Welcome back! We've already explored how to transform datasets using Enum.map. Now, let's take the next logical step in mastering list manipulations in Elixir — by introducing Enum.filter and Enum.reject. Both are powerful tools when you want to work with specific elements of a dataset. Before we dive in, remember that building a TodoList application is just around the corner. Understanding these tools will greatly enhance your capability to manipulate collections as we proceed.

In today's lesson, you'll learn how to streamline your data processing using Enum.filter and Enum.reject. Here's what each one does:

• Enum.filter: Allows you to select elements from a list that satisfy a particular condition.
• Enum.reject: Helps you remove elements from a list based on a condition.

Here's a snippet from our lesson to illustrate these functions:

Elixir
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1defmodule Student do
2  defstruct id: nil, name: "", grade: nil
3
4  def remove_student(students, id) do
5    Enum.reject(students, fn student -> student.id == id end)
6  end
7
8  def get_students_by_grade(students, grade) do
9    Enum.filter(students, fn student -> student.grade == grade end)
10  end
11end
12
13defmodule Main do
14    def run do
15        students = [
16            %Student{id: 1, name: "Alice", grade: :A},
17            %Student{id: 2, name: "Bob", grade: :B},
18            %Student{id: 3, name: "Charlie", grade: :A_plus},
19            %Student{id: 4, name: "David", grade: :A_plus},
20            %Student{id: 5, name: "Eve", grade: :E}
21        ]
22
23        students = Student.remove_student(students, 1)
24        IO.inspect students
25
26        a_plus_students = Student.get_students_by_grade(students, :A_plus)
27        IO.inspect a_plus_students
28    end
29end
30
31Main.run()

In this example, we use Enum.reject to remove a student by their ID and Enum.filter to find students with a specific grade. These functions make your code shorter, clearer, and more efficient.

• Enum.reject/2: This function takes a list and a function as arguments. The function iterates over each element in the list, removing elements for which the provided function returns true. In this example, Enum.reject(students, fn student -> student.id == id end) removes the student whose id matches the specified id. If there are no matches, the original list is returned.

• Enum.filter/2: Similarly, this function also takes a list and a function. It iterates through the list, keeping elements for which the provided function returns true. In the example, Enum.filter(students, fn student -> student.grade == grade end) returns a list of students whose grade matches the specified grade. If there are no matches, an empty list is returned.

Both functions leverage anonymous functions to evaluate each element against a condition, ensuring that the list is processed according to the defined logic — in this case, either removing or filtering elements based on specific criteria.

Knowing how to efficiently filter and exclude items from lists is crucial in data processing and application development. Whether you are cleaning up data or making particular selections, understanding Enum.filter and Enum.reject equips you with the skills to handle data selectively. This makes your applications both performant and easier to maintain — key ingredients in any successful project. Let's get started with the practice section to see these tools in action!