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 consists of these parts :
- Output expression,
- Input sequence,
- A variable representing a member of the 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.
Another example :
lst=[x**2 if x%2==1 else x*2 for x in range(1,11)]
Example 1:
Python3
odd_square = [x ** 2 for x in range(1, 11) if x % 2 == 1]
print (odd_square)
odd_square = []
for x in range(1, 11):
if x % 2 == 1:
odd_square.append(x**2)
print (odd_square)
power_of_2 = [2 ** x for x in range(1, 9)]
print (power_of_2)
lst=[x**2 if x%2==1 else x*2 for x in range(1,11)]
print(lst)
lst1=[]
for x in range(1,11):
if x%2==1:
lst1.append(x**2)
else:
lst1.append(x*2)
print(lst1)
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)
print ([x.lower() for x in ["A","B","C"]] )
string = "my phone number is : 11122 !!"
print("\nExtracted digits")
numbers = [x for x in string if x.isdigit()]
print (numbers)
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:
Python3
lst =list(range(1, 11))
print (lst)
lst1_5 = lst[1 : 5]
print (lst1_5)
lst5_8 = lst[5 : 8]
print (lst5_8)
lst1_ = lst[1 : ]
print (lst1_)
lst_5 = lst[: 5]
print (lst_5)
lst1_8_2 = lst[1 : 8 : 2]
print (lst1_8_2)
lst_rev = lst[ : : -1]
print (lst_rev)
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 the filter function to filter a list based on some condition provided as a lambda expression as the first argument and list as the second argument, an example of which is shown below :
Example 3:
Python3
import functools
lst = filter(lambda x : x % 2 == 1, range(1, 20))
print (list(lst))
lst = filter(lambda x : x % 5 == 0,
[x ** 2 for x in range(1, 11) if x % 2 == 1])
print (list(lst))
lst = filter((lambda x: x < 0), range(-5,5))
print (list(lst))
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
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