List Comprehensions in python for beginners

List Comprehensions

In addition to sequence operations and list methods, Python includes a more advanced operation called a list comprehension.

List comprehensions allow us to build out lists using a different notation. You can think of it as essentially a one-line for loop built inside of brackets. For a simple example:

 

Example 1

# Grab every letter in string
lst = [x for x in 'word']

# Check
print(lst)

OUTPUT

['w', 'o', 'r', 'd']

This is the basic idea of list comprehension. If you're familiar with mathematical notation this format should feel familiar for example: x^2 : x in { 0,1,2...10 }

Let's see a few more examples of list comprehensions in Python:

 

Example 2

# # Square numbers in range and turn into list
lst = [x**2 for x in range(0,11)]

print(lst)

OUTPUT

[0, 1, 4, 9, 16, 25, 36, 49, 64, 81, 100]

 

Example 3

# Check for even numbers in a range
lst = [x for x in range(11) if x % 2 == 0]

print(lst)

OUTPUT

[0, 2, 4, 6, 8, 10]

 

Example 4

Can also do more complicated arithmetic

# Convert Celsius to Fahrenheit
celsius = [0,10,20.1,34.5]




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fahrenheit = [((9/5)*temp + 32) for temp in celsius ]

print(fahrenheit)

OUTPUT

[32.0, 50.0, 68.18, 94.1]

 

Example 5

We can also perform nested list comprehensions, for example:

lst = [ x**2 for x in [x**2 for x in range(11)]]
print(lst)

OUTPUT

[0, 1, 16, 81, 256, 625, 1296, 2401, 4096, 6561, 10000]