集合排列组合的长度为什么是0

Question

为什么集合的排列组合的结果长度为0
scala代码

import scala.collection.immutable

object Test {
  def main(args: Array[String]): Unit = {
    val a=1 to 3
    val b: Iterator[immutable.IndexedSeq[Int]] = a.permutations
    println(b.mkString(","))
    println(b.count(x=>true))
    val c: Iterator[immutable.IndexedSeq[Int]] = a.combinations(2)
    println(c.mkString(","))
    println(c.count(x => true))
  }
}

执行结果
Vector(1, 2, 3),Vector(1, 3, 2),Vector(2, 1, 3),Vector(2, 3, 1),Vector(3, 1, 2),Vector(3, 2, 1)
0
Vector(1, 2),Vector(1, 3),Vector(2, 3)
0
python代码

import itertools

if __name__ == '__main__':
    a=range(1,4)
    b=itertools.permutations(a)
    print(list(b))
    print(len(list(b)))
    c=itertools.combinations(a,2)
    print(list(c))
    print(len(list(c)))

执行结果
[(1, 2, 3), (1, 3, 2), (2, 1, 3), (2, 3, 1), (3, 1, 2), (3, 2, 1)]
0
[(1, 2), (1, 3), (2, 3)]
0

Answer 1

想明白,傻b了,迭代器不可以重复遍历.