Lists

What are lists ?

List is a collection in python. In most languages, it is called an Array. We often need ‘list’ in our programs. Imagine you are writing a program to store the name of every student in a class of 50 .

Elements in a list are stored in sequential order . Every element can be accessed by their index value .

Students                  [ "Chaitya",  "Neel" , "Ankit" , "Siddharth" ]

Forward Indexing           0              1             2               3

Backward Indexing       -4             -3             -2            -1

Declaring and Initializing a list

students = [ ]     #  empty list

students_marks = list( )  #  empty list

player_scores = [ 78, 103, 200 , 57]   # >>> Initialized list with Integers

player_names = [ "Karan" , "Chirag" , "Jay" , "Raj" ]   #  Initialized list with strings

Unlike other programming languages , in python we can Initialize or store elements which are of different datatypes in the same list. For example,

unknown_list = [ "google", "tensorflow", 73 , 37.5 ]  # works :P

print("Number of elements :", len(unknown_list))         # >>> Number of elements : 4

Adding elements to the list

We can add elements to the list by using append & insert methods .

• Insert(index , element) - We use this method to add elements to a specific postion in a list .

• append(element) - We use this method to add elements to the back(last postion) of the list .

scores = [ 1, 2 , 3 , 4 , 5 ]   # Initialized list with Integers

scores.append( 6 )      # score = [ 1, 2, 3, 4, 5, 6]

score.append( 20 )      # score = [ 1, 2, 3, 4, 5, 6, 20]

score.insert( 0 , 8 )   # score = [ 8, 1, 2, 3, 4, 5, 6, 20]

score.insert( 4 , 4.5 )         # score = [ 8, 1, 2, 3, 4, 4.5, 5, 6, 20]

grades = [ "a" , "b" , "e"]     # Initialized list with characters .

grades[2] = "c"         # >>> grades = [ "a", "b", "c"]


updated_grades = grades + [ "d", "e", "f"]              # >>> updated_grades = [ "a" , "b" , "c", "d", "e" ,"f"]

updated_grades.extend("g" ,"h")                 # >>> updated_grades = [ "a" , "b" , "c", "d", "e" ,"f" ,"g", "h"]

Accessing elements from a list

Elements of a list are accessed using their index postion . [ list index starts from 0 ]

 updated_grades = [ "a" , "b" , "c", "d", "e" ,"f"]

print(updated_grades[3])        # >>> d
print(updated_grades[-2])       # >>> e

print(updated_grades[3:])       # >>> ["d", "e", "f"]

print(updated_grades[2:4])      # >>> [ "c", "d"]

print(updated_grades[:])        # >>> [ "a" , "b" , "c", "d", "e" ,"f"]

# Get the index of the first item found
updated_grades.index('c')  # >>> 2
updated_grades.index('j')  # Raises a ValueError as "j" is not in the list

Deleting elements from the list

We use del [delete] , remove and pop methods to delete / remove elements from list .

• del list_name[index]- we use del keyword to remove an element from list which is mentioned / passed as index .

• list_name.remove(element) - here ‘element’ gets removed from the list if it’s present .

• list_name.pop() - removes the last element from the list if the size of list is greater than 1 .

li  = [ 1, 2, 2, 3]

del li[2]  # li is now [1, 2, 3]

li.append(4)    # li is now [1, 2, 3, 4]

li.remove(1)    # li is now [ 2, 3, 4 ]

li.pop()        # li is now [ 2, 3]

Traversing through a list

We traverse ( access element in sequential order ) through a list by using loops .

• Using For loop :

averages = [22.5 , 46.2 , 78.9 , 56.5 , 98.3 ]

for item in averages :
        print(item)

output

22.5
46.2
78.9
56.5
98.3

• Using While loop :

scores  = [25, 89 , 90 , 45 , 87 ]

current = 0
end = len(scores)

while current < end:
        print(scores[current])
        current +=1

output

25
89
90
45
87

We can also traverse through a part of the list by simply using list comprehension techniques (also mentioned above)

grades = ["A+", "A", "B+", "B" , "C+" ,"C" ]

for item in grades[:4]:
        print(item)

output

A+
A
B+
B

Performing various operations on list

• list_name.sort() - Sorts the elements in the list . By default the elements are sorted in ascending order .

• list_name.reverse() - Reverses the order of elements present in the list .

Implementation

li = [ 4, 3, 1, 2 , 5 ]

li.sort()       # li is now [1 ,2 , 3 , 4 , 5]

li.reverse()    # li is now [5, 4, 3, 2, 1]

li = [ 4, 3, 1, 2 , 5 ]

li.sort(reverse=True)   # li is now [5, 4, 3, 2, 1]
print(sum(li))  # >>> 15

li = ["c" ,"d" , "e" , "a" ,"b" ]

li.sort()       # li is now ["a", "b" , "c" , "d" , "e"]

List Comprehension

List comprehensions provide a concise way to create / manipulate lists . For example,,

student_names = ["dustin" , "mark" ,"adrian" ,"thomas"]

initials = []
# creating a list of initails of student names
for student in student_names:
        initials.append(student[0])

print(initials)         # >>> [ 'd', 'm','a', 't']

# However this can be done in a better way
initials = [student[0] for student in student_names]

print(initials)         # >>> [ 'd', 'm','a', 't']

# similarly
name_length = [len(student) for student in student_names]

print(name_length)      # >>> [6, 4 ,6 ,6]      

# list of squares

squares = [i*i for i in range(1,5)]
print(squares)          # >>> [1, 4 ,9 , 16]