Lec 22 Lambdas

Assignment Updates

Stuff due soon:

This is kindof bulky, and we can shrink it with a list comprehension:

def square(x):
    return x * x
my_list = [1, 2, 3, 4, 5]
map_result = [square(num) for num in my_list]

Another way to write this is to use the map() function

Example:

def square(x):
    return x * x
my_list = [1, 2, 3, 4, 5]
map_result = map(square, my_list)

It returns a map object, to get a list we need to cast it to a list

def square(x):
    return x * x
my_list = [1, 2, 3, 4, 5]
map_result = list(map(square, my_list))

The Map function takes a function as its first argument
- We currently have to define a function and then pass it in
- But, is there a way to define a function and pass it in without having to define it above?
- Yes, we can use a lambda function
A lambda function is an anonymous function
- anonymous = It is a function without a name
- It is a function that is defined inline
- Example:
```
my_list = [1, 2, 3, 4, 5]
map_result = list(map(lambda x: x * x, my_list))
```
How do you write it?
- First, you write the keyword lambda
- Second, you write the arguments: lambda x, y, z: (followed by a colon)
- Finally, you write the body of the function: lambda x, y, z: x + y + z
  - The body of the function is a single expression
  - The result of the expression is the return value of the function
- Example: (lambda function that takes two arguments and returns their sum)
```
lambda x, y: x + y
```
- Activity: Write a lambda function that takes a number and returns it doubled
  - Answer: lambda x: x * 2
Why use a lambda function?
- It's more readable (sometimes)
- Can be defined inline (no need to define it above)
Activity: Use map() and a lambda to map each number in a list to that number plus 1
- Answer: list(map(lambda x: x + 1, my_list))

Like map, filter is a top level function
- It takes a function and a collection
- It returns a new collection
- The new collection is the result of applying the function to each element in the original collection. If the function returns True, the element is included in the new collection. If the function returns False, the element is not included in the new collection
Example: (Filters out all even numbers)

my_list = [1, 2, 3, 4, 5]
filter_result = list(filter(lambda x: x % 2 == 1, my_list))

NOTE: Filter returns a filter object (like map), so we need to cast it to a list
Activity: Use filter() and a lambda to filter out all numbers greater than 3
- Answer: list(filter(lambda x: x <= 3, my_list))

my_list = [1, 2, 3, 4, 5]
res = list(map(lambda x: x * x, filter(lambda x: x % 2 == 1, my_list)))

my_list = [1, 2, 3, 4, 5]
res = [x * x for x in my_list if x % 2 == 1]