Python List Comprehension

In this article, you’ll learn about list comprehension.

Syntax:

newList = [ expression(element) for element in oldList if condition ]

For Loop vs List Comprehensions

There are various ways to iterate through a list. However, the most common approach is to use the for loop.

# Empty list
List = []
 
# Traditional approach of iterating
for character in 'Techarge!':
    List.append(character)
 
# Display list
print(List)
Code language: PHP (php)

# Using list comprehension to iterate through loop
List = [character for character in 'Techarge!']




# Displaying list
print(List)
Code language: PHP (php)

Output the both the case are same,

[‘T’, ‘e’, ‘c’, ‘h’, ‘a’, ‘r ‘, ‘g’, ‘e ‘, ‘!’]

Advantages of List Comprehension

• More time efficient and space efficient than loops.
• Require fewer lines of code.
• Transforms iterative statement into a formula.

Nested List Comprehensions

Nested List Comprehensions are nothing but a list comprehension within another list comprehension which is quite similar to nested for loops. Below is the program which implements nested loop:

# Nested list comprehension
matrix = [[j for j in range(5)] for i in range(3)]

print(matrix)
Code language: PHP (php)

Output:

[[0, 1, 2, 3, 4], [0, 1, 2, 3, 4], [0, 1, 2, 3, 4]]

List Comprehensions and Lambda

Lambda Expressions are nothing but shorthand representations of Python functions. Using list comprehensions with lambda creates an efficient combination.

Let us look at the below examples:

Using Loops

# using lambda to print table of 5
numbers = []

for i in range(1, 10):
	numbers.append(i*5)

print(numbers)
Code language: PHP (php)

Using List Comprehension

numbers= [i*5 for i in range(1,10)]

print(numbers)
Code language: PHP (php)

Using List Comprehension and Lambda

# using lambda to print table of 5
numbers = list(map(lambda i: i*5, [i for i in range(1,10)]))

print(numbers)
Code language: PHP (php)

Output in all the case are same .

[5,10,15,20,25,30,35,40,45,50,55,60]

Conditionals in List Comprehension

We can also add conditional statements to the list comprehension. We can create a list using range(), operators, etc. and cal also apply some conditions to the list using the if statement.

Let look on some of the examples

Example 1: Display square of even numbers from 1 to 10.

# Getting square of even numbers from 1 to 10
squares = [n**2 for n in range(1, 11) if n%2==0]

# Display square of even numbers
print(squares)
Code language: PHP (php)

Output

[4, 16, 36, 64, 100]

Example 2: Display cube of odd numbers from 1 to 10.

# Getting cube of odd numbers from 1 to 10
squares = [n**3 for n in range(1, 11) if n%3==0]

# Display square of even numbers
print(squares)
Code language: PHP (php)

Output

[1,27,125,343,729]

Example 3: Toggle case of each character in a string.

# Initializing string
string = 'Techarge'

# Toggle case of each character
List = list(map(lambda i: chr(ord(i)^32), string))

# Display list
print(List)
Code language: PHP (php)

Output

['t','E','C','H','A','R','G','E']

Example 4: Reverse each string in a tuple.

# Reverse each string in tuple
List = [string[::-1] for string in ('Code', 'for', 'Good')]

# Display list
print(List)
Code language: PHP (php)

Output

['edoC', 'rof', 'dooG']

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