Generate a custom pattern number sequence in one go

问题

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)

One way is to do:

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

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?

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

回答1:

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)
}

回答2:

Array(0,0) ++ (0 to 2*n).map(_ / 2 * 3)

回答3:

You can do this:

def create(n: Int): Array[Int] =
  Array.tabulate(n)(i => Array.fill(if (i == 0) 4 else 2)(3 * i)).flatten

However, Arrays are not the best data type for flattening.

回答4:

This is a possible solution using flatMap:

Array.fill[Int](2)(0) ++ Array.tabulate(4)(_*3).flatMap(x => Array(x, x))

回答5:

jmh benchmark of answers

@State(Scope.Benchmark)
@BenchmarkMode(Array(Mode.Throughput))
class So59902144 {
  def Luis(n: Int) = Array.tabulate(n)(i => Array.fill(if (i == 0) 4 else 2)(3 * i)).flatten
  def Dima(n: Int) = Array(0,0) ++ (0 until 2*n).map(_ / 2 * 3)
  def SkyWalker(n: Int) = Array.fill[Int](2)(0) ++ Array.tabulate(n)(_*3).flatMap(x => Array(x, x))
  def MarioGalic(n: 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)
  }

  val n = 10000
  @Benchmark def _Luis = Luis(n)
  @Benchmark def _Dima = Dima(n)
  @Benchmark def _SkyWalker = SkyWalker(n)
  @Benchmark def _MarioGalic = MarioGalic(n)
}

sbt "jmh:run -i 5 -wi 2 -f 1 -t 1 -prof gc bench.So59902144"

[info] Benchmark                   Mode   Cnt        Score       Error   Units
[info] So59902144._Dima            thrpt    5     5178.171 ±   837.780   ops/s
[info] So59902144._Luis            thrpt    5     3649.262 ±   309.027   ops/s
[info] So59902144._MarioGalic      thrpt    5    10926.546 ±  3587.691   ops/s
[info] So59902144._SkyWalker       thrpt    5     6088.413 ±   474.100   ops/s

来源：https://stackoverflow.com/questions/59902144/generate-a-custom-pattern-number-sequence-in-one-go