如果在R中没有按预期工作 [英] ifelse not working as expected in R

问题描述

我正在尝试使用命令 ifelse 在R中创建一个矢量函数，如下所示

I am trying to create a vectorial function in R using the command ifelse as follows

temp <- function(p){
ifelse(p < 0.5 *(1 + 0.5), (1 + 0.5) * qnorm(p/(1 +0.5)), (1 - 0.5) * qnorm((p - 0.5)/(1 - 0.5)))
}

该函数适用于标量值，例如

The function works well with scalar values, for example

> temp(0.1)
[1] -2.251629
> temp(0.9)
[1] 0.4208106

但不适用于矢量：

> temp(c(0.1,0.9))
[1] -2.2516289  0.4208106
Warning message:
In qnorm((p - 0.5)/(1 - 0.5)) : NaNs produced

奇怪的是它返回正确的答案，但表示警告。

The weird thing is that it returns the right answer, but indicating a warning.

我做错了什么？似乎 ifelse 正在评估向量 p 的所有条目中的两个函数，这应该是避免的使用此命令。

What am I doing wrong? It seems that the ifelse is evaluating both functions in all the entries of the vector p, which is supposed to be avoided with this command.

推荐答案

ifelse 基本上是这样做的：

p<- c(.1,.9)
a<-(1 + 0.5) * qnorm(p/(1 +0.5))
b<- (1 - 0.5) * qnorm((p - 0.5)/(1 - 0.5))
c<-NULL
c[which(p < 0.5 *(1 + 0.5))] <-a[which(p < 0.5 *(1 + 0.5))]
c[which(!(p < 0.5 *(1 + 0.5)))] <-b[which(!(p < 0.5 *(1 + 0.5)))]

也就是说，它为'yes'创建了一个向量，为'no'创建了一个向量。它创建的'no'向量会抛出警告。

That is, it creates a vector for 'yes' and a vector for 'no'. The 'no' vector it creates throws the warning.

文档中的示例提到了这一点。

The examples in the documentation allude to this.

x <- c(6:-4)
sqrt(x)  #- gives warning
sqrt(ifelse(x >= 0, x, NA))  # no warning

## Note: the following also gives the warning !
ifelse(x >= 0, sqrt(x), NA)

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