Enum and range

Learn ranges and iteration with enums.

Enum and range: ordered traversal

You can walk through all constants of an enum in order with a for loop over entries.

for (c in Color.entries) {
    println(c)
}
// RED, GREEN, BLUE

Because entries is an immutable list, it does not change during the loop. The order follows the declaration order. You get the same layout every time.

Filtering with the order index

You can pick a subset by using the ordinal of the constants. This is handy for sorting logic.

val firstPair = Color.entries.filter { it.ordinal < 2 }
// [RED, GREEN]

filter keeps each constant that meets a condition. it is each constant in the list. This lets you slice subsets by declaration order.

Exercise Print the sizes

Define an enum class called Size with SMALL, MEDIUM, LARGE. Loop over entries with a for and print each constant on its own line.

Show example solution

enum class Size { SMALL, MEDIUM, LARGE }

fun main() {
    for (s in Size.entries) {
        println(s)
    }
}

Test yourself

Match construct to use

entries

All constants
for (c in ...)

Loop
ordinal

Order number
Color.RED

One constant
What do you use to walk all constants in order?
- Color.each
- for (c in Color.entries)
- Color.loop
- for (c of Color)
Show answer

for (c in Color.entries) — A for loop over entries does the walk.

Fill in the blank: set up the loop

for (c in Color.____) println(c)

Show answer

for (c in Color.entries) println(c)

Next lesson: Summary - Enums

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