# Stack Data Structure In this tutorial, you learn about the stack data structure and its implementation in Python, Java, and C/C++.

Contents

## What is Stack Data Structure?

The stack is an abstract data type with a bounded(predefined) capacity. It is a simple data structure that allows adding and eliminating elements in a specific request. Each time an element is added, it goes on the top of the stack and the only element that can be removed is the element that is at the top of the stack, much the same as a pile of objects.

A stack is a valuable data structure in programming. It is much the same as a heap of plates kept on top of each other.

Consider the things you can do with such a heap of plates

• Put a new plate on top
• Remove the top plate

On the off chance that you need the plate at the bottom, you should initially eliminate all the plates on top. Such a course of action is called Last In First Out – the last item that is the primary item to go out.

## LIFO Principle of Stack

In programming terms, putting an item on top of the stack is called a push, and eliminating an item is called pop.

In the above picture, albeit item 2 was kept last, it was eliminated first – so it follows the Last In First Out(LIFO) standard.

We can execute a stack in any programming language like C, C++, Java, Python, or C#, yet the determination is essentially the equivalent.

## Basic Operations of Stack

A stack is an object (an abstract data type – ADT) that allows the accompanying tasks:

• Push: Add an element to the top of a stack
• Pop: Remove an element from the top of a stack
• IsEmpty: Check if the stack is vacant
• IsFull: Check if the stack is full
• Peek: Get the value of the top element without eliminating it

## Working of Stack Data Structure

The operation works in as follows:

1. A pointer called TOP is used to monitor the top element in the stack.
2. While instating the stack, we set its value to – 1 so we can check if the stack is vacant by comparing TOP == – 1.
3. On pushing an element, we increase the value of TOP and spot the new element in the position highlighted by TOP.
4. On popping an element, we return the element highlighted by TOP and reduce its value.
5. before pushing, we check if the stack is already full
6. before popping, we check if the stack is already vacant

## Stack Implementations in Python, Java, C, and C++

The most common stack implementation is using arrays, but it can also be implemented using lists.

Python

```# Stack implementation in python

# Creating a stack
def create_stack():
stack = []
return stack

# Creating an empty stack
def check_empty(stack):
return len(stack) == 0

# Adding items into the stack
def push(stack, item):
stack.append(item)
print("pushed item: " + item)

# Removing an element from the stack
def pop(stack):
if (check_empty(stack)):
return "stack is empty"

return stack.pop()

stack = create_stack()
push(stack, str(1))
push(stack, str(2))
push(stack, str(3))
push(stack, str(4))
print("popped item: " + pop(stack))
print("stack after popping an element: " + str(stack))
```

Java

```// Stack implementation in Java

class Stack {
private int arr[];
private int top;
private int capacity;

// Creating a stack
Stack(int size) {
arr = new int[size];
capacity = size;
top = -1;
}

public void push(int x) {
if (isFull()) {
System.out.println("OverFlow\nProgram Terminated\n");
System.exit(1);
}

System.out.println("Inserting " + x);
arr[++top] = x;
}

// Remove element from stack
public int pop() {
if (isEmpty()) {
System.out.println("STACK EMPTY");
System.exit(1);
}
return arr[top--];
}

// Utility function to return the size of the stack
public int size() {
}

// Check if the stack is empty
public Boolean isEmpty() {
}

// Check if the stack is full
public Boolean isFull() {
}

public void printStack() {
for (int i = 0; i <= top; i++) {
System.out.println(arr[i]);
}
}

public static void main(String[] args) {
Stack stack = new Stack(5);

stack.push(1);
stack.push(2);
stack.push(3);
stack.push(4);

stack.pop();
System.out.println("\nAfter popping out");

stack.printStack();

}
}
```

C

```// Stack implementation in C

#include <stdio.h>
#include <stdlib.h>

#define MAX 10

int count = 0;

// Creating a stack
struct stack {
int items[MAX];
int top;
};
typedef struct stack st;

void createEmptyStack(st *s) {
s->top = -1;
}

// Check if the stack is full
int isfull(st *s) {
if (s->top == MAX - 1)
return 1;
else
return 0;
}

// Check if the stack is empty
int isempty(st *s) {
if (s->top == -1)
return 1;
else
return 0;
}

void push(st *s, int newitem) {
if (isfull(s)) {
printf("STACK FULL");
} else {
s->top++;
s->items[s->top] = newitem;
}
count++;
}

// Remove element from stack
void pop(st *s) {
if (isempty(s)) {
printf("\n STACK EMPTY \n");
} else {
printf("Item popped= %d", s->items[s->top]);
s->top--;
}
count--;
printf("\n");
}

// Print elements of stack
void printStack(st *s) {
printf("Stack: ");
for (int i = 0; i < count; i++) {
printf("%d ", s->items[i]);
}
printf("\n");
}

// Driver code
int main() {
int ch;
st *s = (st *)malloc(sizeof(st));

createEmptyStack(s);

push(s, 1);
push(s, 2);
push(s, 3);
push(s, 4);

printStack(s);

pop(s);

printf("\nAfter popping out\n");
printStack(s);
}```

C++

```// Stack implementation in C++

#include <stdlib.h>
#include <iostream>

using namespace std;

#define MAX 10
int size = 0;

// Creating a stack
struct stack {
int items[MAX];
int top;
};
typedef struct stack st;

void createEmptyStack(st *s) {
s->top = -1;
}

// Check if the stack is full
int isfull(st *s) {
if (s->top == MAX - 1)
return 1;
else
return 0;
}

// Check if the stack is empty
int isempty(st *s) {
if (s->top == -1)
return 1;
else
return 0;
}

void push(st *s, int newitem) {
if (isfull(s)) {
printf("STACK FULL");
} else {
s->top++;
s->items[s->top] = newitem;
}
size++;
}

// Remove element from stack
void pop(st *s) {
if (isempty(s)) {
printf("\n STACK EMPTY \n");
} else {
printf("Item popped= %d", s->items[s->top]);
s->top--;
}
size--;
cout << endl;
}

// Print elements of stack
void printStack(st *s) {
printf("Stack: ");
for (int i = 0; i < size; i++) {
cout << s->items[i] << " ";
}
cout << endl;
}

// Driver code
int main() {
int ch;
st *s = (st *)malloc(sizeof(st));

createEmptyStack(s);

push(s, 1);
push(s, 2);
push(s, 3);
push(s, 4);

printStack(s);

pop(s);

cout << "\nAfter popping out\n";
printStack(s);
}```

## Stack Time Complexity

For the array-based implementation of a stack, the push and pop operations take constant time, i.e. O(1).

## Applications of Stack Data Structure

Although stack is a simple data structure to actualize, it is extremely amazing. The most common uses of a stack are:

• To reverse a word – Put all the letters in a stack and pop them out. In view of the LIFO request of the stack, you will get the letters backward request.
• In compilers – Compilers use the stack to calculate the value of expressions like 2 + 4/5 * (7 – 9) by changing the expression to prefix or postfix structure.
• In browsers – The back catch in a browsers saves all the URLs you have visited previously in a stack. Each time you visit another page, it is added on top of the stack. At the point when you press the back catch, the current URL is removed from the stack, and the previous URL is accessed.

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