Python List

Python Basics Python Introduction Python Installation Overview of Jupyter IDE Identifiers & Reserved Keywords Python Variables Python Numbers Python Operators Python Operators and Arithmetic Operators Comparison and Logical Operators Assignment and Bitwise Operators Identity and Membership Operators Python Flow Control if else if else statement While Loop Statement Python For Loop Break and Continue Statement Python Data Types Python Strings Python Strings Methods Python Lists Python Tuples Python Dictionary Python Functions Introduction to Python Functions Function Arguments Recursion Function Lambda/Anonymous Function Python - Modules Python Files Python - Files I/O Python - Exceptions Handling Python - Debugging

What is Lists ?

  • A list is a collection which is ordered and changeable.
  • Lists are enclosed in [ ]
  • Each item in a list is separated by a comma
  • Lists are collection of items (Strings, integers or even other lists)
  • Each item in the list has an assigned index value.
  • Lists are MUTABLE, which means they can be changed.
    •        Eg: list1 = [1, 2, 3, 4, 5]

    Creating list in python

    # Create an empty list
list1  = [] 
print(list1)

#list of integers
list2 = [10, 20, 30, 40, 50] 
print(list2)

# list of strings
list3 = ["Python","Java","C"] 
print(list3)

# list of lists
list4 = [[15, 25], ["Python","Data Science"]] 
print(list4)

# list of different datatypes
list5 = [50, 98.3,'Python', [15, 25] ] 
print(list5)

Output:
[]
[10, 20, 30, 40, 50]
['Python', 'Java', 'C']
[[15, 25], ['Python', 'Data Science']]
[50, 98.3, 'Python', [15, 25]]

    List Indexing and Slicing

  • We can access individual elements using indexing and a range of characters using slicing.
  • Index starts from 0
  • Left to right direction


    • Negative Indexing

  • Index starts from -1
  • Right to left direction
  • The index of -1 refers to the last item, -2 to the second last item and so on

    • Sample Code

    #list of integers
list2 = [10, 20, 30, 40, 50] 

#print second element
print(list2[2]) 

#print last element using negative index
print(list2[-2])

#slicing from index 1 to index 3
print(list2[1:3])

# Other ways of slicing - Use it as per the requirement
print(list2[:])
print(list2[1:]) 
print(list2[:3])

Output:
30
40
[20, 30]
[10, 20, 30, 40, 50]
[20, 30, 40, 50]
[10, 20, 30]

    Updating Lists

    # Sample Code
list3 = ["Python","Java","C"] 
print("List Before Updating the item",list3)
list3[2] = "R"
print("List after Updating the item",list3)

Output:
List Before Updating the item ['Python', 'Java', 'C']
List after Updating the item ['Python', 'Java', 'R']

    Delete List Elements

    To remove a list element, you can use the del statement if you know exactly which element(s) you are deleting

    #Sample Code
#del to remove item based on index position
list3 = ["Python","Java","C"]

del list3[0]
print(list3) # Delete a particular item

(or)

#del list3
#print(list3) # Delete the full list

Output:
['Java', 'C']

    Traversing List using for loop

    #loop through a List

Month = ["Jan","Feb","Mar","Apr","May","June","July","Aug","Sep","Oct","Nov","Dec"]

for element in Month:
    print(element)

Output:
Jan
Feb
Mar
Apr
May
June
July
Aug
Sep
Oct
Nov
Dec

    List Methods

    1. Concatenation

    # "+" - It Concatenates two list, List1 and List2
List1 = ["Vinoth","Anand","Kumaran"]
List2 = ["Sathish","Vignesh"]
FinalList = List1 + List2
print(List1)
print(List2)
print(FinalList)

Output:
['Vinoth', 'Anand', 'Kumaran']
['Sathish', 'Vignesh']
['Vinoth', 'Anand', 'Kumaran', 'Sathish', 'Vignesh']

    2. Length

    # Length of sequence, i.e. the number of elements in the list.
List1 = [2,4,5,6,7,8,9,12,34,56,78,45,67,45,86,222]
print("length of the list is",len(List1))

Output:
length of the list is 16

    3. Repetition

    # n copies of list concatenated
List1 = ["Python"]
print(List1 * 7)

Output:
['Python', 'Python', 'Python', 'Python', 'Python', 'Python', 'Python']

    4. Membership

    # Sample Code for 'in' operator - True if element x is in list.
list1 = [26, 34, 24, 61, 132]
print(24 in list1)

# Sample Code for 'not in' operator -True if element x is not in list.
print(35 not in list1)

Output:
True
True

    5. Smallest number in the list

    # Smallest element in list.
list1 = [26, 34, 24, 61, 132]
print("The smallest number in the list is",min(list1))

Output:
The smallest number in the list is 24

    6. Largest number in the list

    #Largest element in list.
list1 = [26, 34, 24, 61, 132]
print("The largest number in the list is",max(list1))

Output:
The largest number in the list is 132

    7. Sum of all the numbers in the list

    # Sum of all numbers in list
List1 = [2,4,5,6,7,8,9,12,34,56,78,45,67,45,86,222]
print("Sum of all the numbers in the list is",sum(List1))

Output:
Sum of all the numbers in the list is 686

    8. Append

    # It Adds an element x to the end of the list and returns None.

#Appending the element
List1 = ["Vinoth","Anand","Kumaran"]
List1.append("Sathish") 
print(List1)

#Appending the list 
List2 = ["Raghul", "Santhosh"]
List1.append(List2)
print(List1)

Output:
['Vinoth', 'Anand', 'Kumaran', 'Sathish']
['Vinoth', 'Anand', 'Kumaran', 'Sathish', ['Raghul', 'Santhosh']]

    9. Extend

    # Appends all the elements from one list to the another list and returns None.
List1 = ["Vinoth","Anand","Kumaran","Sathish"]
List2 = ["Raghul","Santhosh"]

List1.extend(List2)
print(List1)

Output:
['Vinoth', 'Anand', 'Kumaran', 'Sathish', 'Raghul', 'Santhosh']

    10. Insert

    # Inserts an element x at a given index. 
# Note:  The first element in the list has index 0 and returns None..

list1 = [1,2,3,4,6,7,8,9,10]
list1.insert(4,5) # Location , Value as arguments
print(list1)

List2 = ["Vinoth","Anand","Kumaran","Sathish"]
List2.insert(2,"Vignesh")
print(List2)

Output:
[1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
['Vinoth', 'Anand', 'Vignesh', 'Kumaran', 'Sathish']

    11. Remove

    # Removes the first occurrence of element x from the list and returns None

list1 = [1,2,3,4,5,6,7,8,9,10,4]
list1.remove(4) # it will remove first occurence of '4' in a given list
print(list1)

List2 = ["Vinoth","Anand","Kumaran","Sathish"]
List2.remove("Anand")
print(List2)

Output:
[1, 2, 3, 5, 6, 7, 8, 9, 10, 4]
['Vinoth', 'Kumaran', 'Sathish']

    12. Pop

    # Removes the element at the given position and returns it. The parameter i is optional. 
# If it is not specified, pop() removes and returns the last element in the list.

list1 = [1,2,3,4,5,6,7,8,9,10]
a = list1.pop()
print("The value removed is",a)
print(list1)


List2 = ["Vinoth","Anand","Kumaran","Sathish"]
a = List2.pop(2)
print("The value removed is",a)
print(List2)

Output:
The value removed is 10
[1, 2, 3, 4, 5, 6, 7, 8, 9]
The value removed is Kumaran
['Vinoth', 'Anand', 'Sathish']

    13. Count

    #  Returns the number of times element x appears in the list.
numbers = [1, 2, 3, 1, 2, 3, 1, 2]
print(numbers.count(1))

Output:
3

    14. Reverse

    #Reverse the list and returns None

list1 = [1,2,3,4,5,6,7,8,9,10]
list1.reverse()
print(list1)

List2 = ["Vinoth","Anand","Kumaran","Sathish"]
List2.reverse()
print(List2)

Output:
[10, 9, 8, 7, 6, 5, 4, 3, 2, 1]
['Sathish', 'Kumaran', 'Anand', 'Vinoth']

    15. Sort

  • The sort() method sorts the elements of a given list in a specific order - Ascending or Descending.
  • By default in ascending order
  • The original list is not changed
    • # vowels list
vowels = ['e', 'a', 'u', 'o', 'i']

# sort the vowels
vowels.sort()

# print vowels
print('Sorted list:', vowels)

Output:
Sorted list: ['a', 'e', 'i', 'o', 'u']

    # Setting reverse=True sorts the list in the descending order
# vowels list
vowels = ['e', 'a', 'u', 'o', 'i']

# sort the vowels
vowels.sort(reverse=True)

# print vowels
print('Sorted list (in Descending):', vowels)

Output:
Sorted list (in Descending): ['u', 'o', 'i', 'e', 'a']

    # Numbers
list1 = [99, 19, 4, 1, 54, 25, 2]

# sort the vowels
list1.sort()

# print vowels
print('Sorted list:', list1)

Output:
Sorted list: [1, 2, 4, 19, 25, 54, 99]

    String Split to create a list

    split() method breaks up a string at the specified separator and returns a list of strings.

    # splits at space
str1= 'Welcome to Python Class'
print(str1.split())

str2 = "Vinoth,Anand,Santhosh,Raghul"
# splits at ','
print(str2.split(','))

# splits at ':'
str3 = "121212121:1212121:21434322434:4234234"
print(str3.split(':'))

Output:
['Welcome', 'to', 'Python', 'Class']
['Vinoth', 'Anand', 'Santhosh', 'Raghul']
['121212121', '1212121', '21434322434', '4234234']

    List Comprehensions

  • List comprehension is an elegant and concise way to create new list from an existing list in Python.
  • List comprehension consists of an expression followed by for statement inside square brackets.
  • List are faster and should be used instead of for loop + apprend wherever possible 
    • # Without list comprehension
cube = []
for i in range(10):
    cube.append(i**3) 
print(cube)

Output:
[0, 1, 8, 27, 64, 125, 216, 343, 512, 729]

    #Using list comprehension
cube = [i**3 for i in range(10)]
print(cube)

Output:
[0, 1, 8, 27, 64, 125, 216, 343, 512, 729]

    # Print the Even Numbers
list1 = [ x for x in range(20) if x % 2 == 0 ]
print("The Even numbers in the list are",list1)

# Print the Odd Numbers
list2 = [ x for x in range(20) if x % 2 != 0 ]
print("The Odd numbers in the list are",list2)

Output:
The Even numbers in the list are [0, 2, 4, 6, 8, 10, 12, 14, 16, 18]
The Odd numbers in the list are [1, 3, 5, 7, 9, 11, 13, 15, 17, 19]