Python List

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]
    
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