下面的函数沿着目录树向下递归，每找到一个叶子就加1。或者，它会将叶子的名字保存在一个文件中。

count_dirs <- function(path = ".", savefile = FALSE, filename) {
  f <- function(path) {
    if(dir.exists(paths = path)) {
      dir_vec <- list.dirs(path = path, full.names = TRUE, recursive = FALSE)
      if(length(dir_vec) == 1L) {
        e$n <- e$n + 1L
        if(savefile) {
          df1 <- data.frame(directory = dir_vec)
          write.table(
            df1, e$outfile, 
            quote = FALSE, 
            row.names = FALSE, col.names = FALSE,
            append = TRUE
          )
        }
      } else {
        for(d in dir_vec)  Recall(path = d)
      }
    }
  }
  e <- new.env()
  e$n <- 0L
  if(savefile) {
    e$outfile <- filename
    df1 <- data.frame(directory = character(0))
    write.table(df1, e$outfile, quote = FALSE, row.names = FALSE)
  }
  path <- normalizePath(path = path)
  if(.Platform$OS.type == "windows") {
    path <- chartr("\\", "/", path)
  }
  f(path = path)
  e$n
}

# sample usage    
tmpfile <- tempfile(fileext = ".dat")
count_dirs(savefile = TRUE, filename = tmpfile)
# [1] 79

# tidy up
#unlink(tmpfile)