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# Filter Function in Python

Last Updated on July 2, 2023 by Mayank Dham In this article, we will delve into the extensive library of Python, a widely-used programming language known for its convenience in application development. Among the many built-in functions available, we will focus on the filter() function in Python. This particular function serves the purpose of extracting specific data from an iterable by applying a specified condition. By employing the filter() function in Python, a new iterable object is generated, exclusively containing elements that meet the given condition. Throughout this article, we will thoroughly examine the filter function in Python, including its syntax and practical implementation.

## What is the Filter Function in Python?

In Python, the filter function is a pre-existing function designed to extract elements from an iterable based on a specified condition. It necessitates two arguments: the first being a function that outlines the condition, and the second being the iterable from which the elements are to be filtered. Typically, the function argument takes the form of a lambda function, which is a compact, unnamed function capable of accepting multiple arguments and returning a Boolean value. When invoked, the filter function in Python yields a filter object, which is essentially an iterable encompassing the elements from the original iterable that meet the given condition. If desired, the output of the filter function in Python can be transformed into a list using the list() function.

### Syntax of the Filter Function in Python

The syntax of the filter function in python is as follows:

``filter(function, iterable)``

### Parameters of Filter Function in Python

It will take two arguments.

• Function: It is the function that is used to filter the elements.
• Iterable: It is the iterable object that is to be filtered.

### Return Type of Filter Function in Python

The return type of the filter function in Python is a filter object. The filter object is an iterable containing the elements from the original iterable that satisfy the given condition specified by the function argument. The filter object can be converted to other data types, such as a list or a tuple, using built-in functions like list() or tuple(). It’s important to note that the filter function in python doesn’t modify the original iterable, but instead creates a new iterable containing the filtered elements.

Example 1 of Filter Function in Python: Working
The filter function in python works by applying the given function to each element of the iterable object. If the function returns True for an element, it is included in the new iterable object. If the function returns False, the element is excluded from the new iterable.

The filter function in python creates a new iterable object that contains only the elements that satisfy the given condition. The original iterable object remains unchanged.

```def is_even(num):
if num % 2 == 0:
return True
else:
return False

numbers = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
even_numbers = list(filter(is_even, numbers))

print(even_numbers)
```

Output

``[2, 4, 6, 8, 10]``

Explanation of the above example
In this example, we define a function is_even() that takes a number as input and returns True if the number is even, and False otherwise. We then create a list of numbers and apply the filter() function to the list, with the is_even() function as the first argument.

The filter() function returns a new iterable object that contains only the even numbers from the original list. We then convert this iterable to a list using the list() function and print the result.

Example 2 of Filter Function in Python: Using lambda functions with the filter() function
Lambda functions are anonymous functions in Python that can be defined in a single line. The filter() function is often used with lambda functions to filter an iterable object based on a condition.

```numbers = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
even_numbers = list(filter(lambda x: x % 2 == 0, numbers))

print(even_numbers)
```

Output

``[2, 4, 6, 8, 10]``

Explanation of the above code
In this example, we define a lambda function that takes a number as input and returns True if the number is even. We then apply the filter() function to the list of numbers, with the lambda function as the first argument.

The filter() function returns a new iterable object that contains only the even numbers from the original list. We then convert this iterable to a list using the list() function and print the result.

Example 3 of Filter Function in Python: Using the filter() function with strings
In addition to working with numerical values, the filter() function can also be used with strings to filter out characters that meet a certain condition. Let’s look at an example to see how this works:

```string = "Hello, World!"
vowels = list(filter(lambda x: x in ['a', 'e', 'i', 'o', 'u'], string))

print(vowels)
```

Output

``['e', 'o', 'o']``

Explanation of the above example
In this example, we define a string variable string and then use the filter() function to extract all vowels from it. We use a lambda function to check if each character in the string is a vowel or not. The lambda function returns True if the character is a vowel, and False otherwise. The filter() function then applies this lambda function to each character in the string and returns only those characters for which the function returns True.

We convert the resulting iterable object to a list using the list() function and print the result.

Example 4 of Filter Function in Python: Using the filter() function with dictionaries
The filter() function can also be used with dictionaries to filter out key-value pairs that meet a certain condition.

```student_scores = {'John': 85, 'Alice': 92, 'Bob': 77, 'Charlie': 88}

passed_students = dict(filter(lambda x: x >= 80, student_scores.items()))

print(passed_students)
```

Output

``{'Charlie': 88, 'John': 85, 'Alice': 92}``

Explanation of the above code
In this example, we define a dictionary student_scores that contains the scores of four students. We use the filter() function to extract the key-value pairs for which the value (i.e., the score) is greater than or equal to 80. We use a lambda function to check this condition. The lambda function takes a key-value pair as input (i.e., a tuple), and returns True if the value of the key-value pair is greater than or equal to 80, and False otherwise.

The filter() function applies this lambda function to each key-value pair in the dictionary and returns only those key-value pairs for which the function returns True. We convert the resulting iterable object to a dictionary using the dict() function and print the result.

## Applications of Filter Function in Python

The filter() function in Python has many applications in data manipulation and analysis. Here are some common use cases:

• Data Cleaning: The filter() function is often used in data cleaning to remove unwanted data from datasets. For example, we can use the filter() function to remove null or missing values from a dataset.
• Data Analysis: The filter() function is often used in data analysis to select certain subsets of data based on specific conditions. For example, we can use the filter() function to select all the values that are greater than a certain threshold value from a dataset.
• Data Transformation: The filter() function can be used to transform data by selecting specific elements from an iterable based on a given condition. For example, we can use the filter() function to select all the even numbers from a list of integers.
• Data Visualization: The filter() function can be used to filter out unwanted data from datasets before visualizing the data. This can help to create more accurate and meaningful visualizations.
• Machine Learning: The filter() function can be used in machine learning algorithms to preprocess data before training models. For example, we can use the filter() function to remove outliers or noisy data points from a dataset.

Conclusion
The filter() function in Python is a powerful tool for extracting specific elements from an iterable based on a given condition. It simplifies the process of filtering data by providing a concise and efficient solution. By using a function argument, often implemented as a lambda function, the filter() function returns a filter object that contains only the elements satisfying the specified condition. This makes it convenient to work with filtered data in further processing or analysis.

## FAQs related to Filter function in Python:

Q1: Can the filter() function be applied to any type of iterable in Python?
Yes, the filter() function can be used with any iterable, such as lists, tuples, sets, or even custom objects, as long as the condition and function argument are appropriately defined.

Q2: How does the filter() function differ from list comprehensions in Python?
While both filter() and list comprehensions can be used to filter data, they have different approaches. The filter() function is more suitable when you want to apply a specific condition to an iterable and obtain a filter object, while list comprehensions provide a concise way to generate a new list based on a condition.

Q3: Is the filter object returned by the filter() function mutable?
No, the filter object is an iterable and, by itself, does not support direct item modification. However, you can convert it into a list or other mutable data structure if you need to modify the filtered elements.

Q4: Can I chain multiple filter() functions together?
Yes, you can chain multiple filter() functions by passing the filter object of one function as the iterable argument to another filter() function, allowing you to apply multiple conditions sequentially.

Q5: Are there any performance considerations when using the filter() function?
The filter() function is efficient and optimized for performance. However, keep in mind that applying complex or computationally intensive conditions may impact performance, so it’s essential to consider the efficiency of the condition function used.

Q6: Can the filter() function be used with non-Boolean conditions?
No, the condition provided to the filter() function must evaluate to a Boolean value. If you want to apply a non-Boolean condition, you can use other methods like list comprehensions or generator expressions.