在R中，为什么is.numeric(NaN)打印"TRUE"? [英] In R, why does is.numeric(NaN) print "TRUE"?

问题描述

NaN`的意思是不是数字".但是我发现

NaN` means "Not a Number". However I found out that the result of

is.numeric(NaN)

是

[1] "TRUE"

有人知道为什么吗?我认为结果应该是FALSE.

Anybody know why? I think the result should be FALSE.

推荐答案

不是数字"并不表示它不是数字.这是浮点数的特殊编码.请参见 ANSI/IEEE 754浮点标准，或者仅是NaN的Wikipedia页面.

"Not a Number" does not really mean it is not a number. It is a special coding of a floating-point number. See ANSI/IEEE 754 floating-point standard, or simply the Wikipedia page for NaN.

这是所有计算语言的通用标准，R的处理也不例外.

This is a universal standard for all computing languages and R's handling is no exception.

在R中，typeof(NaN)给出双精度"，而mode(NaN)给出数值".现在，如果您阅读?is.numeric，您会发现，如果x的模式为数字"且不是一个因子，则is.numeric(x)会返回TRUE.

In R, typeof(NaN) gives "double", and mode(NaN) gives "numeric". Now if you read ?is.numeric, you see that is.numeric(x) returns TRUE if x has mode "numeric" and is not a factor.

IEEE标准定义了三个特殊的浮点数Inf，-Inf和NaN.

The IEEE standard defines three special floating-point numbers, Inf, -Inf and NaN.

前两个数学意义很明确:正无穷大和负无穷大.尽管它们不是实数的一部分，但它们是定义明确的限制.例如，1 / 0是Inf.

The first two have a clear mathematical meaning: positive infinite and negative infinite. Although they are not part of real number, they are well-defined limit. For example, 1 / 0 is Inf.

那么NaN是什么意思?恰好是定义不明确的实数.简单的例子是

So what does NaN mean? Precisely it is not well-defined real number. Simple cases are

0 / 0  # can be either Inf or -Inf
sqrt(-1)  # not well-defined on real number set
Inf - Inf  # can not be decided, can be 0, Inf or -Inf

在数值计算中，如果结果不能用规则的实数或Inf或-Inf书写，则将其表示为NaN.在这方面，任何浮点数之间的算术都变得完全可表示. NaN在这方面很有参考价值，表示出了点问题".

In numerical computing, if the result can not be written in regular real number, or Inf or -Inf, it is expressed as NaN. In this regard, arithmetics between any floating-point numbers becomes completely representable. NaN is quite informative in this regard, it means "something has gone wrong".

请注意，与NA具有多种类型(NA_real_，NA_integer_等)不同，NaN仅具有一种浮点数类型.任何逻辑，字符，整数值都不是浮点数，因此不能为NaN. NA的定义不是通用标准，R有其自己的定义.

Note that unlike NA which has various types (NA_real_, NA_integer_, etc), NaN has only one type for floating-point numbers. Any logical, character, integer values are not floating-point numbers hence can not be NaN. The definition of NA is not a universal standard and R has its own.

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