List Comprehensions

Creating Lists

• We frequently want to create a list from another list
• This can be accomplished using a for loop

Example

nums = [2, 5, 9]

squares = []

for num in nums:
    squares.append(num * num)

print(squares)

Map

• The task of building one iterable from another is common
• A special function called map is provided to simplify this task
• map takes a function and applies it to all items in an iterable return a new iterable

Example

nums = [2, 5, 9]

def square(n):
    return n * n

squares = map(square, nums)

print(list(squares))

Lambda Functions

• It can sometimes be helpful to create small, inline functions
• The lambda keyword can be used to facilitate this

Example

double = lambda x: x * 2

print(double(4))

Exercise

Create a square function using lambda instead of def.

Map and Lambda

• lambda and map can be combined to quickly perform operations on entire lists

Example

nums = [2, 5, 9]
squares = map(lambda n: n*n, nums)
print(list(squares))

List Comprehensions

• The pattern of creating one list from another is very common
• Python introduces special syntax to simplify these operations
• This is referred to as a list comprehension

Example

nums = [2, 5, 9]
squares = [n*n for n in nums]
print(squares)

Filtering

• Filtering can be applied to list comprehensions using an if clause

Example

nums = list(range(10))
print(nums)
big_squares = [n*n for n in nums if n > 5]
print(big_squares)

Generator Expressions

• Work somewhat like list comprehensions
• Produce generators instead of lists
• Generator wait to compute values until they are needed

Example

# Don't try this with a list
# We'll run out of memory
squares = (i*i for i in range(100**100))

# Find the first square larger than a million
for square in squares:
    if square > 1000000:
        print(square)
        break