R：在 R 中创建递归二叉树

Question

我对编写递归二叉树算法很感兴趣。鉴于以下数据，我已经对协变量进行了排序 x

mydata <- data.frame(x = c(10, 20, 25, 35), y = c(-10.5, 6.5, 7.5, -7.5))
> mydata
   x     y
1 10 -10.5
2 20   6.5
3 25   7.5
4 35  -7.5

我会拆分树，让左节点始终包含父节点的第一个实例，右节点包含父节点中的其余实例（拆分方式很奇怪，但请多多包涵）。本质上，我希望我的树看起来像这样，最大高度 = 3。

          [-10.5, 6.5, 7.5, -7.5]
                /         \
           [-10.5]        [6.5, 7.5, -7.5]
                            /      \
                       [6.5]       [7.5, -7.5]

我希望我的函数的最终输出返回一个包含所有节点的列表：

> final_tree
[[1]]
[[1]][[1]]
   x     y
1 10 -10.5
2 20   6.5
3 25   7.5
4 35  -7.5


[[2]]
[[2]][[1]]
   x     y
1 10 -10.5


[[2]][[2]]
   x     y
1 20   6.5
2 25   7.5
3 35  -7.5


[[3]]
[[3]][[1]]
NULL

[[3]][[2]]
NULL

[[3]][[3]]
   x     y
1 20   6.5


[[3]][[4]]
   x     y
1 25   7.5
2 35  -7.5

这是我目前所拥有的：

# Initialize empty tree
create_empty_tree <- function(max_height) sapply(1:max_height, function(k) replicate(2**(k-1),c()))

# Create empty tree with max_height = 3
tree_struc <- create_empty_tree(max_height = 3)

grow_tree <- function(node_parent, max_height, tree_struc, height){
  # Sort x
  sorted_x <- sort(node_parent$x)

  # Fix best split index at 1
  best_split_ind <- 1

  # Assign instances to left or right nodes
  group <- ifelse(node_parent$x <= node_parent$x[best_split_ind], "left", "right")
  node_left <- node_parent[which(group == "left"), ]
  node_right <- node_parent[which(group == "right"), ]

  # Recursive call on left and right nodes
  if(height < max_height){
  tree_struc[[height]] <- node_parent
  tree_struc[[height + 1]][[1]] <- grow_tree(node_parent = node_left, max_height = max_height, tree_struc = tree_struc, height = height + 1)
  tree_struc[[height + 1]][[2]] <- grow_tree(node_parent = node_right, max_height = max_height, tree_struc = tree_struc, height = height + 1)
  }

  return(tree_struc)
}

grow_tree(node_parent = mydata, max_height = 3, tree_struc = tree_struc, height = 1)

生成的树不正确。我认为这与我如何递归调用左右子节点上的函数有关。谁能指出我正确的方向？

Answer 1

或许你可以试试下面的函数grow_tree

create_empty_tree <- function(max_height) sapply(1:max_height, function(k) replicate(2**(k-1),c()))

grow_tree <- function(node_parent,max_height = nrow(node_parent)) {
  tree_struc <- create_empty_tree(max_height)
  for (i in 1:length(tree_struc)) {
    tree_struc[[i]][[2**(i-1)]] <- `row.names<-`(tail(node_parent,nrow(node_parent)-i+1),NULL)
    if (i > 1) tree_struc[[i]][[2**(i-1)-1]] <- `row.names<-`(node_parent[i-1,],NULL)
  }
  tree_struc
}

示例

> grow_tree(mydata,3)
[[1]]
[[1]][[1]]
   x     y
1 10 -10.5
2 20   6.5
3 25   7.5
4 35  -7.5


[[2]]
[[2]][[1]]
   x     y
1 10 -10.5

[[2]][[2]]
   x    y
1 20  6.5
2 25  7.5
3 35 -7.5


[[3]]
[[3]][[1]]
NULL

[[3]][[2]]
NULL

[[3]][[3]]
   x   y
1 20 6.5

[[3]][[4]]
   x    y
1 25  7.5
2 35 -7.5

和

> grow_tree(mydata)
[[1]]
[[1]][[1]]
   x     y
1 10 -10.5
2 20   6.5
3 25   7.5
4 35  -7.5


[[2]]
[[2]][[1]]
   x     y
1 10 -10.5

[[2]][[2]]
   x    y
1 20  6.5
2 25  7.5
3 35 -7.5


[[3]]
[[3]][[1]]
NULL

[[3]][[2]]
NULL

[[3]][[3]]
   x   y
1 20 6.5

[[3]][[4]]
   x    y
1 25  7.5
2 35 -7.5


[[4]]
[[4]][[1]]
NULL

[[4]][[2]]
NULL

[[4]][[3]]
NULL

[[4]][[4]]
NULL

[[4]][[5]]
NULL

[[4]][[6]]
NULL

[[4]][[7]]
   x   y
1 25 7.5

[[4]][[8]]
   x    y
1 35 -7.5