Grouping classes in kotlin - kotlin

Question

I have a class called as Student, with three fields, I need to convert that class into a GroupedStudent. How can this be achieved?
var students: List<Student> = mutableListOf(Student("A", "X", 1), Student("A", "Y", 2), Student("B", "X", 2), Student("B", "Y", 2))
I need to convert the above list to a GroupedStudent List, how can i do that?
They should be grouped by studentName, and have the SubjectMarks as list.
class Student {
var name: String
var subject: String
var marks: Int
}
class GroupedStudent {
var name: String
var subMarks: MutableList<SubjectMarks>
}
class SubjectMarks {
var subject: String
var marks: Int
}

Answer 1

Use the Kotlin stdlib Collections functions:
var students: List<Student> = mutableListOf(Student("A", "X", 1), Student("A", "Y", 2), Student("B", "X", 2), Student("B", "Y", 2))
val groupedMap = students.groupBy { it.name }
val groupedStudents = mutableListOf<GroupedStudent>()
groupedMap.forEach { key, value ->
val groupedStudent = GroupedStudent(key, value.map { SubjectMarks(it.subject, it.marks) }.toMutableList())
groupedStudents.add(groupedStudent)
}
This will result in a List of GroupedStudent, with each GroupedStudent containing a list of students' marks who have that name.

Answer 2

You can group by the student's name and then do two simple mappings:
students.groupBy(Student::name).map { (name, students) ->
GroupedStudent(name, students.map { SubjectMarks(it.subject, it.marks) }.toMutableList())
}

Answer 3

If your Student class is correct, i.e. marks should be Int and not array or list of integers and you really want to convert it to "GroupedStudent List" you can use sortedWith and map methods:
class Student(var name: String, var subject: String, var marks: Int) {
override fun toString(): String = "name: $name, subject: $subject, marks: $marks"
}
class GroupedStudent(var name: String, var subMarks: MutableList<SubjectMarks>) {
override fun toString(): String = "name: $name, subject: $subMarks"
}
class SubjectMarks(var subject: String, var marks: Int) {
override fun toString(): String = "subject name: $subject, marks: $marks"
}
fun main(args: Array<String>) {
val students: List<Student> = mutableListOf(Student("B", "Y", 2),
Student("A", "X", 1),
Student("B", "X", 2),
Student("A", "Y", 2))
val groupedStudents: List<GroupedStudent> = students.sortedWith(compareBy({ it.name }))
.map { student -> GroupedStudent(student.name, mutableListOf(SubjectMarks(student.subject, student.marks))) }
println(groupedStudents)
}
The result:
[name: A, subject: [subject name: X, marks: 1], name: A, subject: [subject name: Y, marks: 2], name: B, subject: [subject name: Y, marks: 2], name: B, subject: [subject name: X, marks: 2]]
BUT if you want to group your Student objects in a map with name as key and list of subject/marks as list of Pairs, you can use groupBy:
class Student(var name: String, var subject: String, var marks: Int) {
override fun toString(): String = "name: $name, subject: $subject, marks: $marks"
}
class GroupedStudent(var name: String, var subMarks: MutableList<SubjectMarks>) {
override fun toString(): String = "name: $name, subject: $subMarks"
}
class SubjectMarks(var subject: String, var marks: Int) {
override fun toString(): String = "subject name: $subject, marks: $marks"
}
fun main(args: Array<String>) {
val students: List<Student> = mutableListOf(Student("B", "Y", 2),
Student("A", "X", 1),
Student("B", "X", 2),
Student("A", "Y", 2))
val groupedStudentsMap = mutableMapOf<String, List<Pair<String, Int>>>()
students.groupBy { it.name }
.forEach({ (key, value) ->
groupedStudentsMap[key] = value.map { element -> Pair(element.subject, element.marks) } })
println(groupedStudentsMap)
}
The result:
{B=[(Y, 2), (X, 2)], A=[(X, 1), (Y, 2)]}