Introduction to Stack with Example Programs in C,Python

Stack is a linear data structure which follows a particular order in which the operations are performed. The order may be LIFO(Last In First Out) or FILO(First In Last Out).
Mainly the following three basic operations are performed in the stack:
  • Push: Adds an item in the stack. If the stack is full, then it is said to be an Overflow condition.
  • Pop: Removes an item from the stack. The items are popped in the reversed order in which they are pushed. If the stack is empty, then it is said to be an Underflow condition.
  • Peek: Get the topmost item.
Introduction to Stack with Example Programs in C,Python
Introduction to Stack with Example Programs in C,Python

How to understand a stack practically?
There are many real life examples of stack. Consider the simple example of books stacked over one another in library. The books which is at the top is the first one to be removed, i.e. the books which has been placed at the bottom most position remains in the stack for the longest period of time. So, it can be simply seen to follow LIFO/FILO order.
Implementation:
There are two ways to implement a stack:
  • Using array
  • Using linked list
Implementing Stack using Arrays in C



// C program for array implementation of stack
#include <stdio.h>
#include <stdlib.h>
#include <limits.h>
// A structure to represent a stack
struct Stack
{
    int top;
    unsigned capacity;
    int* array;
};
// function to create a stack of given capacity. It initializes size of
// stack as 0
struct Stack* createStack(unsigned capacity)
{
    struct Stack* stack = (struct Stack*) malloc(sizeof(struct Stack));
    stack->capacity = capacity;
    stack->top = -1;
    stack->array = (int*) malloc(stack->capacity * sizeof(int));
    return stack;
}
// Stack is full when top is equal to the last index
int isFull(struct Stack* stack)
{   return stack->top == stack->capacity - 1; }
// Stack is empty when top is equal to -1
int isEmpty(struct Stack* stack)
{   return stack->top == -1;  }
// Function to add an item to stack.  It increases top by 1
void push(struct Stack* stack, int item)
{
    if (isFull(stack))
        return;
    stack->array[++stack->top] = item;
    printf("%d pushed to stack\n", item);
}
// Function to remove an item from stack.  It decreases top by 1
int pop(struct Stack* stack)
{
    if (isEmpty(stack))
        return INT_MIN;
    return stack->array[stack->top--];
}
// Function to get top item from stack
int peek(struct Stack* stack)
{
    if (isEmpty(stack))
        return INT_MIN;
    return stack->array[stack->top];
}
// Driver program to test above functions
int main()
{
    struct Stack* stack = createStack(100);
    push(stack, 10);
    push(stack, 20);
    push(stack, 30);
    printf("%d popped from stack\n", pop(stack));
    printf("Top item is %d\n", peek(stack));
    return 0;
}


Implementing Stack using Arrays in Python




# Python program for implementation of stack
# import maxsize from sys module
# Used to return -infinite when stack is empty
from sys import maxsize
# Function to create a stack. It initializes size of stack as 0
def createStack():
    stack = []
    return stack
# Stack is empty when stack size is 0
def isEmpty(stack):
    return len(stack) == 0
# Function to add an item to stack. It increases size by 1
def push(stack, item):
    stack.append(item)
    print("pushed to stack " + item)
     
# Function to remove an item from stack. It decreases size by 1
def pop(stack):
    if (isEmpty(stack)):
        return str(-maxsize -1) #return minus infinite
     
    return stack.pop()
# Function to get top item from stack
def peek(stack):
    if (isEmpty(stack)):
        return str(-maxsize -1)
     
    return stack[len(stack) - 1]
# Driver program to test above functions   
stack = createStack()
push(stack, str(10))
push(stack, str(20))
push(stack, str(30))
print(pop(stack) + " popped from stack")
print("Top item is " + peek(stack))




Implementing Stack using Linked List in C

// C program for linked list implementation of stack
#include <stdio.h>
#include <stdlib.h>
#include <limits.h>
// A structure to represent a stack
struct StackNode
{
    int data;
    struct StackNode* next;
};
struct StackNode* newNode(int data)
{
    struct StackNode* stackNode =
              (struct StackNode*) malloc(sizeof(struct StackNode));
    stackNode->data = data;
    stackNode->next = NULL;
    return stackNode;
}
int isEmpty(struct StackNode *root)
{
    return !root;
}
void push(struct StackNode** root, int data)
{
    struct StackNode* stackNode = newNode(data);
    stackNode->next = *root;
    *root = stackNode;
    printf("%d pushed to stack\n", data);
}
int pop(struct StackNode** root)
{
    if (isEmpty(*root))
        return INT_MIN;
    struct StackNode* temp = *root;
    *root = (*root)->next;
    int popped = temp->data;
    free(temp);
    return popped;
}
int peek(struct StackNode* root)
{
    if (isEmpty(root))
        return INT_MIN;
    return root->data;
}
int main()
{
    struct StackNode* root = NULL;
    push(&root, 10);
    push(&root, 20);
    push(&root, 30);
    printf("%d popped from stack\n", pop(&root));
    printf("Top element is %d\n", peek(root));
    return 0;
}



Implementing Stack using Linked List in python


# Python program for linked list implementation of stack
# Class to represent a node
class StackNode:
    # Constructor to initialize a node
    def __init__(self, data):
        self.data = data
        self.next = None
class Stack:
     
    # Constructor to initialize the root of linked list
    def __init__(self):
        self.root = None
    def isEmpty(self):
        return True if self.root is None else  False
    def push(self, data):
        newNode = StackNode(data)
        newNode.next = self.root
        self.root = newNode
        print "%d pushed to stack" %(data)
     
    def pop(self):
        if (self.isEmpty()):
            return float("-inf")
        temp = self.root
        self.root = self.root.next
        popped = temp.data
        return popped
     
    def peek(self):
        if self.isEmpty():
            return float("-inf")
        return self.root.data
# Driver program to test above class
stack = Stack()
stack.push(10)       
stack.push(20)
stack.push(30)
print "%d popped from stack" %(stack.pop())
print "Top element is %d " %(stack.peek())
# This code is contributed by Nikhil Kumar Singh(nickzuck_007)
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