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Built-in Map/Reduce functions in Python

Built-in Map/Reduce functions in Python

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Prahlad Inala
ยทJul 16, 2021ยท

2 min read

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What are Built-in Map/Reduce functions in Python?

The map is a built-in function that takes a function and an iterable like a list(or)tuple(or)set and runs the given function on each element in that and returns a new list with the returned values.

Example

let's say we have a list of numbers and we want a new list with squares of these numbers

my_list=[1,2,3]
def square(num):
    return num**2 

in a way we use for loop

my_list=[1,2,3]
def square(num):
    return num**2 
squared=[]
for x in my_list:
    squared.append(square(x))

now we use our function Map to do this in one line

squared=map(square,my_list)
print(type(squared))  

the filter is a built-in function that takes a function and an iterable like a list and returns a new iterable with only those values for which the function returns True...

Example

let us say we have a list of numbers and we want to retain only odd numbers in it.

my_list=[1,2,3,4,5,6,7,8]
def is_odd(num):
    return num%2==1

now we use the filter function

filtered=filter(is_odd,my_list)
print(type(filtered))
#<class 'filter'>

we can iterate over the result using for loop

for x in filtered:
    print(x)
#1
#3
#5
#7

I want to convert it into a list so

my_filtered_list=list(filtered)
print(my_filtered_list)
#[]

Example

my_list=['john','mary','doe','jane']

def lower(str):
    return str.lower()

result=map(lower,my_list)                        
print(type(result))               
#<class 'map'>                                 

result_list=list(result)

print(len(result_list))           
#4
print(result_list[3])             
#jane  

Try it Yourself

write a Python function called filter_list(my_list) which using the filter function to filter the input lists of strings so that only those strings which have the sub-string 'ing' are returned as a list. and also add the 'ing' to the remaining strings using the map function.

my_list=['run','running','code','coding','flinger','ringing']
output_list=filter_list(my_list)
#out_list should be filtered to have only: ing listed.    

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