**Course Title:** Kotlin Programming: From Basics to Advanced Techniques **Section Title:** Working with Collections **Topic:** Using collection functions: filter, map, and reduce In the previous topic, we introduced the basics of collections in Kotlin, including Lists, Sets, and Maps. Now, we'll dive into some of the most powerful and commonly used collection functions: `filter`, `map`, and `reduce`. These functions enable you to transform, manipulate, and extract data from collections in a concise and expressive way. **The filter() Function** The `filter()` function is used to create a new collection containing only the elements that match a specified condition. The syntax is as follows: ```kotlin val result = collection.filter { predicate } ``` Here, `predicate` is a lambda expression that takes an element of the collection as input and returns a boolean value indicating whether the element should be included in the new collection. Example: ```kotlin val numbers = listOf(1, 2, 3, 4, 5) val evenNumbers = numbers.filter { it % 2 == 0 } println(evenNumbers) // [2, 4] ``` **The map() Function** The `map()` function is used to transform each element of a collection into a new value. The syntax is as follows: ```kotlin val result = collection.map { transform } ``` Here, `transform` is a lambda expression that takes an element of the collection as input and returns the transformed value. Example: ```kotlin val numbers = listOf(1, 2, 3, 4, 5) val doubledNumbers = numbers.map { it * 2 } println(doubledNumbers) // [2, 4, 6, 8, 10] ``` **The reduce() Function** The `reduce()` function is used to combine all elements of a collection into a single value. The syntax is as follows: ```kotlin val result = collection.reduce { accumulator, element -> combine(accumulator, element) } ``` Here, `accumulator` is the current accumulated value, `element` is the current element, and `combine` is a function that combines the two values. Example: ```kotlin val numbers = listOf(1, 2, 3, 4, 5) val sum = numbers.reduce { acc, num -> acc + num } println(sum) // 15 ``` Note that `reduce()` can also be used with a specific initial value: ```kotlin val result = collection.reduce(initialValue) { accumulator, element -> combine(accumulator, element) } ``` **Example: Using filter(), map(), and reduce() Together** ```kotlin val numbers = listOf(1, 2, 3, 4, 5) val sumOfEvenNumbers = numbers .filter { it % 2 == 0 } .map { it * 2 } .reduce { acc, num -> acc + num } println(sumOfEvenNumbers) // 20 ``` In this example, we first filter the numbers to include only the even numbers, then map each even number to its double, and finally reduce the resulting collection to calculate the sum. **Key Takeaways** * `filter()` creates a new collection containing only the elements that match a specified condition. * `map()` transforms each element of a collection into a new value. * `reduce()` combines all elements of a collection into a single value. * You can chain these functions together to perform complex operations on collections. **Practice** Try the following exercises to practice using `filter()`, `map()`, and `reduce()`: 1. Filter a list of numbers to include only the odd numbers. 2. Map a list of strings to uppercase strings. 3. Reduce a list of numbers to calculate the maximum value. **Resources** * [Kotlin Documentation: Collection Functions](https://kotlinlang.org/docs/collection-functions.html) * [Kotlin Documentation: filter()](https://kotlinlang.org/api/latest/jvm/stdlib/kotlindart.collections/filter.html) * [Kotlin Documentation: map()](https://kotlinlang.org/api/latest/jvm/stdlib/kotlindart.collections/map.html) * [Kotlin Documentation: reduce()](https://kotlinlang.org/api/latest/jvm/stdlib/kotlindart.collections/reduce.html) **Leave a Comment** If you have any questions or need help with the exercises, feel free to leave a comment below.