一次性生成自定义图案编号序列 [英] Generate a custom pattern number sequence in one go

问题描述

我想使用功能初始化构造一次性生成以下数字序列:

I'd like to generate the following number sequence in one go using a functional initialization construct:

Array(0, 0, 0, 0, 3, 3, 6, 6, 9, 9, ..., n*3, n*3)

一种方法是做

Array.fill[Int](2)(0) ++ Array.tabulate(4)(_*3)

但是我需要将结构第二部分的每个值加倍，即先得到 0，0 然后是 3、3 等.如何复制第二个构造的值?

but I'd need to double each value of the second part of the construct i.e. to get 0, 0 then 3, 3 etc. How can I duplicate the values of the second construct?

我也无法弄清楚会产生这样的序列的数学函数.

I also couldn't figure out a mathematical function that would generate such sequence.

推荐答案

考虑单次通过的尾部递归解决方案

Consider tail-recursive solution for single pass

def pattern(n: Int): List[Int] = {
  @tailrec def run(n: Int, acc: List[Int]): List[Int] = {
    n match {
      case 0 => 0 :: 0 :: 0 :: 0 :: acc
      case i => run(i - 1, (i * 3) :: (i * 3) :: acc)
    }
  }
  run(n-1, Nil)
}

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