Python in 30 Days: Day 13 – List Comprehension

List comprehension in Python is a compact way of creating a list from a sequence. It is a short way to create a new list. List comprehension is considerably faster than processing a list using the for loop.

# syntax
[i for i in iterable if expression]

Example:1

For instance, if you want to change a string to a list of characters. You can use a couple of methods. Let’s see some of them:

# One way
language = 'Python'
lst = list(language) # changing the string to list
print(type(lst))     # list
print(lst)           # ['P', 'y', 't', 'h', 'o', 'n']

# Second way: list comprehension
lst = [i for i in language]
print(type(lst)) # list
print(lst)       # ['P', 'y', 't', 'h', 'o', 'n']

Example:2

For instance, if you want to generate a list of numbers

# Generating numbers
numbers = [i for i in range(11)]  # to generate numbers from 0 to 10
print(numbers)                    # [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

# It is possible to do mathematical operations during iteration
squares = [i * i for i in range(11)]
print(squares)                    # [0, 1, 4, 9, 16, 25, 36, 49, 64, 81, 100]

# It is also possible to make a list of tuples
numbers = [(i, i * i) for i in range(11)]
print(numbers)                             # [(0, 0), (1, 1), (2, 4), (3, 9), (4, 16), (5, 25)]

Example:2

List comprehension can be combined with if expression

# Generating even numbers
even_numbers = [i for i in range(21) if i % 2 == 0]  # to generate even numbers list in range 0 to 21
print(even_numbers)                    # [0, 2, 4, 6, 8, 10, 12, 14, 16, 18, 20]

# Generating odd numbers
odd_numbers = [i for i in range(21) if i % 2 != 0]  # to generate odd numbers in range 0 to 21
print(odd_numbers)                      # [1, 3, 5, 7, 9, 11, 13, 15, 17, 19]
# Filter numbers: let's filter out positive even numbers from the list below
numbers = [-8, -7, -3, -1, 0, 1, 3, 4, 5, 7, 6, 8, 10]
positive_even_numbers = [i for i in range(21) if i % 2 == 0 and i > 0]
print(positive_even_numbers)                    # [2, 4, 6, 8, 10, 12, 14, 16, 18, 20]

# Flattening a three dimensional array
list_of_lists = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]
flattened_list = [ number for row in list_of_lists for number in row]
print(flattened_list)    # [1, 2, 3, 4, 5, 6, 7, 8, 9]

Lambda Function

A lambda function is a small anonymous function without a name. It can take any number of arguments, but can only have one expression. A lambda function is similar to anonymous functions in JavaScript. We need it when we want to write an anonymous function inside another function.

Creating a Lambda Function

To create a lambda function we use lambda keyword followed by a parameter(s), followed by an expression. See the syntax and the example below. The lambda function does not use return but it explicitly returns the expression.

# syntax
x = lambda param1, param2, param3: param1 + param2 + param2
print(x(arg1, arg2, arg3))

Example:

# Named function
def add_two_nums(a, b):
    return a + b

print(add_two_nums(2, 3))     # 5
# Lets change the above function to a lambda function
add_two_nums = lambda a, b: a + b
print(add_two_nums(2,3))    # 5

# Self invoking lambda function
(lambda a, b: a + b)(2,3) # 5 - need to encapsulate it in print() to see the result in the console

square = lambda x : x ** 2
print(square(3))    # 9
cube = lambda x : x ** 3
print(cube(3))    # 27

# Multiple variables
multiple_variable = lambda a, b, c: a ** 2 - 3 * b + 4 * c
print(multiple_variable(5, 5, 3)) # 22

Lambda Function Inside Another Function

Using a lambda function inside another function.

def power(x):
    return lambda n : x ** n

cube = power(2)(3)   # function power now need 2 arguments to run, in separate rounded brackets
print(cube)          # 8
two_power_of_five = power(2)(5) 
print(two_power_of_five)  # 32

 Now do some exercises for your brain and muscles.

Exercises: Python in 30 Days: Day 13 – List Comprehension

1. Filter only negative and zero in the list using list comprehension

   numbers = [-4, -3, -2, -1, 0, 2, 4, 6]

2. Flatten the following list of lists of lists to a one-dimensional list :

   list_of_lists =[[[1, 2, 3]], [[4, 5, 6]], [[7, 8, 9]]]
   
   output
   [1, 2, 3, 4, 5, 6, 7, 8, 9]

3. Using list comprehension create the following list of tuples:

   [(0, 1, 0, 0, 0, 0, 0),
   (1, 1, 1, 1, 1, 1, 1),
   (2, 1, 2, 4, 8, 16, 32),
   (3, 1, 3, 9, 27, 81, 243),
   (4, 1, 4, 16, 64, 256, 1024),
   (5, 1, 5, 25, 125, 625, 3125),
   (6, 1, 6, 36, 216, 1296, 7776),
   (7, 1, 7, 49, 343, 2401, 16807),
   (8, 1, 8, 64, 512, 4096, 32768),
   (9, 1, 9, 81, 729, 6561, 59049),
   (10, 1, 10, 100, 1000, 10000, 100000)]

4. Flatten the following list to a new list:

   countries = [[('Finland', 'Helsinki')], [('Sweden', 'Stockholm')], [('Norway', 'Oslo')]]
   output:
   [['FINLAND','FIN', 'HELSINKI'], ['SWEDEN', 'SWE', 'STOCKHOLM'], ['NORWAY', 'NOR', 'OSLO']]

5. Change the following list to a list of dictionaries:

   countries = [[('Finland', 'Helsinki')], [('Sweden', 'Stockholm')], [('Norway', 'Oslo')]]
   output:
   [{'country': 'FINLAND', 'city': 'HELSINKI'},
   {'country': 'SWEDEN', 'city': 'STOCKHOLM'},
   {'country': 'NORWAY', 'city': 'OSLO'}]

6. Change the following list of lists to a list of concatenated strings:

   names = [[('Tech', 'G')], [('David', 'Smith')], [('Donald', 'Trump')], [('Bill', 'Gates')]]
   output
   ['Tech G', 'David Smith', 'Donald Trump', 'Bill Gates']

7. Write a lambda function that can solve a slope or y-intercept of linear functions.

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