Map, Filter & Reduce

We learned about anonymous functions, but let’s explore 3 of Python’s most powerful anonymous functions and these are map(), filter(), and reduce(). These 3 enable us to approach programming with a functional paradigm {which is another way of saying we use functions}.

Map

We can use the map() function to use all of the provided inputs within a function using. The map() function’s syntax is as follows:

• map(<function>, <list_of_inputs>)

Let’s consider an example to see where this can be useful:

Suppose we need to print the square of a list of items, and based on what we have learned we can approach this likewise:

items = [1, 2, 3, 4, 5]

def sq_function(x):
    return x * x

for item in items:
    value = sq_function(item)
    print(value)

This gives us:

The above approach while correct, is lengthy and with the map() function we can re-write the above code as shown below:

map_items = map(lambda x: x * x, [1, 2, 3, 4, 5])
for values in map_items:
  print(values)

Recall lambda functions? We can use lambda functions to create a function on the spot without having to use the ‘def’ keyword and here ‘x’ is the parameter.

Filter

We can use the filter() function to return a list of items based on whether or not a condition is true. The filter() function’s syntax is as follows:

• filter(<function >, <list_of_inputs>)

Consider an example where we can use this function, to return a list of numbers that are divisible by 5 within 5 to 56. A typical approach is:

number_list = list(range(5, 56))

def divisible_by_five(item):
    if item % 5 == 0:
        print(item)

for number in number_list:
    divisible_by_five(number)

But we can simplify it using filter() as shown below:

number_list = list(range(5, 56))

divisible_by_five = filter(lambda x: x % 5 == 0, number_list)

for numbers in divisible_by_five:
    print(numbers, end=" ")

This should give us:

This is similar to a for loop however filter is a built-in function and is faster.

NOTE: end=“ ” specifies that it should print the output separated by space instead of a newline.

Reduce

Reduce is different from the others in that instead of returning a list of items, it returns a singular value. The reduce() function’s syntax is as follows:

• reduce(<function>, <list_of_inputs>)

Consider an example where we want to print the sum of a list of numbers. We could write a simple program for this as shown below:

list_ = [2, 3, 4, 5, 6, 7]

sum_ = 0

for item in list_:
    sum_ += item

print("Sum is:", sum_)

But using reduce() we can re-write it as:

from functools import reduce

list_ = [2, 3, 4, 5, 6, 7]
sum_ = reduce(lambda a, b: a + b, list_)

print("Sum is:", sum_)

This will give us:

We need to use the “functools” module to work with the reduce() function. Keep in mind, that “functools” is a built-in module so you do not have to install it.

What have we learned?

• What is the map() function?
• What is the filter() function?
• What is the reduce() function?
• Which keyword do we use for these 3 functions?
• How does the reduce() function differ from the map() & filter() function?
• Which built-in module has the reduce() functions?