This article shows how to use filter() function.
Contents
- Basic usage of filter( ) function
- Combination of filter( ) and lambda function
- Applying filter( ) function to a dictionary
- Comparison of filter( ) and map( ) functions
sponsored link
Basic usage of filter( ) function
In the beginning, I show basic usage of filter( ) function.
By using filter( ) function, elements that meet conditions can be extracted from the iterable object.
The filter( ) function is built-in function in Python.
So, we can use filter( ) function without "import".
Specify function as the first argument and iterable object (list, tuple, etc.) as the second argument.
How to write
filter( function, iterable )
The filter() function extracts elements from a list for which a function returns True.
Applying filter() function to a list
I show how to apply filter() function to a list.
This is an example code.
Only odd-numbered elements are extracted from a list.
my_list = list(range(10))
print(my_list)
# [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
def is_odd(x):
return x % 2 == 1
print(filter(is_odd, my_list))
# <filter object at 0x000001A01E164F28>
print(list(filter(is_odd, my_list)))
# [1, 3, 5, 7, 9]> def is_odd(x):
> return x % 2 == 1
The function is defined that returns True if the remainder divided by 2 is 1.
> print(filter(is_odd, my_list))
> # <filter object at 0x000001A01E164F28>
A filter object is generated.
> print(list(filter(is_odd, my_list)))
> [1, 3, 5, 7, 9]
Since the elements cannot be checked as a filter object, they are converted to list type with the list( ) function.
Combination of filter( ) and lambda function
I show how to use filter( ) function in combination with lambda.
If a function is simple, lambda can be used to write concise code.
This is an example code.
my_list = list(range(10))
print(my_list)
# [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
print(list(filter(lambda x : x % 2 == 1, my_list)))
# [1, 3, 5, 7, 9]> print(list(filter(lambda x : x % 2 == 1, my_list)))
The function is defined using lambda.
How to use lambda is explained in this article.
Extract elements with a specified number of characters using filter() and len() functions.
I show another example code.
By using len( ) function, elements with a specified number of characters are extracted.
list_fruits = ['Apple', 'Banana', 'Orange', 'Grape', 'Mango', 'Lemon', 'Peach', 'Plum']
print(list(filter(lambda x : len(x) == 5, list_fruits)))
# ['Apple', 'Grape', 'Mango', 'Lemon', 'Peach']sponsored link
Applying filter( ) function to a dictionary
I show how to apply filter() function to a dictionary.
Applying filter( ) function to dictionary keys
This is an example code.
dict_fruits1 = {'Apple':100, 'Banana':200, 'Grape':500, 'Orange':100}
keys = set(['Banana', 'Orange'])
dict_fruits2 = dict(filter(lambda fruit : fruit[0] in keys, dict_fruits1.items()))
print(dict_fruits2)
# {'Banana': 200, 'Orange': 100}> keys = set(['Banana', 'Orange'])
Describe the keys to be extracted.
set( ) function is used to convert a list to set type.
> dict_fruits2 = dict(filter(lambda fruit : fruit[0] in keys, dict_fruits1.items()))
It is in the form of "dict( filter( function, iterable ) )"
Elements extracted by filter( ) function are converted to dictionary type by dict( ) function.
As "iterable", "dict_fruits1.items()" is specified.
Applying items( ) method to a dictionary allows you to get all keys and elements contained in a dictionary.
As "function", "lambda fruit : fruit[0] in keys" is specified.
The function returns True if "fruit[0]" is contained in "keys".
Applying filter( ) function to dictionary values
This is an example code.
dict_fruits1 = {'Apple':100, 'Banana':200, 'Grape':500, 'Orange':100}
dict_fruits2 = dict(filter(lambda fruit : fruit[1] >= 200, dict_fruits1.items()))
print(dict_fruits2)
# {'Banana': 200, 'Grape': 500}Comparison of filter( ) and map( ) functions
As a supplement, I show difference between filter( ) and map( ) function.
Like filter( ) function, map( ) also takes "function" and "iterable" as arguments.
How to write
map( function, iterable )
By using map(), function can be applied to all elements of the iterable object( list, tuple, etc.).
This is an example code.
The same user-defined function is specified as argument for filter( ) and map( ) function.
dict_fruits1 = {'Apple':100, 'Banana':200, 'Grape':500, 'Orange':100}
def my_func(x) :
return x[1] >= 200
print(list(map(my_func, dict_fruits1.items())))
# [False, True, True, False]
print(list(filter(my_func, dict_fruits1.items())))
# [('Banana', 200), ('Grape', 500)]How to use map( ) function is explained in this article.
sponsored link