Python List Comprehension

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List Comprehension :

• List comprehensions provide a concise and elegant way to create lists.

• List comprehension is more compact and faster than normal functions and loops for creating list.

• We can create lists just like mathematical statements and in one line only

• The list comprehension always returns a result list.

Syntax :

It consists of brackets containing an expression followed by a for clause, then zero or more for or if clauses. The expressions can be anything, meaning you can put in all kinds of objects in lists.

The result will be a new list resulting from evaluating the expression.

A list comprehension generally consist of these parts :
1. Output expression, 
2. input sequence, 
3. a variable representing member of input sequence and
   an optional predicate part. 
    
  [ expression for item in list if conditional ]
         
              Above Syntax is equivalent to:

for item in list:
    if conditional:
        expression
                

Example : Create a list that contains number from 0 to 9.

my_list = []
for i in range(10):
    my_list.append(i)
print(my_list)
                
[OUTPUT]
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
                

We can do the same by list comprehension in one line as

my_list = [i for i in range(10)]
print(my_list)
                
[OUTPUT]
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
                

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Conditions in List Comprehension

# 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)
                
[OUTPUT]
[1, 9, 25, 49, 81]

# Without list comprehension 
odd_square = [] 
for x in range(1, 11): 
	if x % 2 == 1: 
		odd_square.append(x**2) 
print(odd_square)
                   

The list ,odd_square, will be populated square of number, by the items in range from 1-11 if the item's value is not divisible by 2.

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Nested if with List Comprehension

# below list contains numbers from 0 to 100
# which are divided by 2 and 5
num_list = [x for x in range(100) if x % 2 == 0 if x % 5 == 0]
print(num_list)
                
[OUTPUT]
[0, 10, 20, 30, 40, 50, 60, 70, 80, 90]

# Without list comprehension 
num_list = [] 
for x in range(100): 
    if x % 2 == 0:
        if x % 5 == 0:
            num_list.append(x)
print(num_list)
                   

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if...else with List Comprehension

my_list = ["even" if i%2==0 else "odd" for i in range(10)]
print(my_list)
                
[OUTPUT]
['even', 'odd', 'even', 'odd', 'even', 'odd', 'even', 'odd', 'even', 'odd']
   

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Nested loops in List Comprehension

Example :Write a Python program to find Transpose of matrix :

transposed = []
matrix = [[1, 2, 3, 4], [4, 5, 6, 8]]

for i in range(len(matrix[0])):
    transposed_row = []

    for row in matrix:
        transposed_row.append(row[i])
    transposed.append(transposed_row)

print(transposed)
                
[OUTPUT]
[[1, 4], [2, 5], [3, 6], [4, 8]]
   

As we can see, in the above code we are using two for loops to find the transpose of matrix.

Let's see how to do nested iteration inside a list comprehension.

Transpose of a Matrix using List Comprehension

matrix = [[1, 2, 3, 4], [4, 5, 6, 8]]
transpose = [[row[i] for row in matrix] for i in range(4)]
print (transpose)
                
[OUTPUT]
[[1, 4], [2, 5], [3, 6], [4, 8]]
   

The nested loops in list comprehension don’t work like normal nested loops. In the above program, for i in range(4) is executed before row[i] for row in matrix. Hence at first, a value is assigned to i then item directed by row[i] is appended in the transpose variable.

NOTE: Remember, every list comprehension can be rewritten in for loop, but every for loop can’t be rewritten in the form of list comprehension.

Some Python program to demonstrate list comprehension

# 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)
                
[OUTPUT]
[1, 9, 25, 49, 81]
                        

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# 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) 

[OUTPUT]
[2, 4, 8, 16, 32, 64, 128, 256]
  

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# 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)

[OUTPUT]
[2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47]
 

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# list for lowering the characters 
print([x.lower() for x in ["A","B","C"]])

[OUTPUT]
['a', 'b', 'c']
  

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# list which extracts number 
string = "my phone number is : 11122 !!"

print("\nExtracted digits") 
numbers = [x for x in string if x.isdigit()] 
print(numbers) 

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

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# 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]
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] 
  

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