What I found is that written evidence for knocking on wood (and for the history of gesture as a whole) is remarkably sparse. In the Penguin Guide to Superstitions, folklorist Steve Roud offers a skeptical take on the provenance of knocking on wood due to the near total absence of written records about it:
* @param n 数组长度
，这一点在heLLoword翻译官方下载中也有详细论述
常用于： 深层 CNN、MLP。
Interaction — Draggable, Hoverable, Clickable mobjects