Filtering Collections in Kotlin

Introduction

Filtering Collections is a fundamental concept every Kotlin developer should understand. List, Map, and Set are the three core collection types. Choose List for ordered sequences, Set for unique elements, and Map for key-value lookups.

Filter returns elements matching a condition. In this tutorial you will learn the syntax, walk through a complete example program, study the sample output, and review best practices so you can apply the concept confidently in your own projects.

Definition

• filter returns elements matching a condition.
• filterNot returns elements that fail the condition.
• Predicates are passed as lambda expressions.

Syntax

list.filter { it > 10 }

Filtering Collections in Kotlin Example Program in Kotlin

fun main(args: Array<String>) {
    val marks = listOf(45, 78, 92, 61)
    val passed = marks.filter { it >= 60 }
    println("Passed marks: $passed")
}

Sample Output

Passed marks: [78, 92, 61]

When to use

Pick List for ordered data, Set when uniqueness matters, Map when you look up values by a key.

How it works

1. The program starts with a main function — the entry point that runs when you execute the file.

2. val marks = listOf(45, 78, 92, 61) assigns or updates a value used later in the program.

3. val passed = marks.filter { it >= 60 } assigns or updates a value used later in the program.

4. The println("Passed marks: $passed") statement writes a line to the console — this produces part of the sample output below.

5. Filter returns elements matching a condition.

6. Run the program in IntelliJ IDEA, Android Studio, or with the Kotlin command-line compiler (kotlinc / kotlin). Compare your console output with the sample output shown below.

Best Practices

• Understand the core idea: filter returns elements matching a condition.
• Prefer readable names and small functions so examples map directly to real projects.
• Run and modify the example — change values and observe how the output changes.

Common Mistakes

• Skipping the example and only reading the definition — hands-on practice cements the concept.
• Copying syntax without understanding nullable vs non-nullable types or scope rules.
• Ignoring compiler warnings that often point to safer alternatives.

Key Points

• filter returns elements matching a condition.
• filterNot returns elements that fail the condition.
• Predicates are passed as lambda expressions.
• Test the example locally and verify the output matches the sample.
• Experiment by changing input values to see how behaviour changes.

Notes

• Semicolons at the end of statements are optional in Kotlin.