如果下限大于上限，则创建一个空序列的序列构造 [英] Sequence construction that creates an empty sequence if lower is greater than upper bound

问题描述

在 lower == upper - 1 的极端情况下，R 的 seq 函数的聪明才智"不止一次地打击了我:

More than once the "cleverness" of R's seq function has hit me badly in the corner case when lower == upper - 1:

> 1:0
[1] 1 0
> seq(1, 0)
[1] 1 0
> seq(1, 0, 1)
Error in seq.default(1, 0, 1) : wrong sign in 'by' argument

我不是在问这种奇怪行为的原因——我认为它现在只是我们必须忍受的遗产.相反，我想知道是否有任何包实现了在这种情况下返回空序列的 seq 运算符，如下所示:

I'm not asking for the reasons of this odd behavior -- I assume it's now just a legacy that we have to live with. Instead, I'd like to know if any package implements a seq operator that returns an empty sequence in this case, like the following:

safe.seq.int <- function(from, to, by=1) {
  if (from > to) integer(0) else seq.int(from, to, by)
}

> safe.seq.int(1, 0)
integer(0)

推荐答案

正是出于这个原因，最好使用 seq_len(n) 而不是 1:n -如果 n=0 那么你会得到一个空序列而不是 c(1,0).

It's good practice to use seq_len(n) instead of 1:n for exactly this reason - if n=0 then you get an empty sequence rather than c(1,0).

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