Python Programming Language List Comprehensions

What is list comprehensions?

• List comprehension provides the option to create lists. Creating new lists where each element is the result of some operations applied to each member of another sequence or iterable, or to create a subsequence of those elements that satisfy a certain condition.
• List comprehension provides the new list result based on evaluating the expression in the context of the for loop and if clauses.
• List comprehension consists of brackets (starting '[' and ending ']')containing an expression followed by a for clause, then zero or more for or if clauses.

List comprehensions syntax:

[ expression for item in list if conditional ]

lists = [val*2 for val in range(6)]
print lists;

[0, 2, 4, 6, 8, 10]

lists = []
for val in range(10):
    lists.append(val*2)

print lists;

List comprehensions with filter if clauses

lists = [val*3 for val in range(10) if val%2 == 0]                              
print lists; 

[0, 6, 12, 18, 24]

List comprehensions general format

*new_list*  = [*transform*    *iteration*     *filter*]

List comprehensions with calling method

numbers = ['  one', '  two ', 'three  ']                                        
new_list = [number.strip() for number in numbers]                               
print new_list; 

$ python list-coprehension.py
['one', 'two', 'three']

List comprehensions for each element starts with capital first letter in list

numbers = ['one', 'two', 'three']                                               
new_list = [(number[0].upper() + number[1:]) for number in numbers]             
print new_list;   

$ python list-coprehension.py
['One', 'Two', 'Three']

List comprehensions to extract numbers in string

string = "test 2345 numbers 322 43!";                                           
new_list = [num for num in string if num.isdigit()]                             
print new_list;  

$ python list-coprehension.py
['2', '3', '4', '5', '3', '2', '2', '4', '3']

List comprehensions to extract letters in string

string = "test 2345 numbers 322 43!";                                           
new_list = [ch for ch in string if ch.isalpha()]                                
print new_list;                                                                 

$ python list-coprehension.py
['t', 'e', 's', 't', 'n', 'u', 'm', 'b', 'e', 'r', 's']

List comprehensions to extract lower letters in string

string = "Test 2345 Numbers 322 43!";                                           
new_list = [ch for ch in string if ch.islower()]                                
print new_list; 

$ python list-coprehension.py
['e', 's', 't', 'u', 'm', 'b', 'e', 'r', 's']

List comprehensions with tuple values

new_list = [num, num**3 for num in range(1, 10)]                                
print new_list; 

$ python list-coprehension.py
  File "list-coprehension.py", line 1
    new_list = [num, num**3 for num in range(1, 10)]
                              ^
SyntaxError: invalid syntax

new_list = [(num, num**3) for num in range(1, 10)]                              
print new_list;

$ python list-coprehension.py
[(1, 1), (2, 8), (3, 27), (4, 64), (5, 125), (6, 216), (7, 343), (8, 512), (9, 729)]

List comprehensions with array values

new_list = [[num, num**2, num**3] for num in range(1, 10)]                      
print new_list;

$ python list-coprehension.py
[[1, 1, 1], [2, 4, 8], [3, 9, 27], [4, 16, 64], [5, 25, 125], [6, 36, 216], [7, 49, 343], [8, 64, 512], [9, 81, 729]]

List comprehensions with math function

from math import pi                                                             
new_list = [str(round(pi, i)) for i in range(5)]                                
print new_list;  

$ python list-coprehension.py
['3.0', '3.1', '3.14', '3.142', '3.1416']

Nested List Comprehensions

Finding Average Mark with different rounding decimals

marks = [[67,87,54], [98, 56, 78, 86], [56, 89]];                               
new_list = [[round(float(sum(mark))/len(mark), i) for i in range(1,4)] for mark in marks]
print new_list;  

$ python list-coprehension.py
[[69.3, 69.33, 69.333], [79.5, 79.5, 79.5], [72.5, 72.5, 72.5]]

Finds prime numbers

no_primes = [no_prime for div in range(2, 8) for no_prime in range(div*2, 100, div)]
primes = [num for num in range(2, 100) if num not in no_primes]                 
print primes;  

$ python list-coprehension.py
[2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47, 53, 59, 61, 67, 71, 73, 79, 83, 89, 97]