List Comprehension

Source: List Comprehension in Python

List comprehensions are a tool for creating lists in a concise expression. They allow for simple manipulation of all items in a list.

Syntax

`my_new_list = [expression for item in list]

A python list comprehension requires 3 ingredients:

1. The expression to carry out (expression)
2. The object to perform the expression on (item)
3. An iterable list of objects to build from (list)

Creating lists

List comprehensions can be used with the range() function.

Per the reading, something that would normally be accomplished with loops…

squares = []

for x in range(10):
    # raise x to the power of 2
    squares.append(x**2)

print(squares)
#[0, 1, 4, 9, 16, 25, 36, 49, 64, 81]

…can be rewritten with a list comprehension:

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

print(squares)
#[0, 1, 4, 9, 16, 25, 36, 49, 64, 81]

Functions and if statements

An if statement can be used at the end of a list comprehension, to act as a filter for the expression.

From the reading:

even_numbers = [ x for x in range(1,20) if x % 2 == 0]

print(even_numbers)
#[2, 4, 6, 8, 10, 12, 14, 16, 18]

Functions can also be called within the expression of a list comprehension. The return of the function is what will be placed in the output array.

From the reading:

def double(x):
    return x*2

nums = [double(x) for x in range(1,10)]
print(nums)
#[2, 4, 6, 8, 10, 12, 14, 16, 18]