LIST OPERATION

LIST OPERATION

• Creating and accessing list
• List operation
• Working with list
• List Function and Method
• 
  

   What is Python List ?

• Python offers a range of compound data types often referred to as   sequences.
•  List is one of the most frequently used and very versatile data types used in Python

  

How to create a list?

• In Python programming, a list is created by placing all the items (elements) inside square brackets [], separated by commas.
• It can have any number of items and they may be of different types (integer, float, string etc.)

     

# empty list
list = []

# list of integers
list = [1, 2, 3]

# list with mixed data types
list = [1, "Hello", 3.4]

What is nested List ?

• A list can also have another list as an item. This is called a nested list

     

    # nested list

   list = ["mouse", [8, 4, 6], ['a']]

How to access elements from a list?

There are various ways in which we can access the elements of a list.

1.List Index

  

• We can use the index operator [] to access an item in a list. In Python, indices start at 0. So, a list having 5 elements will have an index from 0 to 4.
• Trying to access indexes other than these will raise an IndexError. The index must be an integer. We can't use float or other types, this will result in TypeError.
• Nested lists are accessed using nested indexing.

# List indexing

list = ['p', 'r', 'o', 'b', 'e']

# Output: p
print(list[0])

# Output: o
print(list[2])

# Output: e
print(list[4])

# Nested List
n_list = ["Happy", [2, 0, 1, 5]]

# Nested indexing
print(n_list[0][1])

print(n_list[1][3])

# Error! Only integer can be used for indexing
print(list[4.0])

  Output

    p

o
e
a
5
Traceback (most recent call last):
  File "<string>", line 21, in <module>
TypeError: list indices must be integers or slices, not float

2.Negative indexing

• Python allows negative indexing for its sequences. The index of -1 refers to the last item, -2 to the second last item and so on.
• 
  

 # Negative indexing in lists

list = ['p','r','o','b','e']

print(list[-1])

print(list[-5])

When we run the above program, we will get the following output:

e
p

How to slice lists in Python?

We can access a range of items in a list by using the slicing operator :(colon).

# List slicing in Python

list = ['p','r','o','g','r','a','m','i','z']

# elements 3rd to 5th
print(list[2:5])

# elements beginning to 4th
print(list[:-5])

# elements 6th to end
print(list[5:])

# elements beginning to end 

print(list[:])

 Output

['o', 'g', 'r']
['p', 'r', 'o', 'g']
['a', 'm', 'i', 'z']
['p', 'r', 'o', 'g', 'r', 'a', 'm', 'i', 'z']

How to change or add elements to a list?

Lists are mutable, meaning their elements can be changed unlike string or tuple.

We can use the assignment operator (=) to change an item or a range of items.

# Correcting mistake values in a list
odd = [2, 4, 6, 8]

# change the 1st item    
odd[0] = 1            

print(odd)

# change 2nd to 4th items
odd[1:4] = [3, 5, 7]  

print(odd)              

 Output

[1, 4, 6, 8]
[1, 3, 5, 7]

List Comprehension: Elegant way to create new List

• List comprehension is an elegant and concise way to create a new list from an existing list in Python.
• A list comprehension consists of an expression followed by for statement inside square brackets.
• A list comprehension can optionally contain more for or if statements. An optional if statement can filter out items for the new list.
• Here is an example to make a list with each item being increasing power of 2.

    pow2 = [2 ** x for x in range(10)]

 print(pow2)

    Output

[1, 2, 4, 8, 16, 32, 64, 128, 256, 512]

>>> pow2 = [2 ** x for x in range(10) if x > 5]
>>> pow2
[64, 128, 256, 512]
>>> odd = [x for x in range(20) if x % 2 == 1]
>>> odd
[1, 3, 5, 7, 9, 11, 13, 15, 17, 19]
>>> [x+y for x in ['Python ','C '] for y in ['Language','Programming']]
['Python Language', 'Python Programming', 'C Language', 'C Programming']

Operation on list

• We can also use + operator to combine two lists. This is also called concatenation.
• The * operator repeats a list for the given number of times.

    # Concatenating and repeating lists

odd = [1, 3, 5]

print(odd + [9, 7, 5])

print(["pre"] * 3)

 Output

[1, 3, 5, 9, 7, 5]
['pre', 'pre', 'pre']

• List Membership Operator

We can test if an item exists in a list or not, using the keyword in.

my_list = ['p', 'r', 'o', 'b', 'l', 'e', 'm']

# Output: True
print('p' in my_list)

# Output: False
print('a' in my_list)

# Output: True
print('c' not in my_list)

Output

True
False
True

Iterating Through a List

Using a for loop we can iterate through each item in a list.

for fruit in ['apple','banana','mango']:
    print("I like",fruit)

Output

I like apple
I like banana
I like mango

Python List Methods

• Methods that are available with list objects in Python programming are tabulated below.
• They are accessed as list.method()

        

SHARE

Milan Tomic

Hi. I’m Designer of Blog Magic. I’m CEO/Founder of ThemeXpose. I’m Creative Art Director, Web Designer, UI/UX Designer, Interaction Designer, Industrial Designer, Web Developer, Business Enthusiast, StartUp Enthusiast, Speaker, Writer and Photographer. Inspired to make things looks better.

• 
• 
• 
• 
•