Python List Comprehension and Slicing

2.5

List comprehension is an elegant way to define and create 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 :
   Output expression, 
   input sequence, 
   a variable representing member of input sequence and
   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.
# 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

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 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.

# Let us first create a list to demonstrate slicing
# lst contains all number from 1 to 10
lst = 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

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 :

#  filtering odd numbers
lst = filter(lambda x : x % 2 == 1, range(1, 20))
print lst

#  filtering odd square which are divisble by 5
lst = filter(lambda x : x % 5 == 0, 
      [x ** 2 for x in range(1, 11) if x % 2 == 1])
print lst

#   filtering negative numbers
lst = filter((lambda x: x < 0), range(-5,5))
print lst

#  implementing max() function, using
print reduce(lambda a,b: a if (a > b) else b, [7, 12, 45, 100, 15])

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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