R 中的连续引用编号：如果是连续的，则用连字符分隔数字 - 如果不是，则添加逗号

我想生成连续引用数对于 R 中的数字。如果数字是连续的，则应使用连字符分隔它们。否则，数字之间用逗号分隔。例如，数字1, 2, 3, 5, 6, 8, 9, 10, 11 and 13应该出来为1-3,5,6,8-11,13.

这个问题已经被之前回答过c#，并且我编写了一个适用于 R 的函数，但是这个函数可以改进。我发布这个问题作为可能有类似需求的其他人的参考。如果您发现 R 的类似问题（我没有），请投票关闭，我将删除该问题。

下面的函数不是很优雅，但似乎可以完成工作。如何让函数变得更短、更优雅？

x <- c(1,2,3,5,6,8,9,10,11,13)

library(zoo) ## the function requires zoo::na.approx function 

##' @title Generate hyphenated sequential citation from an integer vector
##' @param x integer vector giving citation or page numbers
##' @importFrom zoo na.approx

seq.citation <- function(x) {

## Result if lenght of the integer vector is 1. 
if(length(x) == 1) return(x) else {

## Sort
x <- sort(x)

## Difference
df <- diff(x)

## Index to determine start and end points
ind <- c("start", rep("no", length(df)-1), "end")
ind[which(df > 1)] <- "end"

## Temporary start point vector
sts <- which(ind == "end") + 1
ind[sts[sts < length(ind)]] <- "start"

## Replace the first index element
ind[1] <- "start"

## Replace the last index element, if preceding one is "end"
if(ind[length(ind)-1] == "end") ind[length(ind)] <- "start"

## Groups for comma separation using "start" as the determining value.
grp <- rep(NA, length(x))
grp[which(ind == "start")] <- 1:length(grp[which(ind == "start")])
grp <- zoo::na.approx(grp, method = "constant", rule = 2)

## Split sequences by group
seqs <- split(x, grp)

seqs <- lapply(seqs, function(k) {
  if(length(k) == 1) k else {
    if(length(k) == 2) paste(k[1], k[2], sep = ",") else {
  paste(k[1], k[length(k)], sep = "-")
  }}
})

## Result
return(do.call("paste", c(seqs, sep = ",")))
}
}

seq.citation(x)
# [1] "1-3,5,6,8-11,13"

您可以通过基本 R 轻松完成此操作tapply,

paste(tapply(x, cumsum(c(1, diff(x) != 1)), function(i) 
    ifelse(length(i) > 2, paste0(head(i, 1), '-', tail(i, 1)), 
                            paste(i, collapse = ','))), collapse = ',')

[1] "1-3,5,6,8-11,13"