Nulls in collections in Kotlin

Parse messy input without a single try/catch.

fun main() {
    val logged = listOf("12.4", "n/a", "9.8", "", "15.1")

    // toDoubleOrNull turns bad input into null instead of throwing.
    val parsed = logged.map { it.toDoubleOrNull() }
    println(parsed)

    // mapNotNull parses and drops the failures in one pass.
    val depths = logged.mapNotNull { it.toDoubleOrNull() }
    println("Mean of ${depths.size} readings: ${depths.average()}")

    println("First good one: ${parsed.firstOrNull { it != null }}")
}

How it works

1. toDoubleOrNull maps bad strings to null instead of exceptions.
2. mapNotNull transforms and drops the nulls in one pass.
3. firstOrNull with a predicate finds the first survivor, or null.

Keywords and builtins used here

• fun
• it
• main
• val

The run, in numbers

Lines

13

Characters to type

440

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98

Three-star pace

70 tpm

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Type this snippet

Step 3 of 3 in Null safety, step 18 of 27 in Language basics.

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