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Mastering Collection Sorting in Kotlin

Sorting a Collection in Kotlin

Sorting a collection is a common operation in programming. It helps in organizing data in a meaningful way, making it easier to read and manipulate.

In Kotlin, sorting a collection is simple and straightforward. In this article, we will discuss two ways of sorting a collection in Kotlin: sorting by individual properties and sorting by multiple properties.

Sorting by Individual Properties

Sorting by individual properties involves sorting a collection based on one property. For instance, sorting books based on the year they were published.

Kotlin provides two methods for sorting a collection by individual properties: sortBy() and sortedBy(). What is sortBy()?

sortBy() is a method provided by Kotlin that sorts a collection based on the values of a specific property. It sorts the collection in ascending order.

Consider the following code snippet:

“`

data class Book(val name: String, val author: String, val yearPublished: Int)

fun main(args: Array) {

val books = mutableListOf(

Book(“The Alchemist”, “Paulo Coelho”, 1988),

Book(“The Catcher in the Rye”, “J.D. Salinger”, 1951),

Book(“To Kill a Mockingbird”, “Harper Lee”, 1960)

)

books.sortBy { it.yearPublished }

books.forEach { println(it.name) }

}

“`

In the code above, we have a collection of three books, and we want to sort them based on the year they were published. We call the sortBy() method on the books collection and pass it a lambda expression that returns the value of the yearPublished property.

The sorted collection will be printed to the console. The output of the code above will be:

“`

The Catcher in the Rye

To Kill a Mockingbird

The Alchemist

“`

What is sortedBy()? sortedBy() works similarly to sortBy(), but it returns a new sorted collection instead of sorting the original collection in place.

sortedBy() method sorts the collection in ascending order. Consider the following code snippet:

“`

data class Book(val name: String, val author: String, val yearPublished: Int)

fun main(args: Array) {

val books = mutableListOf(

Book(“The Alchemist”, “Paulo Coelho”, 1988),

Book(“The Catcher in the Rye”, “J.D. Salinger”, 1951),

Book(“To Kill a Mockingbird”, “Harper Lee”, 1960)

)

val sortedByYear = books.sortedBy { it.yearPublished }

sortedByYear.forEach { println(it.name) }

}

“`

In the code above, we have a collection of three books, and we want to sort them based on the year they were published.

We call the sortedBy() method on the books collection and pass it a lambda expression that returns the value of the yearPublished property. The sorted collection will be printed to the console.

The output of the code above will be:

“`

The Catcher in the Rye

To Kill a Mockingbird

The Alchemist

“`

Sorting by Multiple Properties

Sorting by multiple properties involves sorting a collection based on more than one property. For instance, sorting books based on their names and the year they were published.

Kotlin provides a method for sorting a collection by multiple properties: sortedWith(). What is sortedWith()?

sortedWith() is a method provided by Kotlin that sorts a collection based on a custom Comparator that compares the values of two or more properties. It returns a new sorted collection instead of sorting the original collection in place.

Consider the following code snippet:

“`

data class Book(val name: String, val author: String, val yearPublished: Int)

fun main(args: Array) {

val books = mutableListOf(

Book(“The Alchemist”, “Paulo Coelho”, 1988),

Book(“The Catcher in the Rye”, “J.D. Salinger”, 1951),

Book(“To Kill a Mockingbird”, “Harper Lee”, 1960)

)

val sortedByNameAndYear = books.sortedWith(

compareBy { it.name }

.thenBy { it.yearPublished }

)

sortedByNameAndYear.forEach { println(it.name) }

}

“`

In the code above, we have a collection of three books, and we want to sort them based on their names and the year they were published. We call the sortedWith() method on the books collection and pass it a custom Comparator that compares the values of the name and yearPublished properties.

The sorted collection will be printed to the console. The output of the code above will be:

“`

The Alchemist

The Catcher in the Rye

To Kill a Mockingbird

“`

Generating a Kotlin Project

Creating a project is the first step in any software development process. In Kotlin, creating a project is simple and straightforward.

In this section, we will discuss how to create a Kotlin project in IntelliJ.

Creating a project in IntelliJ

IntelliJ IDEA is a popular IDE for Kotlin development. You can easily create a Kotlin project in IntelliJ by following the steps below:

1.

Open IntelliJ IDEA. 2.

Click on “Create New Project” from the Welcome screen. 3.

At the left side of the “New Project” window, select “Kotlin”. 4.

Choose the project location, name, and SDK. 5.

Click “Finish”.

Creating a file and adding code

After creating a Kotlin project in IntelliJ, the next step is to create a file and add some Kotlin code. In this section, we will create a file named “Main.kt” and add a method named “fetchBooks()”.

The steps to create a file and add code are as follows:

1. Right-click on the “src” folder and select “New” “Kotlin File/Class”.

2. Enter “Main” for the File name and click “OK”.

3. Add the following code to the Main.kt file:

“`

fun fetchBooks(): List {

// code to fetch books

}

“`

In the code above, we have defined a function named fetchBooks() that returns a list of Book objects.

You can replace the comment with the code to fetch the books.

Conclusion

In this article, we have discussed two topics related to Kotlin development: sorting a collection and generating a Kotlin project. We have covered the different methods provided by Kotlin for sorting a collection, including sortBy(), sortedBy(), and sortedWith().

We have also demonstrated how to create a Kotlin project in IntelliJ and how to add Kotlin code to a project file. With this knowledge, you are ready to start developing Kotlin applications with ease.

3) Using the sortBy() Method

Sorting a collection is an essential operation in programming, allowing data to be organized in a meaningful order. Kotlin provides several methods to sort collections, including the sortBy() method.

In this section, we’ll explore how to use the sortBy() method to sort a collection.

Sorting a collection using sortBy()

The sortBy() method is an extension function that sorts a collection in place. To use the sortBy() method, you need a MutableList of Comparable objects.

Comparable is an interface that defines a compareTo() method, which returns an integer value to indicate the order of two objects:

“`

interface Comparable {

fun compareTo(other: T): Int

}

“`

To sort a collection of objects that implement the Comparable interface, Kotlin’s sortBy() function can be called directly on the MutableList. Consider the following example:

“`

data class Product(val name: String, val price: Double): Comparable {

override fun compareTo(other: Product): Int {

return price.compareTo(other.price)

}

}

fun main() {

val products = mutableListOf(

Product(“Keyboard”, 50.0),

Product(“Mouse”, 25.0),

Product(“Monitor”, 150.0)

)

products.sortBy { it.price }

products.forEach { println(it.name) }

}

“`

In the code above, a MutableList of Product objects is created, where each product has a name and a price.

The Product class implements the Comparable interface, which means we can use the sortBy() method directly on the list. In the main function, the sortBy() method is called, passing a lambda expression that specifies the property to sort by.

In this case, we sort the list of products by the price property. The forEach() method then prints the sorted elements in the console.

The output of the code above will be:

“`

Mouse

Keyboard

Monitor

“`

4) Using the sortedBy() Method

Kotlin also provides a sortedBy() method to sort a collection of elements, which returns a new sorted collection rather than sorting the original in place. The sortedBy() method is an extension function available on any Iterable object.

Sorting a collection using sortedBy()

To sort elements in a collection using sortedBy() method, we need to create an Iterable object containing the elements to be sorted. Iterable is an interface representing a collection of elements that can be iterated.

In Kotlin, we can use various classes that implement the Iterable interface. Consider the following example:

“`

data class User(val name: String, val age: Int)

fun main() {

val users = listOf(

User(“Alice”, 25),

User(“Bob”, 30),

User(“Charlie”, 20)

)

val sortedUsers = users.sortedBy { it.age }

sortedUsers.forEach { println(it.name) }

}

“`

In the code above, we have a list of User objects containing their names and ages.

The sortedBy() method is called on the list with a lambda expression, that specifies the property to sort by. In this case, we sort the list of users by age, returning a new sorted list stored in the sortedUsers variable.

The forEach() method is used to print the names of the sorted users to the console. The output of the code above will be:

“`

Charlie

Alice

Bob

“`

In summary, we have explored two methods to sort collections in Kotlin. The sortBy() method is used to sort a collection in place, while the sortedBy() method is used to return a new sorted collection.

With these methods, we can sort any collection of elements in Kotlin easily and efficiently.

5) Using the sortWith() Method

In Kotlin, the sortWith() method is used to sort a collection of elements using a custom comparison function. This method provides greater customization opportunities than other sorting methods.

In this section, we’ll explore how to use the sortWith() method to sort a collection.

Sorting a collection using sortWith()

The sortWith() method is an extension function that sorts a collection in place. To use the sortWith() method, you need a MutableList of elements that implement the Comparable interface.

However, instead of implementing the Comparable interface, you can provide a custom comparison function through a Comparator object. Consider the following example:

“`

data class Employee(val name: String, val age: Int, val salary: Int)

fun main() {

val employees = mutableListOf(

Employee(“Alice”, 25, 5000),

Employee(“Bob”, 30, 7000),

Employee(“Charlie”, 20, 3000)

)

val comparator = Comparator { employee1, employee2 ->

compareBy { it.salary }

.thenBy { it.age }

.compare(employee1, employee2)

}

employees.sortWith(comparator)

employees.forEach { println(it.name) }

}

“`

In the code above, we have a MutableList of Employee objects containing their names, ages, and salaries.

To sort the list of employees, we define a Comparator object called ‘comparator,’ which sorts employees first by their salary and then by their age using the thenBy() method. The sortWith() method is called on the list of employees, passing the comparator object as its parameter.

Finally, the forEach() method is used to print the names of the sorted employees to the console. The output of the code above will be:

“`

Charlie

Alice

Bob

“`

Using sortedWith()

The sortedWith() method is an extension function available on any Iterable object. The sortedWith() method sorts the elements in the collection based on a provided Comparator object and returns a new sorted list.

Consider the following example:

“`

data class Person(val name: String, val age: Int)

fun main() {

val people = listOf(

Person(“Alice”, 25),

Person(“Bob”, 30),

Person(“Charlie”, 20)

)

val comparator = Comparator { person1, person2 ->

compareBy { it.age }

.thenBy { it.name }

.compare(person1, person2)

}

val sortedPeople = people.sortedWith(comparator)

sortedPeople.forEach { println(it.name) }

}

“`

In the code above, we have a list of Person objects containing their names and ages. We define a comparator object as a lambda expression, which sorts the people first by their age and then by their name using the thenBy() method.

The sortedWith() method is called on the list of people, passing the comparator object as its parameter and returns a new sorted list. Finally, the forEach() method is used to print the names of the sorted people to the console.

The output of the code above will be:

“`

Charlie

Alice

Bob

“`

6)

Conclusion

In conclusion, Kotlin provides several methods for sorting collections, depending on the developer’s requirements. The sortBy() method sorts a collection in-place, whereas the sortedBy() method returns a new sorted collection.

The sortWith() method sorts a collection in-place using a custom comparison function, while the sortedWith() method sorts a collection and returns a new sorted collection using a custom comparison function. Furthermore, these methods can be used to sort elements in a descending order by using descending() in the lambda expressions, which inverts the order.

With the knowledge gained in this article, developers can sort collections in Kotlin efficiently and effectively. In conclusion, this article focused on various ways to sort collections in Kotlin programming language.

The discussion included the sortBy(), sortedBy(), sortWith(), and sortedWith() methods with relevant examples for each. While sortBy() sorts any mutable list in place, sortedBy() returns a new list of elements sorted based on a predefined property.

On the other hand, sortWith() and sortedWith() use custom comparator functions to sort elements in a mutable collection and a new sorted list, respectively. Finally, developers can use the descending() function to sort elements in descending order, opposite the default manner.

Sorting collections in Kotlin is an essential operation that helps organize and manipulate data better. Knowing the various approaches to sorting methods can improve coding efficiency and simplify sorting tasks in programming.

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