Python List Comprehension and Slicing

List comprehension is an elegant way to define and create a list in python. We can create lists just like mathematical statements and in one line only. The syntax of list comprehension is easier to grasp.

A list comprehension generally consist of these parts :

  1. Output expression,
  2. Input sequence,
  3. A variable representing a member of the input sequence and
  4. An optional predicate part.
For example :

lst  =  [x ** 2  for x in range (1, 11)   if  x % 2 == 1] 

here, x ** 2 is output expression, 
      range (1, 11)  is input sequence, 
      x is variable and   
      if x % 2 == 1 is predicate part.

Example 1:

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# Python program to demonstrate list comprehension in Python 
  
# below list contains square of all odd numbers from
# range 1 to 10
odd_square = [x ** 2 for x in range(1, 11) if x % 2 == 1]
print odd_square
  
# for understanding, above generation is same as,
odd_square = []
for x in range(1, 11):
    if x % 2 == 1:
        odd_square.append(x**2)
print odd_square
  
# below list contains power of 2 from 1 to 8
power_of_2 = [2 ** x for x in range(1, 9)]
print power_of_2
  
# below list contains prime and non-prime in range 1 to 50
noprimes = [j for i in range(2, 8) for j in range(i*2, 50, i)]
primes = [x for x in range(2, 50) if x not in noprimes]
print primes
  
# list for lowering the characters
print [x.lower() for x in ["A","B","C"]]
  
# list which extracts number
string = "my phone number is : 11122 !!"
  
print("\nExtracted digits")
numbers = [x for x in string if x.isdigit()]
print numbers
  
# A list of list for multiplication table
a = 5
table = [[a, b, a * b] for b in range(1, 11)]
  
print("\nMultiplication Table")
for i in table:
    print i

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Output:

[1, 9, 25, 49, 81]
[1, 9, 25, 49, 81]
[2, 4, 8, 16, 32, 64, 128, 256]
[2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47]
['a', 'b', 'c']

Extracted digits
['1', '1', '1', '2', '2']

Multiplication Table
[5, 1, 5]
[5, 2, 10]
[5, 3, 15]
[5, 4, 20]
[5, 5, 25]
[5, 6, 30]
[5, 7, 35]
[5, 8, 40]
[5, 9, 45]
[5, 10, 50]

After getting the list, we can get a part of it using python’s slicing operator which has the following syntax:




[start : stop : steps]  

which means that slicing will start from index start
 will go up to stop in step of steps. 
 Default value of start is 0, stop is last index of list
 and for step it is 1 

So [: stop] will slice list from starting till stop index and [start : ] will slice list from start index till end Negative value of steps shows right to left traversal instead of left to right traversal that is why [: : -1] prints list in reverse order.

Example 2:

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# Let us first create a list to demonstrate slicing
# lst contains all number from 1 to 10
lst =list(range(1, 11))
print (lst)
   
#  below list has numbers from 2 to 5
lst1_5 = lst[1 : 5]
print (lst1_5)
   
#  below list has numbers from 6 to 8
lst5_8 = lst[5 : 8]
print (lst5_8)
   
#  below list has numbers from 2 to 10
lst1_ = lst[1 : ]
print (lst1_)
   
#  below list has numbers from 1 to 5
lst_5 = lst[: 5]
print (lst_5)
   
#  below list has numbers from 2 to 8 in step 2
lst1_8_2 = lst[1 : 8 : 2]
print (lst1_8_2)
   
#  below list has numbers from 10 to 1
lst_rev = lst[ : : -1]
print (lst_rev)
   
#  below list has numbers from 10 to 6 in step 2
lst_rev_9_5_2 = lst[9 : 4 : -2]
print (lst_rev_9_5_2)

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Output:

[1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
[2, 3, 4, 5]
[6, 7, 8]
[2, 3, 4, 5, 6, 7, 8, 9, 10]
[1, 2, 3, 4, 5]
[2, 4, 6, 8]
[10, 9, 8, 7, 6, 5, 4, 3, 2, 1]
[10, 8, 6]

We can use filter function to filter a list based on some condition provided as a lambda expression as first argument and list as second argument, example of which is shown below :

Example 3:

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import functools
  
#  filtering odd numbers
lst = filter(lambda x : x % 2 == 1, range(1, 20))
print (list(lst))
   
#  filtering odd square which are divisible by 5
lst = filter(lambda x : x % 5 == 0
      [x ** 2 for x in range(1, 11) if x % 2 == 1])
print (list(lst))
   
#   filtering negative numbers
lst = filter((lambda x: x < 0), range(-5,5))
print (list(lst))
   
#  implementing max() function, using
print (functools.reduce(lambda a,b: a if (a > b) else b, [7, 12, 45, 100, 15]))

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Output:

[1, 3, 5, 7, 9, 11, 13, 15, 17, 19]
[25]
[-5, -4, -3, -2, -1]
100

This article is contributed by Utkarsh Trivedi. Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above




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