为什么x [0]返回零长度向量? [英] why does `x[0]` return a zero-length vector?

问题描述

假设我有一个向量，例如x <- 1:10，那么x[0]返回一个与x相同类别(此处为integer(0))的零长度向量.

Say I have a vector, for example, x <- 1:10, then x[0] returns a zero-length vector of the same class as x, here integer(0).

我想知道该选择背后是否有原因，而不是抛出错误或像x[11]那样返回NA?另外，如果您想到让x[0]返回integer(0)有用的情况，则感谢您将其包含在答案中.

I was wondering if there is a reason behind that choice, as opposed to throwing an error, or returning NA as x[11] would? Also, if you can think of a situation where having x[0] return integer(0) is useful, thank you for including it in your answer.

推荐答案

在?"["

NA和零值是允许的:包含 零将被忽略，而包含NA的行将在 结果.

NA and zero values are allowed: rows of an index matrix containing a zero are ignored, whereas rows containing an NA produce an NA in the result.

因此索引0会被忽略.我们可以在下面看到它

So an index of 0 just gets ignored. We can see this in the following

x <- 1:10
x[c(1, 3, 0, 5, 0)]
#[1] 1 3 5

因此，如果我们给它的唯一索引是0，那么适当的响应是返回一个空向量.

So if the only index we give it is 0 then the appropriate response is to return an empty vector.

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