R中是否存在三元运算符? [英] Does the ternary operator exist in R?

问题描述

正如问题所问，R中是否存在类似于C的三元运算符的控制序列?如果是这样，您如何使用它?谢谢！

As the question asks, is there a control sequence in R similar to C's ternary operator? If so, how do you use it? Thanks!

推荐答案

由于if是R中的函数，并且返回最新的求值，如果if-else等效于?:.

As if is function in R and returns the latest evaluation, if-else is equivalent to ?:.

> a <- 1
> x <- if(a==1) 1 else 2
> x
[1] 1
> x <- if(a==2) 1 else 2
> x
[1] 2

R的幂是矢量化.三元运算符的向量化为ifelse:

The power of R is vectorization. The vectorization of the ternary operator is ifelse:

> a <- c(1, 2, 1)
> x <- ifelse(a==1, 1, 2)
> x
[1] 1 2 1
> x <- ifelse(a==2, 1, 2)
> x
[1] 2 1 2

---

开个玩笑，您可以定义c样式?::

`?` <- function(x, y)
    eval(
      sapply(
        strsplit(
          deparse(substitute(y)), 
          ":"
      ), 
      function(e) parse(text = e)
    )[[2 - as.logical(x)]])

在这里，您无需注意括号:

here, you don't need to take care about brackets:

> 1 ? 2*3 : 4
[1] 6
> 0 ? 2*3 : 4
[1] 4
> TRUE ? x*2 : 0
[1] 2
> FALSE ? x*2 : 0
[1] 0

但是您需要用括号括起来:(

but you need brackets for assignment :(

> y <- 1 ? 2*3 : 4
[1] 6
> y
[1] 1
> y <- (1 ? 2*3 : 4)
> y
[1] 6

---

最后，您可以使用c进行非常相似的操作:

---

Finally, you can do very similar way with c:

`?` <- function(x, y) {
  xs <- as.list(substitute(x))
  if (xs[[1]] == as.name("<-")) x <- eval(xs[[3]])
  r <- eval(sapply(strsplit(deparse(substitute(y)), ":"), function(e) parse(text = e))[[2 - as.logical(x)]])
  if (xs[[1]] == as.name("<-")) {
    xs[[3]] <- r
        eval.parent(as.call(xs))
  } else {
    r
  }
}       

您可以删除括号:

> y <- 1 ? 2*3 : 4
> y
[1] 6
> y <- 0 ? 2*3 : 4
> y
[1] 4
> 1 ? 2*3 : 4
[1] 6
> 0 ? 2*3 : 4
[1] 4

这些不是日常使用，但可能对学习R语言的某些内部知识很有帮助.

These are not for daily use, but maybe good for learning some internals of R language.

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