kotlin - 两个列表的所有可能组合

Question

鉴于我有两个列表:

val ints = listOf(0, 1, 2)
val strings = listOf("a", "b", "c")

我想要它们元素的所有可能组合

0a、1a、2a、0b等

有没有比这样更优雅的方式:

ints.forEach { int -> 

    strings.forEach { string ->  


        println("$int $string")

    }

}

Answer 1

您可以基于 flatMap stdlib 函数编写这些扩展函数:

// Extensions
fun <T, S> Collection<T>.cartesianProduct(other: Iterable<S>): List<Pair<T, S>> {
    return cartesianProduct(other, { first, second -> first to second })
}

fun <T, S, V> Collection<T>.cartesianProduct(other: Iterable<S>, transformer: (first: T, second: S) -> V): List<V> {
    return this.flatMap { first -> other.map { second -> transformer.invoke(first, second) } }
}

// Example
fun main(args: Array<String>) {
    val ints = listOf(0, 1, 2)
    val strings = listOf("a", "b", "c")

    // So you could use extension with creating custom transformer
    strings.cartesianProduct(ints) { string, int ->
        "$int $string"
    }.forEach(::println)

    // Or use more generic one
    strings.cartesianProduct(ints)
            .map { (string, int) ->
                "$int $string"
            }
            .forEach(::println)
}