List comprehensions python code example

Example 1: list comprehension python

# All of the possibilies that can be done with the List Comprehension

vec = [-4, -2, 0, 2, 4]
# create a new list with the values doubled

doubled = [x*2 for x in vec]
# [-8, -4, 0, 4, 8]

# filter the list to exclude negative numbers
greater_thatn_0 = [x for x in vec if x >= 0]
# output [0, 2, 4]

# apply a function to all the elements
positive = [abs(x) for x in vec]
#  output [4, 2, 0, 2, 4]

# call a method on each element
freshfruit = ['  banana', '  loganberry ', 'passion fruit  ']
fruits_nospaces = [weapon.strip() for weapon in freshfruit]
# output ['banana', 'loganberry', 'passion fruit']

# create a list of 2-tuples like (number, square)
squares = [(x, x**2) for x in range(6)]
# output [(0, 0), (1, 1), (2, 4), (3, 9), (4, 16), (5, 25)]

# the tuple must be parenthesized, otherwise an error is raised
# error = [x, x**2 for x in range(6)]
  #   error = [x, x**2 for x in range(6)]
                     ^
# SyntaxError: invalid syntax

# flatten a list using a listcomp with two 'for'
vec = [[1,2,3], [4,5,6], [7,8,9]]
unpacking_tuple = [num for elem in vec for num in elem]
# output [1, 2, 3, 4, 5, 6, 7, 8, 9]

Example 2: python list comprehension

nums = [4, -7, 9, 1, -1, 8, -6]
half_of_nums = [x/2 for x in nums] #[2, -3.5, 4.5, 0.5, -0.5, 4, -3]

#optionally you can add an if statement like this
half_of_positive_nums = [x/2 for x in nums if x>=0] #[2, 4.5, 0.5, 4]

Example 3: python list comprehension

#example: removing common elements found in `a` from `b`.
a = [1,2,3,4,5]
b = [5,6,7,8,9]
# desired output: [1,2,3,4]

# gets each item found in `a` AND not in `b`
print([i for i in a if i not in b])

Example 4: list comprehension

# List comprehension 

        
list_comp = [i+3 for i in range(20)]

# above code is similar to 

for i in range(20):
	print(i + 3)

Example 5: python list comprehension

lst=[1,2,3,4,5]
lst2=[item for item in lst if <condition>]
# generates a list based on another list and an if statement. the code above is a replacement for:
lst=[1,2,3,4,5]
lst2=[]
for item in lst:
  if <condition>:
    lst2.append(item)

Example 6: list comprehension python 3

number_list = [x for x in range(10) if x % 2 == 0]
print(number_list)