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 |
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.
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:
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)