Fragmented - An Android Developer Podcast

In this episode of Fragmented, we go back to learning some Kotlin and look at the Iterable like data structure introduced called "Sequences". What is a sequence? How is it different from Iterable? When should I use it?

Show Notes

• Kotlin Sequence
• Java Iterable vs Iterator - stackoverflow.com

Eager/Lazy

Eager evaluation:

val lst = listOf(1, 2) val lstMapped: List = lst.map { print("$it "); it * it } print("before sum ") val sum = lstMapped.sum() // prints "1 2 before sum"

Lazy evaluation:

val seq = sequenceOf(1, 2) val seqMapped: Sequence = seq.map { print("$it "); it * it } print("before sum ") val sum = seqMapped.sum() // prints "before sum 1 2"

Source stackoverflow.com answer

Intermediate and terminal operations

Notice that at each chain operation, a new temporary list is created:

data class Person(val name: String, val age: Int) fun main(args: Array) { val people = listOf(Person("Chris Martin", 31), Person("Will Champion", 32), Person("Jonny Buckland", 33), Person("Guy Berryman", 34), Person("Mhris Cartin", 30)) println(people .filter { it.age > 30 } // new temp. list .map { it.name.split(" ").map {it[0]}.joinToString("") } // new temp. list .map { it.toUpperCase() }) // new temp. list }

Using a sequence:

println(people .asSequence() // convert to sequence .filter { it.age > 30 } // lazy eval (intermediate op) .map { it.name.split(" ").map {it[0]}.joinToString("") } // lazy eval (intermediate op) .map { it.toUpperCase() } // lazy eval (intermediate op) .toList() // terminal operation )

Without a terminal operation, Sequences won't print anything:

val seq = sequenceOf(1, 2, 3) println(seq) // prints address println(seq.toList()) // [1, 2, 3]

You can't pick an index from a sequence:

println(seq[0]) // throws ERROR "No get method providing array access" println(seq.toList()[0]) // 1 Sponsors

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Contact

• @fragmentedcast [twitter.com]
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