LIST COMPREHENSION IN PYTHON

In this article, we are going to be be looking into list comprehension, how to use it with some examples and some key points to note when making use of list comprehension.

List comprehension is a short and precise way of creating list based on existing list(it can also be any iterables). It combines both the for loop and creation of elements into one line and append the elements into a new list automatically which will be return as the result. Note that list comprehension always return a list as the output.

Syntax (without conditions):

[ exp for item in iterable]

With conditions:

[ exp for item in iterable if exp]

Example:

Supposed we were asked to write a code that returns a list containing the square of the first 10 integers. Using list comprehension

squares = [ x**2 for x in range (10)]

Explanation

1) To create a list comprehension, we start with the name of the new list we are creating. In this case, the name of the new list is squares

2) open a set of brackets and define the expression for the elements you want to store in the new list. In the example above, the expression is x**2

3) Then write a for loop to generate number you want to feed into the expression. In this case we have for x in range(10) . Note: the for loop will not end with a semi-colon(:)

List comprehension vs python for loop

Example

write a code to separate letter from string

using python for loop

letters = []
for i in "human":
    letters.append(i)
print(letters)

Output

['h', 'u', 'm', 'a', 'n']

Using list comprehension

letters = [i for i in "human"]

List comprehension with if statement

Example

Write a code to output the first 5 even number

Using python for loop

even = []
for i in range(1,11):
    if i%2==0:
        even.append(i)
print(even)

Output

[2,4,6,8,10]

Using list comprehension

even = [i for i in range(1,11) if i%2==0]

List comprehension with nested for loop

Example

even = [2,4,6,8]
odd = [1,3,5,7]
even_odd = [(x,y) for x in even for y in odd]
print(even_odd)

Output

[(2, 1), (2, 3), (2, 5), (2, 7), (4, 1), (4, 3), (4, 5), (4, 7), (6, 1), (6, 3), (6, 5), (6, 7), (8, 1), (8, 3), (8, 5), (8, 7)]

List comprehension with if and else statement

Example

even_odd = ["even" if i%2 == 0 else "odd" for i in range(50)]
print(even_odd)

Output

['even', 'odd', 'even', 'odd', 'even', 'odd', 'even', 'odd', 'even', 'odd', 'even', 'odd', 'even', 'odd', 'even', 'odd', 'even', 'odd', 'even', 'odd', 'even', 'odd', 'even', 'odd', 'even', 'odd', 'even', 'odd', 'even', 'odd', 'even', 'odd', 'even', 'odd', 'even', 'odd', 'even', 'odd', 'even', 'odd', 'even', 'odd', 'even', 'odd', 'even', 'odd', 'even', 'odd', 'even', 'odd']

NOTE

I) Avoid writing very long list comprehensions in one line ii) Every list comprehension can be rewritten in for loop, but it is not true for the other way round