Python - Filter Vs Map Vs Reduce Vs List Comprehension

Today we discuss about the following topics :
1) Filter
2) Map
3) List Comprehension

Lets discuss with few examples:

Filter :
filter(function, sequence) returns a sequence consisting of those items from the sequence for which function(item) is true

Map :
map(function, sequence) calls function(item) for each of the sequence's items and returns a list of the return values

List Comprehension :
List comprehension in Python provides a clear and concise syntax for creating lists from other lists


filter_test.py

#filter(function, sequence) returns a sequence consisting of those items from the sequence for which function(item) is true
def isPythonFile(list_1):
    if list_1.find(".py") == -1:
        return False
    else:
        return True
 
list_1 = ["1.py","2.pl", "3.zip", "4.py","5.php" ]
py_files = filter(isPythonFile, list_1)   #Function is called for each element of list
 
for item in py_files:
    print("Each item in Filetr :" , item)  

Output:

Each item in Filetr : 1.py
Each item in Filetr : 4.py

filter_lambda_test.py

foo = [2, 18, 9, 22, 17, 24, 8, 12, 27]
print("Before Lambda :", list(foo))
print("After  Lambda :", list(filter(lambda x: x % 3 == 0, foo)))

Output:

Before Lambda : [2, 18, 9, 22, 17, 24, 8, 12, 27]
After  Lambda : [18, 9, 24, 12, 27]  

map_test.py

print("Before Map :", list(foo))
print("After  Map :", list(map(lambda x: x * 2 + 10, foo)))  

Output:

Before Map : [2, 18, 9, 22, 17, 24, 8, 12, 27]
After  Map : [14, 46, 28, 54, 44, 58, 26, 34, 64]  

smallest_no_using_reduce_test.py
#how to use if else in lambda

from functools import reduce

ll = [10, 12, 45, 2, 100]
out = reduce(lambda x, y: x  if x < y  else y,  ll)
print(out)

Output:
2

largest_no_using_reduce_test.py
#how to use if else in lambda

from functools import reduce

ll = [10, 12, 45, 2, 100]
out = reduce(lambda x, y:  x  if x > y  else y,  ll)
print(out)

Output:
100

list_comprehension_test1.py

input_arr = [2, 3, 4]
output_arr = [2*i for i in input_arr if i > 2]
print("List Comprehension Test 1 :", output_arr)  

Output:

List Comprehension Test 1 : [6, 8]

list_comprehension_test2.py

input_arr = ['Mother Teresa', 'Abraham Lincoln', 'Nelson Mandela']
output_arr = ['Dear...' + i  for i in input_arr if len(i) > 5]
print("List Comprehension Test 2 :",output_arr)  

Output:

List Comprehension Test 2 : ['Dear...Mother Teresa', 'Dear...Abraham Lincoln', 'Dear...Nelson Mandela']  

list_comprehension_test3.py  (if-else inside list comprehension)

input_arr = [20, 30, 40, 33, 55]
output_arr = [2*i if i%2 == 0 else i for i in input_arr]
print("List Comprehension Test 3 :", output_arr)  

Output:

List Comprehension Test 3 :  [40, 60, 80, 33, 55]

Please refer to Regular Expressions Concepts :
Brief on Regular Expressions
Greedy Operators in Regular Expressions in Perl
Modifiers in Regular Expressions in Perl
Capturing concept in Regular Expressions in Perl
Capture Pre Match ,Post Match, Exact match in Regular Expressions in Perl
Non Capturing Paranthesis in Regular Expressions in Perl
Substitute nth occurance in Regular Expressions in Perl
All Topics in Regular Expressions in Perl

You might also wish to read other topics like :
Python Class and Object Example
Inheritance in Python
Packages in Python
Exceptions in Python
How to remove duplicate lines from a file in Perl
How to remove duplicate lines from a file in Pyhton