Python List: Declaring adding searching and removing elements

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List is one of the four data types which are used to store multiple data using a single variable.
For comparison all four such data types are listed here.

...	LIST	Tuple	Set	Dictionary
Mutable	Mutable	Immutable	Mutable	Mutable
index	yes my_list[1]	yes my_tuple[1]	No	Yes my_dict['key']
Duplicate	Yes	Yes	No	No ( key )
Changeable	Yes	No	No	Yes
Example	['a','c','e']	('a','x','y')	{'a','c','f'}	{'a':'one','b':'two','c':'three'}

Declaring a List 🔝

Declaring an empty list

my_list=list()# declare an empty list  using list constructor

my_list=[]    # declare an empty list

my_list=['Alex','Ronald','John']
print(my_list)

Output

['Alex','Ronald','John']

Python data structure List tuple set and dictionary using List methods and functions

Using range to create list 🔝

Create one list using a range of values. It should start from 5 , stops before 50 with increment of 10.

x=range(5,50,10)
my_list=list(x)
print(my_list)

Output

[5, 15, 25, 35, 45]

List by using for loop with condition 🔝

my_list=[ r**2 for r in range(5)] # Square of the numbers 
## List of square of the even numbers 
my_list=[ pow(r,2) for r in range(5) if r%2==0]
print(my_list)

Unpacking and listing elements 🔝

my_list=[5,50,10]
for i in range(*my_list):
  print(i)

Output is

Read how list is used after unpacking to create a range

my_list=['Alex','Ronald','John']
print(*my_list)

Output

Alex Ronald John

Using string to create List 🔝

Using split() we can break a string. By default split() uses space as delimiter to break a string and returns a list.

str="Welcome to Python"
print(str.split())

Output

['Welcome', 'to', 'Python']

Displaying elements based on position 🔝

my_list=['Alex','Ronald','John']
print(my_list[2])

Output is here

John

The position starts from 0 and first element value is my_list[0], so the data at my_list[2] gives us John.

my_list=['Ronald','John','King','Ravi','Alex']
print(my_list[-1]) # last element will be displayed : Alex
print(my_list[-2]) # 2nd from last element will be displayed : Ravi
print(my_list[0:])  # All elements starting from 0 position till end.
print(my_list[:3])  # All elements starting from 0 position till 3.
print(my_list[1:4])  # All elements starting from 1 position till 4.

Output is here

Alex
Ravi
['Ronald', 'John', 'King', 'Ravi', 'Alex']
['Ronald', 'John', 'King']
['John', 'King', 'Ravi']

We can go beyond the last element ( No error here )

print(my_list[1:8])  # All elements starting from 1 position till end.

Output

['John', 'King', 'Ravi', 'Alex']

Displaying all items by looping 🔝

We used for loop to display all items of the list.

my_list=['Alex','Ronald','John']
for i in my_list:
    print(i)

Output is here

Alex
Ronald
John

Methods we can use with list. 🔝

Method	Description
append(x)	Add element x at the end of the list
clear()	Removes all elements of the list
copy()	Returns a copy of the list
count(x)	Count the number of matching input value x present inside list
extend(iterable)	Add one more list ( any iterable ) at the end
index(x)	Returns the position of first matching element x
insert(i,x)	Add element x at given position i
pop([i])	Removes element from given position i
remove(x)	Remove the first matching element x
reverse()	Reverse the sequence of elements
sort()	Sort the list

Searching element using in 🔝

Collecting matching elements by filtering 🔝

Create a list by filtering matching string only.

def my_check(x):
    if x.find('xy') <0: # check for string xy
        return False
    else:
        return True
list_source=['abcd.php','xyabcd','pqrxy','dataxy'] # sample list
list_filter=filter(my_check,list_source) # used filter 
print(list(list_filter))

len(), max() , min() 🔝

len() : Total Number of elements present in a list

my_list=['Alex','Ronald','John']
print("Number of elements:",len(my_list))

Output

Number of elements: 3

max() : Maximum value of elements present in a list
min() : Minimum value of elements present in a list

my_list=[4,2,8,6]
print("Maximum value : ", max(my_list)) 
print("Minimum value : ", min(my_list))

Output

Maximum value :  8
Minimum value :  2

We can get mean by dividing sum of values with number of elements.

print("Mean value : ", sum(my_list)/len(my_list)) # 5.0

We can import statistical module and use mean() in Python 3.4+

import statistics 
print("mean value : ",statistics.mean(my_list)) # 5

High School Python ( part of Syllabus)

Creating a list with distinct elements 🔝

We can use a set to store unique elements as set don't accept duplicate elements.

my_list=[1,2,3,4,4,5,5,6]
print(set(my_list))

Output

{1, 2, 3, 4, 5, 6}

Using this set we can create the list again.

my_list=[2,4,1,2,4,5,3]
my_set=set(my_list)
print(my_set)
my_list=list(my_set)
print(my_list)

Removing duplicate by using for loop

my_list=[1,2,3,4,4,5,5,6]
my_list2=[]
for i in my_list:
    if i not in my_list2:
        my_list2.append(i)
print(my_list2)

Output

[1, 2, 3, 4, 5, 6]

List of duplicate elements in a list

a = [1,2,3,2,1,5,6,5,5,5]
import collections
print([item for item, count in collections.Counter(a).items() if count > 1])
## [1, 2, 5]

Getting unique elements and counting the occurrence by using Counter.

Frequency of elements in a list 🔝

Counting the frequency of elements in a list using a dictionary.

my_list=['a','z','c','z','z','c','b','a']
my_dict={}
for i in my_list:
  if i in my_dict:
    my_dict[i] = my_dict[i] + 1
  else:
    my_dict[i]  = 1
print(my_dict)

Output

{'a': 2, 'z': 3, 'c': 2, 'b': 1}

High School Python ( part of Syllabus)

Sum of elements of a list 🔝

As list is an iterator Object , we can use sum to calculate sum of all elements.

my_list=[1,2,5,6]
my_list_sum=sum(my_list)
print(my_list_sum)
print("Sum by looping")
my_sum=0
for i in my_list:
    my_sum=my_sum+i
print(my_sum)

Output

14
Sum by looping
14

Common elements between two Lists 🔝

list1=[3,2,5,8]
list2=[5,2,8,9]
print(set(list1) & set(list2))

Output

{8, 2, 5}

Difference between two lists 🔝

l1=[5,10,20,25,15,10,5]
l2=[20,10]
l3=list(set(l1)-set(l2)) # removes the duplicates 

#Checks all elements by looping and without removing duplicates
#l3=[i for i in l1 + l2 if i not in l1 or i not in l2]

print(l3)

Highest and lowest Common elements

print(max(set(list1) & set(list2)))
print(min(set(list1) & set(list2)))

Output

8
2

From List to String 🔝

We can create a string by using all elements of a list. While creating string if any integer ( or other type of data ) is there then we will get an error message. So we used map() to convert all elements to string object before using join() to join them.

my_list = ['One',2,'Three']       
my_str=",".join(map(str,my_list))
print(my_str)

map() to apply function or lambda to each element of a list

Creating a list by using data from JSON file 🔝

Here is the sample JSON file.

import json
path="D:\\my_data\\student.json" # use your sample file. 
fob=open(path,)
data=json.load(fob)
names=[]
for student in data:
    #print(student['name'])
    names.append(student['name'])
print(names)
fob.close()

Using one line for loop in above code.

names=[r['name'] for r in data]

Random element from List 🔝

import random
my_list=['Alex','Ron','Ravi','Geek','Rbindra']
random_element = random.choice(my_list)
print(random_element)

2D List 🔝

Create one List using all the 2nd item of each inner List

l1=[['abc','def','ghi','jkl'],
    ['mno','pkr','frt','qwr'],
    ['asd','air','abc','zpq'],
    ['zae','vbg','qir','zab']]
my_name=[r[1] for r in l1]
print(my_name)# Output is : ['def', 'pkr', 'air', 'vbg']

Searching for matching string inside 2D list 🔝


l1=[['abc','def','ghi','jkl'],
    ['mno','pkr','frt','qwr'],
    ['asd','air','abc','zpq'],
    ['zae','vbg','qir','zab']]
for r in l1:
    if 'pkr' in r:
        print(r)

Output

['mno', 'pkr', 'frt', 'qwr']

We will get same output by using this one-liner

my_row = next((r for r in l1 if 'pkr' in r), None)
print(my_row)

Getting a list from data returned from database table 🔝

This code connects to a SQLite database using SQLAlchemy, retrieves all table names from the database schema, and stores them in a Python list. It uses the SQL query SELECT name FROM sqlite_master WHERE type = 'table' to fetch the names of all tables. A list comprehension is employed to extract each table name from the query result and organize it into a list, which is then printed.

from sqlalchemy import create_engine, text

# Assuming my_conn is a connection to your database
# Replace 'sqlite:///your_database.db' with your actual database connection string
engine = create_engine('sqlite:///your_database.db')
my_conn = engine.connect()

# Query to fetch table names from SQLite database
r_set = my_conn.execute(text("SELECT name FROM sqlite_master WHERE type = 'table'"))

# Create a list from the query result
table_names = [row[0] for row in r_set]

# Output the list
print(table_names)

Questions 🔝

What is a list in Python?
How do you add an item to the end of a list?
What is the difference between the append() and extend() methods in lists?
How can you remove items from a list in Python?
Explain how to sort a list in Python. What methods are available?
How do you reverse a list in Python?
What is slicing in Python lists? Provide an example of how to slice a list.
How would you access the last element of a list?
Can a Python list contain elements of different data types?
What is the difference between shallow copy and deep copy in the context of Python lists?
Explain the use of the del keyword with Python lists.
How do you find the index of an item in a Python list?
What happens if you try to access an index that is out of range in a Python list?
How can you concatenate two or more lists in Python?
What is list comprehension and provide an example of its usage?
How does the remove() method work, and what happens if the element does not exist in the list?
What is the purpose of the pop() method in Python lists?
Explain how to use a loop to iterate through all items in a list.
What does the count() method do in a list?
How can you convert a list into a tuple?

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Subhendu Mohapatra

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LIST in Python

Declaring a List 🔝

Using range to create list 🔝

List by using for loop with condition 🔝

Unpacking and listing elements 🔝

Using string to create List 🔝

Displaying elements based on position 🔝

Displaying all items by looping 🔝

Methods we can use with list. 🔝

Searching element using in 🔝

Collecting matching elements by filtering 🔝

len(), max() , min() 🔝

Creating a list with distinct elements 🔝

Frequency of elements in a list 🔝

Sum of elements of a list 🔝

Common elements between two Lists 🔝

Difference between two lists 🔝

From List to String 🔝

Creating a list by using data from JSON file 🔝

Random element from List 🔝

2D List 🔝

Searching for matching string inside 2D list 🔝

Getting a list from data returned from database table 🔝

Questions 🔝

Subhendu Mohapatra