[Python] How to Use Comprehension [List, Dictionary, if]

This article shows how to use comprehension.

Comprehension in Python provide us with a concise way to construct new sequences (such as list, set, dictionary etc.).

Basics of Comprehension

In the beginning, I explain briefly basics of comprehension.

Comprehension is often used to construct new list.

The syntax of list comprehension is as stated below.

The syntax includes "for statement".

Elements are taken from iterable(list, tuple, etc.) and calculated based on "expression".

A new list is created from the calculation results.

This is an example code.

num_list = list(range(6))
print(num_list)
# [0, 1, 2, 3, 4, 5]

double_list = [num*2 for num in num_list]

print(double_list)
# [0, 2, 4, 6, 8, 10]

> double_list = [num*2 for num in num_list]

Elements are taken from "num_list" with the variable name "num" and multiplied by 2.

[ Supplement ] The same process without list comprehension.

num_list = list(range(6))
print(num_list)
# [0, 1, 2, 3, 4, 5]

double_list = []
for num in num_list :
    double_list.append(num*2)

print(double_list)
# [0, 2, 4, 6, 8, 10]

Use of comprehension allows for concise descriptions.

[ Supplement ] Combination of map( ) function and lambda.

num_list = list(range(6))
print(num_list)
# [0, 1, 2, 3, 4, 5]

double_list = map(lambda num : num*2, num_list)
print(list(double_list))
# [0, 2, 4, 6, 8, 10]

This method can also be described on a single line.

It is a matter of taste, but I think list comprehension is more concise.

How to use of map( ) function and lambda are explained in this article.

How to Use map( ) Function
How to Use Lambda

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Combination of comprehension and if statement

Comprehension can be combined with if statement.

By using "if statement", a new list can be created using only elements that satisfy the condition.

This is an example code.

Only odd-numbered elements are doubled.

num_list = list(range(6))
print(num_list)
# [0, 1, 2, 3, 4, 5]

double_list = [num*2 for num in num_list if num%2 == 1]

print(double_list)
# [2, 6, 10]

> if num%2 == 1

Even and odd numbers are determined by whether the remainder divided by 2 is 1 or not.

This is another example code.

A new list is created with elements that contain certain letters.

fruits = ['Apple', 'Banana', 'Orange', 'Peach', 'Lemon']

fruits_new = [ fruit for fruit in fruits if 'n' in fruit]

print(fruits_new)
# ['Banana', 'Orange', 'Lemon']

> if 'n' in fruit

The "if statement" become True if the string contains "n".

Combination of comprehension and if-else statement

Comprehension can be combined with if-else statement.

The syntax of list comprehension with if-else statement is as stated below.

This is an example code.

By determining whether the elements of the original list are even or odd, a new list is created using the strings "odd" and "even".

num_list = list(range(6))
print(num_list)
# [0, 1, 2, 3, 4, 5]

num_types = ["odd" if num %2 == 1 else "even" for num in num_list]
print(num_types)
# ['even', 'odd', 'even', 'odd', 'even', 'odd']

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Combination of comprehension and enumerate() function

I show combination of comprehension and enumerate( ) function.

By using the enumerate( ) function, the elements and indices of iterable objects (such as list, tuple, etc.) are got simultaneously.

This is an example code.

fruits = ['Apple', 'Banana', 'Orange', 'Peach', 'Lemon']

list_fruits = [(num, fruit) for num, fruit in enumerate(fruits) if 'n' in fruit]
print(list_fruits)
# [(1, 'Banana'), (2, 'Orange'), (4, 'Lemon')]

How to use of enumerate( ) function is explained in this article.

How to use of enumerate( ) function

Combination of comprehension and zip() function

I show combination of comprehension and zip( ) function.

The zip( ) function creates an iterator that will aggregate elements from multiple iterables.

This is an example code.

fruits = ['Apple', 'Banana', 'Orange', 'Peach', 'Lemon']
prices = [100, 200, 100, 200, 100]

list_fruits = [(fruit, price) for fruit, price in zip(fruits, prices) if 'n' in fruit]
print(list_fruits)
# [('Banana', 200), ('Orange', 100), ('Lemon', 100)]

How to use of zip( ) function is explained in this article.

How to use of zip( ) function

Dictionary comprehension

Dictionary can also be created by using comprehension.

Specify "key" and "value" in comprehension.

This is an example code.

fruits_list = ['Apple', 'Banana', 'Orange']

fruits_dict = {fruit : len(fruit) for fruit in fruits_list}

print(type(fruits_dict))
# <class 'dict'>
print(fruits_dict)
# {'Apple': 5, 'Banana': 6, 'Orange': 6}

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