Map, Filter, Reduce

Map, Filter, and Reduce are paradigms of functional programming. They allow the programmer (you) to write simpler, shorter code, without neccessarily needing to bother about intricacies like loops and branching.

Essentially, these three functions allow you to apply a function across a number of iterables, in one fell swoop. map and filter come built-in with Python (in the __builtins__ module) and require no importing. reduce, however, needs to be imported as it resides in the functools module. Let's get a better understanding of how they all work, starting with map.

Map

The map() function in python has the following syntax:

map(func, *iterables)

Where func is the function on which each element in iterables (as many as they are) would be applied on. Notice the asterisk(*) on iterables? It means there can be as many iterables as possible, in so far func has that exact number as required input arguments. Before we move on to an example, it's important that you note the following:

1. In Python 2, the map() function returns a list. In Python 3, however, the function returns a map object which is a generator object. To get the result as a list, the built-in list() function can be called on the map object. i.e. list(map(func, *iterables))
2. The number of arguments to func must be the number of iterables listed.

Let's see how these rules play out with the following examples.

Say I have a list (iterable) of my favourite pet names, all in lower case and I need them in uppercase. Traditonally, in normal pythoning, I would do something like this:

my_pets = ['alfred', 'tabitha', 'william', 'arla']
uppered_pets = []

for pet in my_pets:
    pet_ = pet.upper()
    uppered_pets.append(pet_)

print(uppered_pets)

Which would then output ['ALFRED', 'TABITHA', 'WILLIAM', 'ARLA']

With map() functions, it's not only easier, but it's also much more flexible. I simply do this:

# Python 3
my_pets = ['alfred', 'tabitha', 'william', 'arla']

uppered_pets = list(map(str.upper, my_pets))

print(uppered_pets)

Which would also output the same result. Note that using the defined map() syntax above, func in this case is str.upper and iterables is the my_pets list -- just one iterable. Also note that we did not call the str.upper function (doing this: str.upper()), as the map function does that for us on each element in the my_pets list.

What's more important to note is that the str.upper function requires only one argument by definition and so we passed just one iterable to it. So, if the function you're passing requires two, or three, or n arguments, then you need to pass in two, three or n iterables to it. Let me clarify this with another example.

Say I have a list of circle areas that I calculated somewhere, all in five decimal places. And I need to round each element in the list up to its position decimal places, meaning that I have to round up the first element in the list to one decimal place, the second element in the list to two decimal places, the third element in the list to three decimal places, etc. With map() this is a piece of cake. Let's see how.

Python already blesses us with the round() built-in function that takes two arguments -- the number to round up and the number of decimal places to round the number up to. So, since the function requires two arguments, we need to pass in two iterables.

# Python 3

circle_areas = [3.56773, 5.57668, 4.00914, 56.24241, 9.01344, 32.00013]

result = list(map(round, circle_areas, range(1, 7)))

print(result)

See the beauty of map()? Can you imagine the flexibility this evokes?

The range(1, 7) function acts as the second argument to the round function (the number of required decimal places per iteration). So as map iterates through circle_areas, during the first iteration, the first element of circle_areas, 3.56773 is passed along with the first element of range(1,7), 1 to round, making it effectively become round(3.56773, 1). During the second iteration, the second element of circle_areas, 5.57668 along with the second element of range(1,7), 2 is passed to round making it translate to round(5.57668, 2). This happens until the end of the circle_areas list is reached.

I'm sure you're wondering: "What if I pass in an iterable less than or more than the length of the first iterable? That is, what if I pass range(1, 3) or range(1, 9999) as the second iterable in the above function". And the answer is simple: nothing! Okay, that's not true. "Nothing" happens in the sense that the map() function will not raise any exception, it will simply iterate over the elements until it can't find a second argument to the function, at which point it simply stops and returns the result.

So, for example, if you evaluate result = list(map(round, circle_areas, range(1, 3))), you won't get any error even as the length of circle_areas and the length of range(1, 3) differ. Instead, this is what Python does: It takes the first element of circle_areas and the first element of range(1,3) and passes it to round. round evaluates it then saves the result. Then it goes on to the second iteration, second element of circle_areas and second element of range(1,3), round saves it again. Now, in the third iteration (circle_areas has a third element), Python takes the third element of circle_areas and then tries to take the third element of range(1,3) but since range(1,3) does not have a third element, Python simply stops and returns the result, which in this case would simply be [3.6, 5.58].

Go ahead, try it.

# Python 3

circle_areas = [3.56773, 5.57668, 4.00914, 56.24241, 9.01344, 32.00013]

result = list(map(round, circle_areas, range(1, 3)))

print(result)

The same thing happens if circle_areas is less than the length of the second iterable. Python simply stops when it can't find the next element in one of the iterables.

To consolidate our knowledge of the map() function, we are going to use it to implement our own custom zip() function. The zip() function is a function that takes a number of iterables and then creates a tuple containing each of the elements in the iterables. Like map(), in Python 3, it returns a generator object, which can be easily converted to a list by calling the built-in list function on it. Use the below interpreter session to get a grip of zip() before we create ours with map()

# Python 3

my_strings = ['a', 'b', 'c', 'd', 'e']
my_numbers = [1, 2, 3, 4, 5]

results = list(zip(my_strings, my_numbers))

print(results)

As a bonus, can you guess what would happen in the above session if my_strings and my_numbers are not of the same length? No? try it! Change the length of one of them.

Onto our own custom zip() function!

# Python 3

my_strings = ['a', 'b', 'c', 'd', 'e']
my_numbers = [1, 2, 3, 4, 5]

results = list(map(lambda x, y: (x, y), my_strings, my_numbers))

print(results)

Just look at that! We have the same result as zip.

Did you also notice that I didn't even need to create a function using the def my_function() standard way? That's how flexible map(), and Python in general, is! I simply used a lambda function. This is not to say that using the standard function definition method (of def function_name()) isn't allowed, it still is. I simply preferred to write less code (be "Pythonic").

That's all about map. Onto filter()

Filter

While map() passes each element in the iterable through a function and returns the result of all elements having passed through the function, filter(), first of all, requires the function to return boolean values (true or false) and then passes each element in the iterable through the function, "filtering" away those that are false. It has the following syntax:

filter(func, iterable)

The following points are to be noted regarding filter():

1. Unlike map(), only one iterable is required.
2. The func argument is required to return a boolean type. If it doesn't, filter simply returns the iterable passed to it. Also, as only one iterable is required, it's implicit that func must only take one argument.
3. filter passes each element in the iterable through func and returns only the ones that evaluate to true. I mean, it's right there in the name -- a "filter".

Let's see some examples

The following is a list (iterable) of the scores of 10 students in a Chemistry exam. Let's filter out those who passed with scores more than 75...using filter.

# Python 3
scores = [66, 90, 68, 59, 76, 60, 88, 74, 81, 65]

def is_A_student(score):
    return score > 75

over_75 = list(filter(is_A_student, scores))

print(over_75)

The next example will be a palindrome detector. A "palindrome" is a word, phrase, or sequence that reads the same backwards as forwards. Let's filter out words that are palindromes from a tuple (iterable) of suspected palindromes.

# Python 3
dromes = ("demigod", "rewire", "madam", "freer", "anutforajaroftuna", "kiosk")

palindromes = list(filter(lambda word: word == word[::-1], dromes))

print(palindromes)

Which should output ['madam', 'anutforajaroftuna'].

Pretty neat huh? Finally, reduce()

Reduce

reduce applies a function of two arguments cumulatively to the elements of an iterable, optionally starting with an initial argument. It has the following syntax:

reduce(func, iterable[, initial])

Where func is the function on which each element in the iterable gets cumulatively applied to, and initial is the optional value that gets placed before the elements of the iterable in the calculation, and serves as a default when the iterable is empty. The following should be noted about reduce(): 1. func requires two arguments, the first of which is the first element in iterable (if initial is not supplied) and the second element in iterable. If initial is supplied, then it becomes the first argument to func and the first element in iterable becomes the second element. 2. reduce "reduces" (I know, forgive me) iterable into a single value.

As usual, let's see some examples.

Let's create our own version of Python's built-in sum() function. The sum() function returns the sum of all the items in the iterable passed to it.

# Python 3
from functools import reduce

numbers = [3, 4, 6, 9, 34, 12]

def custom_sum(first, second):
    return first + second

result = reduce(custom_sum, numbers)
print(result)

The result, as you'll expect is 68.

So, what happened?

As usual, it's all about iterations: reduce takes the first and second elements in numbers and passes them to custom_sum respectively. custom_sum computes their sum and returns it to reduce. reduce then takes that result and applies it as the first element to custom_sum and takes the next element (third) in numbers as the second element to custom_sum. It does this continuously (cumulatively) until numbers is exhausted.

Let's see what happens when I use the optional initial value.

# Python 3
from functools import reduce

numbers = [3, 4, 6, 9, 34, 12]

def custom_sum(first, second):
    return first + second

result = reduce(custom_sum, numbers, 10)
print(result)

The result, as you'll expect, is 78 because reduce, initially, uses 10 as the first argument to custom_sum.

That's all about Python's Map, Reduce, and Filter. Try on the below exercises to help ascertain your understanding of each function.

Exercise

In this exercise, you'll use each of map, filter, and reduce to fix broken code.

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