How to print middle element in list Python

This is a Python program to print the middle node(s) of a linked list.

The program creates a linked list using data items input from the user and prints the middle node(s) of the linked list.

1. Create a class Node with instance variables data and next. 2. Create a class LinkedList with instance variables head and last_node. 3. The variable head points to the first element in the linked list while last_node points to the last. 4. Define methods append and display inside the class LinkedList to append data and display the linked list respectively. 5. Define a function print_middle that prints the middle element(s) of the list. 6. The function print_middle iterates through the list to find its length and then iterates again to half its length. If the length of the list is even, it prints two middle elements.

7. Create an instance of LinkedList, append data to it and print its middle element(s).

Here is the source code of a Python program to print the middle element(s) of a linked list.

class Node: def __init__(self, data): self.data = data self.next = None class LinkedList: def __init__(self): self.head = None self.last_node = None def append(self, data): if self.last_node is None: self.head = Node(data) self.last_node = self.head else: self.last_node.next = Node(data) self.last_node = self.last_node.next def print_middle(llist): current = llist.head length = 0 while current: current = current.next length = length + 1 current = llist.head for i in range((length - 1)//2): current = current.next if current: if length % 2 == 0: print('The two middle elements are {} and {}.' .format(current.data, current.next.data)) else: print('The middle element is {}.'.format(current.data)) else: print('The list is empty.') a_llist = LinkedList() data_list = input('Please enter the elements in the linked list: ').split() for data in data_list: a_llist.append(int(data)) print_middle(a_llist)

1. An instances of LinkedList is created. 2. The user is prompted to enter the data items for the list.

3. The function print_middle is called to print the middle element(s) of the list.

Case 1: Please enter the elements in the linked list: 1 2 3 4 5 6 7 8 The two middle elements are 4 and 5. Case 2: Please enter the elements in the linked list: 5 The middle element is 5. Case 3: Please enter the elements in the linked list: 3 1 0 4 2 The middle element is 0.

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middle value of a list in python

python by Yucky Yak on Dec 09 2020 Comment

Source: stackoverflow.com

middle value of a list in python

python by Yucky Yak on Dec 09 2020 Comment

Source: stackoverflow.com

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by · Published February 6, 2021 · Updated October 25, 2021

If you want to find the middle element from a list then we can use following code.

My_List=[1,2,3,4,5,6,7] Middle_index=int((len(My_List)/2)) print(My_List[Middle_index]) #4

First, we will find the middle index of the list by dividing the length of the list by 2 then by the middle index, we can access a middle element of the list. But what if the length of the list is even. For example, if the length is odd then we can easily decide the middle number i.e. if there are 5 items then 2 items will be on the left side and 2 will be on the right side and 1 will be on the middle.

if the length of the list is even then we pick the first element of the second half i.e. if there are 4 items(even number ) then the first item of the second half will be item 3. Look at the following example.

My_List=[1,2,3,4,5,6] Middle_index=int((len(My_List)/2)) print(My_List[Middle_index]) #4

1,2,3 is the first half and 4,5,6 is the second half part so according to the above code 4(first element of the second half) will be the middle element of the list.

Element-wise list operations in python:
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Extended List slicing in python:
check if a number is even or odd in python:
Python List:
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python program to find the area of a triangle:
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Given a singly linked list, find the middle of the linked list. For example, if the given linked list is 1->2->3->4->5 then the output should be 3.
If there are even nodes, then there would be two middle nodes, we need to print the second middle element. For example, if given linked list is 1->2->3->4->5->6 then the output should be 4.

Recommended: Please solve it on “PRACTICE” first, before moving on to the solution.

Method 1:
Traverse the whole linked list and count the no. of nodes. Now traverse the list again till count/2 and return the node at count/2.

Method 2:
Traverse linked list using two pointers. Move one pointer by one and the other pointers by two. When the fast pointer reaches the end slow pointer will reach the middle of the linked list.

Below image shows how printMiddle function works in the code :

Python3

# Python3 program to find middle of 
# the linked list 
# Node class 

class Node:

# Function to initialise the

# node object

def __init__(self, data):

# Assign data

self.data = data

# Initialize next as null

self.next = None
# Linked List class contains a 
# Node object 

class LinkedList:

# Function to initialize head

def __init__(self):

self.head = None

# Function to insert a new node at

# the beginning

def push(self, new_data):

new_node = Node(new_data)

new_node.next = self.head

self.head = new_node

# Print the linked list

def printList(self):

node = self.head

while node:

print(str(node.data) +

"->", end = "")

node = node.next

print("NULL")

# Function that returns middle.

def printMiddle(self):

# Initialize two pointers, one will go

# one step a time (slow), another two

# at a time (fast)

slow = self.head

fast = self.head

# Iterate till fast's next is null (fast

# reaches end)

while fast and fast.next:

slow = slow.next

fast = fast.next.next

# Return the slow's data, which would be

# the middle element.

print("The middle element is ", slow.data)
# Driver code 

if __name__=='__main__':

# Start with the empty list

llist = LinkedList()

for i in range(5, 0, -1):

llist.push(i)

llist.printList()

llist.printMiddle()
# This code is contributed by Kumar Shivam (kshivi99)

Output:

5->NULL The middle element is [5] 4->5->NULL The middle element is [5] 3->4->5->NULL The middle element is [4] 2->3->4->5->NULL The middle element is [4] 1->2->3->4->5->NULL The middle element is [3]

Method 3:Initialize mid element as head and initialize a counter as 0. Traverse the list from head, while traversing increment the counter and change mid to mid->next whenever the counter is odd. So the mid will move only half of the total length of the list.

Thanks to Narendra Kangralkar for suggesting this method.

Python3

# Python program to implement 
# the above approach 
# Node class 

class Node:

# Function to initialise the

# node object

def __init__(self, data):

# Assign data

self.data = data

# Initialize next as null

self.next = None
# Linked List class contains a 
# Node object 

class LinkedList:

# Function to initialize head

def __init__(self):

self.head = None

# Function to insert a new node at

# the beginning

def push(self, new_data):

new_node = Node(new_data)

new_node.next = self.head

self.head = new_node

# Print the linked list

def printList(self):

node = self.head

while node:

print(str(node.data) +

"->", end = "")

node = node.next

print("NULL")

# Function to get the middle of

# the linked list

def printMiddle(self):

count = 0

mid = self.head

heads = self.head

while(heads != None):

# Update mid, when 'count'

# is odd number

if count & 1:

mid = mid.next

count += 1

heads = heads.next

# If empty list is provided

if mid != None:

print("The middle element is ",

mid.data)
# Driver code 

if __name__=='__main__':

# Start with the empty list

llist = LinkedList()

for i in range(5, 0, -1):

llist.push(i)

llist.printList()

llist.printMiddle()

# This code is contributed by Manisha_Ediga

Output:

5->NULL The middle element is [5] 4->5->NULL The middle element is [5] 3->4->5->NULL The middle element is [4] 2->3->4->5->NULL The middle element is [4] 1->2->3->4->5->NULL The middle element is [3]

Please refer complete article on Find the middle of a given linked list for more details!

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