Python: Lists

Lists

 

1. Introduction to Lists

i. Definition of List

List is a sequence of values written in a square bracket and separated by commas.

For example

How to create list?

>>>a=['AAA','BBB','CCC']

>>>b=[10,20,30,40]

>>> 

Here a and b are the lists data structures. Following screenshot illustrate this.

ii. Accessing Values in List

Individual element of the list can be accessed using index of the list. For example ‒

>>> rollNo = [1,2,3,4,5]

>>> name=['Shilpa', 'Chinmaya', 'Akash', 'Aditya', 'Swati']

>>> print(rollNo[0],name[0])

1 Shilpa

>>> print(rollNo[1],name[1])

2 Chinmaya

>>> print(rollNo[1:4])

[2, 3, 4]

>>> 

The above code can be illustrated on IDE as follows ‒

In [2]:  rollNo = [1,2,3,4,5]

name=['Shilpa', 'Chinmaya', 'Akash', 'Aditya','Swati']

print (rollNo[0], name[0])

1 Shilpa

In [3]: print(rollNo[1], name[1])

2 Chinmaya

In [4]: print(rollNo [1:4])

[2, 3, 4]

Using the in operator we can check whether particular element belongs to the list or not. If the given element is present in the list it returns true otherwise false. For example

>>> a = ['AAA', 'XXX', 'CCC']

>>> 'XXX' in a

True

>>> 'BBB' in a

False

iii. Deleting Values in List

• The deletion of any element from the list is carried out using various functions like pop, remove, del.

• The pop function: If we know the index of the element to be deleted then just pass that index as an argument to pop function.

o For example

>>> a=['u', 'v', 'w','x','y','z']

>>> val=a.pop(1)   #the element at index 1 is v, it is deleted

>>> a

['u', 'w', 'x', 'y', 'z']    #list after deletion

>>> val                   #deleted element is present in variable val

'v'

>>> 

• If we do not provide any argument to the pop function then the last element of the list will be deleted.

。 For example

>>> a =['u', 'v', 'w', 'x','y','z']

>>> val=a.pop()

>>> a

['u', 'v', 'w', 'x', 'y'l

>>> val

'z'

>>> 

The remove function: If we know the value of the element to be deleted then the remove function is used. That means the parameter passed to the remove function is the actual value that is to be removed from the list. Unlike, pop function the remove function does not return any value.

o The execution of remove function is shown by following illustration ‒

>>> a=['a','b','c','d','e']

>>> a.remove('c')

>>> a

['a', 'b', 'd', 'e']

>>> 

• The del function: In python, it is possible to remove more than one element at a time using del function.

o For example

>>> a=['a','b','c','d','e']

>>> del a[2:4]

>>> a

['a', 'b', 'e']

>>> 

iv. Updating List

• Lists are mutable. That means it is possible to change the values of list.

• If the bracket operator is present on the left hand side, then that element is identified and the list element is modified accordingly.

• For example

>>> a=['AAA', 'BBB','CCC']

>>> a[1]='XXX'

>>> a

['AAA', 'XXX', 'CCC']

 >>>

• Using the in operator we can check whether particular element belongs to the list or not. If the given element is present in the list it returns true otherwise false.

• For example

a = ['AAA', 'XXX', 'CCC']

>>> 'XXX' in a

True

>>> 'BBB' in a

False

>>> 

 

2. Use for and while Loops along with Useful Built‒In Functions to Iterate over and Manipulate Lists

1) Traversing a List

The loop is used in list for traversing purpose. The for loop is used to traverse the list elements.

Syntax

for VARIABLE in LIST:

BODY

Example

>>> a=['a','b','c', 'd','e'] # List a is created

>>> for i in a:

print(i)

will result into

a

b

C

d

e

>>> 

There are some useful functions using which the we can traverse the list. These are discussed below ‒

i) Using the range() function

Using range() function we can access each element of the list using index of a list.

If we want to increment each element of the list by one, then we must pass index as argument to for loop. This can be done using range() function as follows ‒

>>> a=[10,20,30,40]

>>> for i in range(len(a)):

a[i]=a[i]+1 #incremented each number by one

>>> a

[11, 21, 31, 41]

>>> 

 (ii) Using enumerate() function

Using enumerate() function we can print both index and item of the list.

Example:1

Write a program to iterate through list using enumerate() function.

Solution:

myList =[10,20,30]

for item in enumerate(myList):

print(item)

Output

(0, 10)

(1,20)

(2, 30)

2) Concatenation using +

The two lists can be created and can be joined using + operator. Following screenshot illustrates it

In [8]:

L1 = [10,20,30]

L2 = [40,50,60]

L = L1+L2

L

Out[8]: [10, 20, 30, 40, 50, 60]

3) Repetition using

The * is used to repeat the list for number of times. Following screenshot illustrates it.

In [9]:  [10]*3

Out[9]: [10, 10, 10]

In [10]: [10,20,30]*3

Out[10]: [10, 20, 30, 10, 20, 30, 10, 20, 30]

In [ ]:

4) Finding length of a list

The len() function is used to find the number of elements present in the list. For example

In [11]:

mylist = [10, 20, 30, 40, 50]

print(len(mylist))

5

5) Membership

• Using the in operator we can check whether particular element belongs to the list or not. If the given element is present in the list it returns true otherwise false.

• For example

>>> a = ['AAA', 'XXX', 'CCC']

>>> 'XXX' in a

True

>>> 'BBB' in a

False

>>> 

6) List slices

• The operator used within the square bracket that it is a list slice and not the index of the list.

>>> a=[10,20,30,40,50,60]

>>> a[1:4]

[20, 30, 40]

>>> a[:5]

[10, 20, 30, 40, 50]

>>> a[4:]

[50, 60]

>>> a[:]

[10, 20, 30, 40, 50, 60]

>>> 

• If we omit the first index then the list is considered from the beginning. And if we omit the last second index then slice goes to end.

If we omit both the first and second index then the list will be displayed from the beginning to end.

Lists are mutable. That means we can change the elements of the list. Hence it is always better to make a copy of the list before performing any operation.

For example

>>> a=[10,20,30,40,50]

>> a[2:4]=[111,222,333]

>> a

[10, 20, 111, 222, 333, 50] >>>

6) List methods

There are various methods that can work on the list. Let us understand these method with the help of illustrative examples.

i) append

The append method adds the element at the end of the list. For example

>>> a=[10,20,30]

>>> a.append(40)    #adding element 40 at the end

>>> a

 [10, 20, 30, 40]

>>> b=['A','B','C']

>>>b.append('D')   #adding element D at the end

>>> b

['A', 'B', 'C', 'D']

>>> 

ii) extend

The extend function takes the list as an argument and appends this list at the end of old list. For example

>>> a=[10,20,30]

>>> b=['a','b','c']

>>> a.extend(b)

>>> a

[10, 20, 30, 'a', 'b', 'c']

>>> 

iii) sort

The sort method arranges the elements in increasing order. For example

>>> a=['x','z','u', 'v','y','w']

>>> a.sort()

>>> a

['u', 'v', 'w', 'x', 'y', 'z']

>>> 

The methods append, extend and sort does not return any value. These methods simply modify the list. These are void methods.

3. Built In List Functions

There are various built in functions in python for supporting the list operations. Following table shows these functions:

Function       ‒     Purpose

all()     ‒   If all the elements of the list are true or if the list is empty then this function returns true.

any()    ‒   If the list contains any element true or if the list is empty then this function returns true.

len()     ‒    This function returns the length of the string.

max()    ‒   This function returns maximum element present in the list.

min()    ‒     This function returns minimum element present in the list.

sum()    ‒    This function returns the sum of all the elements in the list.

sorted()  ‒   This function returns a list which is sorted one.

Following screenshot illustrates the use of some built‒in functions of list

In [12]:

 mylist [1,2,3,4,5]

print(max(mylist))

5

In [13]:

print(min(mylist))

1

In [14]: print(sum(mylist))

15

Functions related to strings

1) List function

• String is a sequence of characters and list is sequence of values.

• But list of characters is not the string.

• We can convert the string to list of characters.

For example ‒

>>> str='hello'

anivo:>>> myList=list(str)

>>> print(myList)

['h', 'e', '1', '1', 'o']

>>> 

• Note that we have used list function to split the characters of the string which ultimately forms the list. The list is a built in function.

2) Split function

• If the string contains multiple words then we need to use the built in function split to split the words into the list.

For example

>>> msg="I love Python Programming very much"

>>> myList=msg.split()

>>> print(myList)

['I', 'love', 'Python', 'Programming', 'very', 'much']

>>> 

• We can pass argument to the split function as some delimiter. For instance: If we have following string.

For example ‒

>>> msg="I#love#Python#programming"

>>> myList=msg.split('#')

>>> print(myList)

['I', 'love', 'Python', 'programming']

>>> 

3) Join function

• The join function is exactly reverse to the split function. That means the join function takes the list of strings and concatenate to form a string.

• The join is basically a string method.

• The use of join method is as shown below ‒

>>> msg=['I', 'love', 'Python', 'programming']

>>> ch='#'

>>> ch.join(msg)

'I#love#Python#programming'

>>> 

Note that the string is joined, used the delimiting character #

List comprehensions

• List comprehension is an elegant way to create and define new lists using existing lists.

• This is mainly useful to make new list where each element is obtained by applying some operations to each member of another sequence.

Syntax

List=[expression for item in the list]

Example:2

Write a python program to create a list of even numbers from 0 to 10.

Solution :

even = [] #creating empty list

for i in range(11):

    if i%2= = 0:

        even.append(i)

print("Even Numbers List: ",even)

    even = [] #creating empty list

for i in range(11):

    if i%2= = 0:

        even.append(i)

print("Even Numbers List: ",even)

Output

Even Numbers List: [0, 2, 4, 6, 8, 10]

Program explanation: In above program,

1) We have created an empty list first.

2) Then using the range of numbers from 0 to 11 we append the empty list with even numbers. The even number is test using the if condition. i.e. if I %2==0. If so then that even number is appended in the list.

3) Finally the comprehended list will be displayed using print statement.

Example:3

Write a python program to combine and print two lists using list comprehension.

Solution :

print([(x,y)for x in['a','b'] for y in ['b','d'] if x!=y])

Output

[('a', 'b'), ('a', 'd'), ('b', 'd')]

Example:4

To accept N numbers from user. Compute and display maximum in list, minimum in list, sum and average of numbers.

Solution :

mylist = [] # start an empty list

print("Enter the value of N: ")

N = int(input()) # read number of element in the list

for i in range(N):

    print("Enter the element: ")

    new_element = int(input()) # read next element

    mylist.append(new_element) # add it to the list

print("The list is: ")

print(mylist)

print("The maximum element from list is: ")

max_element=mylist[0]

for i in range(N): #iterating throu list

    if(mylist[i]>max_element):

        max_element=mylist [i] #finding the maximum element

print(max_element)#printing the maximum element

print("The minimum element from list is: ")

min_element=mylist[0]

for i in range(N): #iterating throu list

    if(mylist[i]<min_element):

        min_element=mylist [i] #finding the minimum element

print(min_element)#printing the minimum element

print("The sum of all the numbers in the list is: ")

sum=0

for i in range(N): #iterating throu list

    sum sum+mylist[i] # finding the sum

print(sum)

avg=sum/N # computing the average

print("The Average of all the numbers in the list is: ",avg)

Output

Enter the value of N:

5

Enter the element:

33

Enter the element:

22

Enter the element:

55

Enter the element:

11

Enter the element:

44

The list is:

[33, 22, 55, 11, 44]

The maximum element from list is:

55

The minimum element from list is:

11

The sum of all the numbers in the list is:

165

The Average of all the numbers in the list is: 33.0

>>> 

 

Review Questions

1.What is list? Explain how to access the list elements.

2.Write a python program to delete and update list elements.