如何在R中为函数使用条件语句和返回值? [英] How to use conditional statement and return value for a function in R?

问题描述

我必须创建一个函数:ans(x)，它返回值 2*abs(x)，如果 x 是负数，否则为 x 值.我可以使用什么命令?

I have to create a function as: ans(x) which returns the value 2*abs(x), if x is negative, and the value x otherwise. What command could i use?

谢谢

推荐答案

ans <- function(x){
  ifelse(x < 0, 2*abs(x), x)
}

会做的.

> ans(2)
[1] 2

> ans(-2)
[1] 4

说明:我们可以使用内置的基本 R 函数 ifelse().逻辑很简单:

Explanation: We can use the built-in base R function ifelse(). The logic is pretty simple:

ifelse(条件，条件为真时输出，条件为假时输出)

因此，ifelse(x <0, 2*abs(x), x) 将执行以下操作:

Therefore, ifelse(x < 0, 2*abs(x), x) will do the following:

1. 评估值 x 是否为负 (<0)
2. 如果TRUE，返回2*abs(x)
3. 如果FALSE，返回x

1. evaluate whether value x is negative (<0)
2. if TRUE, return 2*abs(x)
3. if FALSE, return x

ifelse() 相对于传统的 if() 的优势在于向量化.if() 只能处理单个值，ifelse() 将评估作为输入给出的任何向量.

The advantage of ifelse() over traditional if() is the vectorization. if() can only handle a single value, ifelse() will evaluate any vector given as input.

对比:

ans_if <- function(x){
  if(x < 0){2*abs(x)}else{x}
}

这是同一个函数，使用传统的 if() 结构.将单个值作为输入将导致两个函数的输出相同:

This is the same function, using a traditional if() structure. Giving a single value as input will result in the same output for both functions:

> ans(-2)
[1] 4
> ans_if(-2)
[1] 4

但是如果你想输入多个值，比方说

But if you want to input multiple values, let's say

test <- c(-1, -2, 3, -4)

ifelse() 变体将评估向量的每个元素并生成正确的输出作为相同长度的向量:

the ifelse() variant will evaluate every element of the vector and generate the correct output as a vector of the same length:

> ans(test)
[1] 2 4 3 8

而 if() 变体会抛出警告

> ans_if(test)
[1] 2 4 6 8
Warning message:
In if (x < 0) { :
  the condition has length > 1 and only the first element will be used

并返回错误的输出，因为只有第一个值用于评估 (-1) 并且对整个向量的操作基于此评估.

and return the wrong output, as only the first value was used for evaluation (-1) and the operation over the whole vector was based on this evaluation.

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