What is a Clojure filter?

Key takeaways

• In Clojure, filter function is a higher-order function used to modify collection by selecting elements from an existing one based on a specified predicate.

• The filter function takes two parameters: a predicate function and a collection to filter.

• Common use cases include filtering file types (e.g., extracting .txt files) and cleaning data by removing invalid or nil values.

The Clojure is a dynamically typed functional programming language. Clojure is well known for its sophisticated method of handling data. The filter function is one of its most effective tools for modifying collections. We'll dive deep into filter, exploring its syntax, applications, and real-world examples.

Basics of filter

In Clojure, filter is a higher-order function, which means it has two arguments:

1. Another function

2. A collection

By choosing elements from an existing collection that meet a specified predicate, a filter creates a new collection.

Syntax

• predicate is a function that takes an element from the collection and returns true or false based on a condition.

• collection is the input collection from which we will filter elements.

Code example

In this example, filter uses the even? predicate to select even numbers from the numbers collection.

• Line 1: def is used to define a new variable and numbers is the name of the variable. Here, [1 2 3 4 5 6 7 8 9 10] is a vector containing the numbers 1 to 10.

• Line 3: even-numbers is the new variable and (filter even? numbers) applies the filter function to numbers, keeping only the elements for which the even? predicate returns true (i.e., the even numbers).

• Line 5: println is a function that prints the given value to the console and even-numbers is the value being printed, which is the list of even numbers from the numbers vector.

Real-world applications

The power of filter becomes evident when applied to real-world problems. Let's explore some common use cases where filter shines.

Filtering a list of files

Imagine you have a list of files and you want to extract only the text files. This is where filter comes in handy.

• Line 1: ["document.txt" "image.jpg" "spreadsheet.xlsx" "readme.md"] is a vector containing the names of four files.

• Line 2: is-text-file? is the new variable and (fn [file] (.endsWith file ".txt")) is an anonymous function (lambda) that takes a file name as an argument (file) and returns true if the file name ends with ".txt".

• Line 3: text-files is the variable and (filter is-text-file? files) applies the filter function to files, keeping only the elements for which the is-text-file? function returns true (i.e., the files ending with ".txt").

• Line 5: println is a function that prints the given value to the console and text-files is the value being printed, which is the list of files that end with ".txt".

Removing invalid data

When working with data, it's common to have invalid or null values. filter can help you clean your data by removing these unwanted elements.

• Line 1: data is the variable name and [1 2 3 nil 5 nil 7 8 nil] is a vector containing numbers and nil values.

• Line 2: valid-data is the variable and (filter identity data) applies the filter function to data, keeping only the elements for which the identity function returns true (i.e., non-nil values). The identity function returns its argument, so it effectively removes nil values.

• Line 4: println is a function that prints the given value to the console and valid-data is the value being printed, which is the list of non-nil values from the data vector.

In this case, the identity function is used as the predicate, effectively removing all nil values from the data collection.