Lists in python

February 4, 2021

Lists in python

Introduction to lists 

Lists are used to store data in a single variable. Lists in python is same as arrays , the only difference is arrays store only homogeneous type of elements (say all elements must only be integers , float , string) but in lists it stores heterogeneous type of elements. Lists are mutable and indexed where index starts from zero.

Example lists: Lst1=[“a”,”b”,”c”,”d”]
                          Lst2=[1,2,3,4,5,6]
                          Lst3=[True,”learn python” ,”with”,”PrepInsta”,440,403] 

Keyword used for lists is “list”

Properties of lists

  • Lists are indexed
  • Lists are mutable
  • Lists allow duplicates
  • Lists can store heterogeneous datatypes
  • Lists can be used as nested format 

Declaration of lists in python

An empty list can be declared as two types

  • using keyword list()
  • using square braces [ ]
L1=list()
L2=[]
print(“List L1 is :”,L1) 
print(“List L2 is :”,L2)
List L1 is : []
List L2 is : []

List functions

some of the important functions in python are

  • len
  • append
  • insert
  • count
  • index
  • sort
  • reverse
  • pop

Len:

len function is used to find the length of the list. It returns integer.
Syntax:  len(Lst)

append:

append means adding . append function is used to add elements to the list. Generally it adds elements to the end of the list.
Syntax: Lst.append(element)

insert:

insert function is also used for same purpose as append. But it adds element at the desired place. It takes two arguments , element and index where to be inserted.
Syntax: Lst.insert(index,element)

count:

count function returns the count of particular element that is passed as arguments.
Syntax: Lst.count(element) 

index:

index function returns the index of the first occurred particular element from the list.
Syntax: Lst.index(element)

sort:

sort function sorts the list in ascending order
Syntax: Lst.sort(reverse=True/False,key=func)
here reverse and key is optional . If reverse is True then it sort in descending order.
key is parameter on which sorting criteria depends.

reverse:

reverse function will reverse the order of the list.
Syntax: Lst.reverse()

pop:

pop will delete the elements from the list and stores it in a variable. It works on index
Syntax:  x=Lst.pop(index_of_element)

Lets check a program using all the above functions

Lst=list()
n=5
print(“Give inputs to the list :”)
for i in range(n):
    a=input()
    #adds elements to the list
    Lst.append(a)

#insert element at index 1
Lst.insert(1,“bat”) 
print(“List after inserting :”,Lst)  

print(“Length of the list :”,len(Lst)) 
print(“count of apple in the list :”,Lst.count(“apple”))

#sorts in ascending order
Lst.sort()
print(“List after sorting :”,Lst)

Lst.reverse()
print(“List after reversing :”,Lst)

x=Lst.pop(4)
print(“Popped element “,x)
print(“Final list :”,Lst) 
Give inputs to the list :
apple
cat
dog
eye
apple
List after inserting : ['apple', 'bat', 'cat', 'dog', 'eye', 'apple']
Length of the list : 6
count of apple in the list : 2
List after sorting : ['apple', 'apple', 'bat', 'cat', 'dog', 'eye']
List after reversing : ['eye', 'dog', 'cat', 'bat', 'apple', 'apple']
Popped element apple
Final list : ['eye', 'dog', 'cat', 'bat', 'apple']