Filter NumPy Array with Lambda Functions

In this tutorial, you’ll learn how to filter NumPy with lambda functions.

Lambda functions in Python are small anonymous functions defined with the keyword lambda.

We’ll explore various methods such as using numpy.vectorize() with lambda functions, list comprehension, boolean indexing, and the filter() function.



Using numpy.where()

You can apply numpy.where() with a lambda function to filter data:

import numpy as np
data_usage = np.array([1.5, 4.8, 3.1, 0.9, 5.7, 2.3, 3.8])
heavy_users = np.where(lambda x: x > 3, data_usage, False)


[array([4.8, 5.7, 3.8])]

The lambda function lambda x: x > 3 acts as a filter criterion, selecting only those elements in data_usage that satisfy this condition.


Using numpy.vectorize()

numpy.vectorize() transforms a Python function into a vectorized function that can operate on NumPy arrays.

When combined with lambda functions, numpy.vectorize() provides a way to apply operations element-wise on arrays.

Suppose you want to categorize these scores into ‘Low’, ‘Medium’, and ‘High’.

Here’s how numpy.vectorize() with a lambda function can be used for this purpose:

import numpy as np
satisfaction_scores = np.array([7, 4, 9, 6, 3, 8, 5])
categorize = lambda x: 'High' if x > 6 else ('Medium' if x > 3 else 'Low')
vectorized_categorize = np.vectorize(categorize)
categories = vectorized_categorize(satisfaction_scores)


['High' 'Medium' 'High' 'Medium' 'Low' 'High' 'Medium']

numpy.vectorize(categorize) transforms this lambda function into a vectorized function that can be applied to the entire satisfaction_scores array.


List Comprehension with a Lambda Function

This method is useful in cases where you need to apply a simple function to each element in a list or array.

Here’s how you can do this using list comprehension with a lambda function:

monthly_expenses = [50, 75, 30, 45, 60]
apply_discount = lambda x: x * 0.9
discounted_expenses = [apply_discount(expense) for expense in monthly_expenses]


[45.0, 67.5, 27.0, 40.5, 54.0]

In this example, the lambda function apply_discount calculates 90% of the original expense, effectively applying a 10% discount.

The list comprehension [apply_discount(expense) for expense in monthly_expenses] iterates over monthly_expenses, applying the discount to each element.


Boolean Indexing with a Lambda Function

Boolean indexing in NumPy allows for the selection of elements based on specific conditions.

Here’s how you can use boolean indexing and a lambda function:

import numpy as np
customer_ages = np.array([25, 67, 50, 72, 34, 58, 62])
is_senior = lambda x: x >= 60
senior_citizens = customer_ages[is_senior(customer_ages)]


[67 72 62]

This code uses the lambda function is_senior to create a boolean array, where each element in customer_ages is checked against the condition x >= 60.

This boolean array is then used to index customer_ages, resulting in an array senior_citizens that contains only the ages of customers who are 60 years or older.


Using filter() Function

The filter() function in Python is used to construct an iterator from elements of an iterable for which a function returns true.

When filter() function combined with a lambda function, it allows you to filter data based on custom criteria.

Here’s how you can use the filter() function with a lambda:

import numpy as np
call_durations = np.array([3, 12, 1, 9, 5, 2, 8])
effective_call = lambda x: x >= 5
effective_calls = list(filter(effective_call, call_durations))


[12, 9, 5, 8]

In this code, the lambda function effective_call defines the criteria for filtering: x >= 5.

The filter() function then applies this criterion to each element in call_durations.

The result is an iterator, which we convert into a list effective_calls.

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