基于R中的辅助向量中的值有条件地连接向量中的字符串

这感觉效率不是很高.我正在try 想象一个很好的嵌套列表对象，或者其他对象.也许一个更好的答案会出现.

这就是说，这样的东西可能会起作用，尽管我可能会测试一些边缘情况.它只是沿着行往下移动，并存储需要使用的以前的标题.缩进0到4存储在stored_headings向量中的索引位置1到5.

contents <- c(
  "Heading 1", "Subheading 1", "Item 1", "Item 2", "Subheading 2", "Item 1",
  "Item 2", "Heading 2", "Subheading 1", "Item 1", "Item 2"
)
indents <- c(0, 1, 2, 2, 1, 2, 2, 0, 2, 4, 4)
df <- data.frame(indents, contents)

# Store up to 5 levels of headings in memory (in this case for 0 through 4)
# Use extra level at end to avoid error in assignment below,
# so we use a vector of length 6
stored_headings <- rep(NA, 6) |> as.character()

# Logic according to indent level, using +1 on everything to
# agree with R indexing starting at 1
for (i in seq_len(nrow(df))) {

  # Store heading at appropriate level in stored_headings
  stored_headings[df$indents[i] + 1] <- df$contents[i]

  # Remove all headings below (greater indent) than current
  stored_headings[(df$indents[i] + 1 + 1):length(stored_headings)] <- NA

  # Concatenate headings, removing NA headings between or below
  df$headings[i] <- stored_headings[!is.na(stored_headings)] |>
    paste(collapse = " | ")

}

df

   indents     contents                          headings
1        0    Heading 1                         Heading 1
2        1 Subheading 1          Heading 1 | Subheading 1
3        2       Item 1 Heading 1 | Subheading 1 | Item 1
4        2       Item 2 Heading 1 | Subheading 1 | Item 2
5        1 Subheading 2          Heading 1 | Subheading 2
6        2       Item 1 Heading 1 | Subheading 2 | Item 1
7        2       Item 2 Heading 1 | Subheading 2 | Item 2
8        0    Heading 2                         Heading 2
9        2 Subheading 1          Heading 2 | Subheading 1
10       4       Item 1 Heading 2 | Subheading 1 | Item 1
11       4       Item 2 Heading 2 | Subheading 1 | Item 2