Kotlin flatten + map of nested objects - kotlin

Question

I have a list of objects that looks like:
data class Classroom(
val id: Int,
val name: String,
val students: List<Student>
)
data class Student(
val id: Int,
val name: String,
val age: Int,
val gpa: Long,
)
I wanna have a hash map that maps Student.name -> Student of all the students that are in all of the classes (regardless of which student belongs to each class).
My input is a List of Classroom
How can i achieve that elegantly?

Answer 1

val studentMap: Map<String, Student> = classrooms
.flatMap { it.students }
.map { student -> student.name to student }
.toMap()
Use flatMap to extract all the students
map them to name/student pairs
Convert to a map
Here is the the setup and test code, that you should have provided in the question.
fun generateClassrooms() : List<Classroom> {
val classANames = listOf("foo", "bar", "baz")
val classBNames = listOf("baz", "asdf", "ghjk")
val classCNames = listOf("asdf", "ghjk", "bar", "qwerty")
val student = Student(0, "name", 0, 0)
val classAStudents = classANames.map { student.copy(name = it) }
val classBStudents = classBNames.map { student.copy(name = it) }
val classCStudents = classCNames.map { student.copy(name = it) }
val classroom = Classroom(0, "classroom", emptyList())
val classA = classroom.copy(students = classAStudents)
val classB = classroom.copy(students = classBStudents)
val classC = classroom.copy(students = classCStudents)
return listOf(classA, classB, classC)
}
fun main() {
val classrooms = generateClassrooms()
val studentMap: Map<String, Student> = classrooms
.flatMap { it.students }
.map { student -> student.name to student }
.toMap()
for (student in studentMap) println(student)
}
Running this produces:
foo=Student(id=0, name=foo, age=0, gpa=0)
bar=Student(id=0, name=bar, age=0, gpa=0)
baz=Student(id=0, name=baz, age=0, gpa=0)
asdf=Student(id=0, name=asdf, age=0, gpa=0)
ghjk=Student(id=0, name=ghjk, age=0, gpa=0)
qwerty=Student(id=0, name=qwerty, age=0, gpa=0)