Python Map & Filter Functions

Map

Map is a built-in function in python. It is used for transformation of the objects in a list or tuple.

Map function iterates through a list or tuple (iterable objects) implicitly and applies a function in each element.

Returns a map value.

It takes 2 arguments:

1. Function to be applied
2. Iterable Object

Lets take an example. There is a list of pet animals and birds. We need to convert all the string values in the list to uppercase.

Without using Map:

pets = ['Cat','Dog','Parrot','Ant','Bird']
uppercase = []

for names in pets:
    name_upper = names.upper()
    uppercase.append(name_upper)

print(uppercase)

['CAT', 'DOG', 'PARROT', 'ANT', 'BIRD']

With Map:

myfriends = ['Cat','Dog','Parrot','Ant','Bird']
uppercase = list(map(str.upper,myfriends))
print(uppercase)

['CAT', 'DOG', 'PARROT', 'ANT', 'BIRD']

In the map function, we sent the first argument as a function str.upper. The second argument is an iterable object on which the function to be applied.

Another example: Lets say we have a list of bill price values. We need to round the values.

floatNum = [4.77,2.33,9.02,10.67,0.66,4.67]
print(floatNum)
intNew = list(map(round,floatNum))
print(intNew)

[4.77, 2.33, 9.02, 10.67, 0.66, 4.67]
[5, 2, 9, 11, 1, 5]

Filter

The Filter function applies a boolean function to each item of the iterable object and return the items only if the function returns True for it.

• Init signature: filter(self, /, *args, **kwargs)
• Docstring: filter(function or None, iterable) –> filter object
• Return an iterator yielding those items of iterable for which function(item) is true.
• If function is None, return the items that are true.
• It is similar to map but the only difference is that, filter will return the element of list only if the condition is true.

Without Filter:

Lets say we have a list of people’s age. We need to filter only the age that is eligible for voting (> 18).

# Prints the age which is >= 18
#Filter ages without filter() function
age_list = [15,18,45,90,5]
def eligibility_vote(age):
    return age >= 18

for age in age_list:
    if(eligibility_vote(age)):
        print(age)     

18
45
90

With Filter Function:

#With Filter
age_list = [15,18,45,90,5]
def eligibility_vote(age):
    return age >= 18

list(filter(eligibility_vote, age_list))

[18, 45, 90]

What if the function is not a Boolean function?

It process the function but returns all the elements as it is.

As Filter function is not a transformation function, it will not change the items of the iterable object. It will just filter the items if the function is a Boolean function.

#Filter
age_list = [15,18,45,90,5]

def eligibility_vote_all(age):
    if age >= 18:
        return age * 10
    else:
        return age * 10

#Without Filter
for age in age_list:
    if(eligibility_vote_all(age)):
        print(age)
        
new_list = list(filter(eligibility_vote_all,age_list))
print(new_list)

15
18
45
90
5
[15, 18, 45, 90, 5]

Another Example:

words = ['Myth','Eat','Cry','Mass']
vowels = ['a','e','i','o','u']

def withVowel(a):
    for c in a:
        if c in vowels:
            return True        
    return False
    
list(filter(withVowel,words))

['Eat', 'Mass']

Filter Vs Map In terms of for loops:

• Filter returns the value only if the boolean function returns True.
• Map function applies a function to all the items regardless of the return value of the function and creates a new iterable object with the result.

Map:

list(map(eligibility_vote,age_list))

[False, True, True, True, False]

Filter:

list(filter(eligibility_vote,age_list))

[18, 45, 90]