structure that holds the data and a reference to the next node. The top of the stack is represented by the first node in the linked list.

struct Node {
int data;
Node* next;
};

Node* top = nullptr;

void push(int element) {
Node* newNode = new Node();
newNode->data = element;
newNode->next = top;

top = newNode;
}

int pop() {
if (top == nullptr) {
cout << "Stack Underflow" << endl; return -1; } Node* temp = top; int poppedValue = temp->data;

top = top->next;

delete temp;

return poppedValue;
}

int main() {
push(10);
push(20);
push(30);

cout << "Popped: " << pop() << endl; // Outputs: Popped: 30 cout << "Popped: " << pop() << endl; // Outputs: Popped: 20 return 0; }

## Applications of Stacks

Stacks find applications in various domains, including:

• Function call stack: Stacks are used to keep track of function calls in most programming languages. Each function call is added to the stack, and when a function completes its execution, it is removed from the stack.
• Expression evaluation: Stacks are utilized for evaluating arithmetic expressions, such as converting infix expressions to postfix or prefix form.
• Backtracking algorithms: Stacks are an essential component of backtracking algorithms that involve exploring all possible solutions by maintaining a stack of choices made at each step.

## In Conclusion

A stack is a powerful data structure that follows the Last-In-First-Out (LIFO) principle. It allows efficient addition and removal of elements at one end, making it suitable for numerous applications like function call tracking and expression evaluation.

If you want to dive deeper into data structures, understanding stacks is crucial. With the knowledge gained from this article, you can confidently implement stacks in your programming endeavors.