Using filter() with Lambda in Python

In Python, the filter() function is used to filter elements of an iterable (e.g., a list) based on a certain condition. When combined with the lambda function, you can create a concise and inline way to specify the filtering condition. Python filter() with the lambda expression is the most used way to filter the elements from the iterable like set, list, tuple, and dictionary.

Let’s first quickly see the filter() function and lambda expression usage separately in Python and then we will learn how to use them together with an example.

Related: Python filter() with List

1. Quick Examples of filter() with Lambda

If you are in a hurry, below are some quick examples of using filter() and lambda together.

# Quick examples of filter() with lambda 

# Example 1: Lambda example 
# With single argument
square = lambda x: x * x
print(square(4))

# Example 2: Using filter() 
# With user defined function
numbers = [1,2,3,4,5,6,7,8]
def even(i):
    return (i%2 == 0)
print(list(filter(even, numbers)))

# Example 3: Lambda with filter()
numbers = [1, 2, 3, 4, 5, 6, 7, 8]
even_numbers = list(filter(lambda x: x % 2 == 0, numbers))

# Example 4: Filter list from another list
numbers = [1, 2, 3, 4, 5, 6, 7, 8]
filter_list = [3,5,7,8]
result = list(filter(lambda x: x not in filter_list, numbers))

2. Usage of Lambda Expression

A lambda is a small anonymous function, an anonymous meaning function without a name. It can take a single argument, executes the expression, and returns a result.

In the below example, the lambda function lambda x: x * x takes a single argument x and returns the square of that argument (x*x). When you call the lambda function with the argument 4, it calculates and returns the square of 4, which is 16.

# Syntax of lambda
lambda arguments: expression

# Lambda example with single argument
square = lambda x: x * x
print(square(4))

# Output:
# 16

3. Usage of filter()

The filter() is a built-in function that is used to filter the elements from the iterable object like list, set, tuple, etc. This function will directly filter the elements in an iterable by taking the condition as a function. Here is an example.

# Syntax of filter()
filter(function, iterable)

# Usage of filter()
# Create numbers list
numbers = [1,2,3,4,5,6,7,8]
print("Numbers:",numbers)

# Function that return True for even.
def even(i):
    return (i%2 == 0)

# Using filter() with user defined function
print("Even Numbers: ",list(filter(even, numbers)))

Yields below output.

4. Python Lambda with Filter() Example

Let’s see the same above example by using the Python filter() with lambda, By using these two together you can write the above example(finding even numbers) in a single line.

In the below example, we are filtering out the odd numbers from the list and converting the result into a list using the list() function. Here, the lambda function takes one argument x and returns True if x is even, False otherwise.

# Lambda with filter()
numbers = [1, 2, 3, 4, 5, 6, 7, 8]
even_numbers = list(filter(lambda x: x % 2 == 0, numbers))
print(even_numbers) 

# Output: 
# [2, 4, 6, 8]

5. Get List Values not Present in Another List

Let’s see another example of filtering list values using the python filter with lambda, here, I have taken two lists named numbers and filter_list, Lambda function is executed for each element from the numbers and in lambda expression we check if this element is not in filter_list, if it is not it returns True otherwise False. Finally, it returns all values from numbers list that are not present in filter_list.

# Filter list from another list
numbers = [1, 2, 3, 4, 5, 6, 7, 8]
filter_list = [3,5,7,8]
result = list(filter(lambda x: x not in filter_list, numbers))
print(result) 

# Output: 
# [1, 2, 4, 6]

Frequently Asked Questions on filter() with Lambda in Python

Conclusion

By using the python filter() and lambda together you can filter the iterable elements in a single line. Here, you have learned the usage of lambda and filter() function separately with an example and learned how to rewrite the same example by using these two together.

Happy Learning!!

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References

• https://docs.python.org/3/reference/expressions.html#lambda