正如问题所问，R中是否存在一个类似于C的三元算子的控制序列?如果有，你如何使用它?谢谢!
当前回答
我会看一下ifelse命令。我认为它更好，因为它也是向量化的。一个使用cars数据集的例子:
> cars$speed > 20
[1] FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE
[13] FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE
[25] FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE
[37] FALSE FALSE FALSE FALSE FALSE FALSE FALSE TRUE TRUE TRUE TRUE TRUE
[49] TRUE TRUE
> ifelse(cars$speed > 20, 'fast', 'slow')
[1] "slow" "slow" "slow" "slow" "slow" "slow" "slow" "slow" "slow" "slow"
[11] "slow" "slow" "slow" "slow" "slow" "slow" "slow" "slow" "slow" "slow"
[21] "slow" "slow" "slow" "slow" "slow" "slow" "slow" "slow" "slow" "slow"
[31] "slow" "slow" "slow" "slow" "slow" "slow" "slow" "slow" "slow" "slow"
[41] "slow" "slow" "slow" "fast" "fast" "fast" "fast" "fast" "fast" "fast"
其他回答
它并不明确存在，但你可以这样做:
set.seed(21)
y <- 1:10
z <- rnorm(10)
condition1 <- TRUE
x1 <- if(condition1) y else z
or
condition2 <- sample(c(TRUE,FALSE),10,TRUE)
x2 <- ifelse(condition2, y, z)
两者之间的区别在于，condition1必须是长度为1的逻辑向量，而condition2必须是与x、y和z长度相同的逻辑向量。第一个将返回y或z(整个对象)，而第二个将返回y (condition2==TRUE)或z (condition2==FALSE)的对应元素。
还要注意，ifelse将比if / else if条件慢，y和z都是长度为1的向量。
我会看一下ifelse命令。我认为它更好，因为它也是向量化的。一个使用cars数据集的例子:
> cars$speed > 20
[1] FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE
[13] FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE
[25] FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE
[37] FALSE FALSE FALSE FALSE FALSE FALSE FALSE TRUE TRUE TRUE TRUE TRUE
[49] TRUE TRUE
> ifelse(cars$speed > 20, 'fast', 'slow')
[1] "slow" "slow" "slow" "slow" "slow" "slow" "slow" "slow" "slow" "slow"
[11] "slow" "slow" "slow" "slow" "slow" "slow" "slow" "slow" "slow" "slow"
[21] "slow" "slow" "slow" "slow" "slow" "slow" "slow" "slow" "slow" "slow"
[31] "slow" "slow" "slow" "slow" "slow" "slow" "slow" "slow" "slow" "slow"
[41] "slow" "slow" "slow" "fast" "fast" "fast" "fast" "fast" "fast" "fast"
就像其他人说的，使用ifelse，但是你可以定义操作符，这样你就几乎有了三元操作符语法。
`%?%` <- function(x, y) list(x = x, y = y)
`%:%` <- function(xy, z) if(xy$x) xy$y else z
TRUE %?% rnorm(5) %:% month.abb
## [1] 0.05363141 -0.42434567 -0.20000319 1.31049766 -0.31761248
FALSE %?% rnorm(5) %:% month.abb
## [1] "Jan" "Feb" "Mar" "Apr" "May" "Jun" "Jul" "Aug" "Sep" "Oct" "Nov" "Dec"
# or, more generally
condition %?% value1 %:% value2
如果在定义操作符时不使用%符号，它实际上是可以工作的
`?` <- function(x, y) if(x) y[[1]] else y[[2]]
`:` <- function(y, z) list(y, z)
TRUE ? rnorm(5) : month.abb
## [1] 1.4584104143 0.0007500051 -0.7629123322 0.2433415442 0.0052823403
FALSE ? rnorm(5) : month.abb
## [1] "Jan" "Feb" "Mar" "Apr" "May" "Jun" "Jul" "Aug" "Sep" "Oct" "Nov" "Dec"
(这是因为:的优先级低于?)
不幸的是，这将破坏现有的帮助和序列操作符。
我写了一个小的语言扩展，它在r中模拟C的条件三元运算符。它可以作为一个包从这里安装
实现是基于@kohske给出的答案，但我做了一些更改，使其健壮到if_true和if_false参数包含冒号的情况下，允许条件语句被链接，并保留的基本功能?操作符。
我将参考其他人关于重新定义操作符的危险的警告，但这是R语言可以多么动态的一个很好的例子!
你的链接指向一个if语句。
> x <- 1
> if(x < 2) print("Less than") else print("Greater than")
[1] "Less than"
如果你的输入变量是一个向量，那么ifelse可能更适合:
> x <- 1:3
> ifelse(x<=2, "Less than or equal", "Greater than")
[1] "Less than or equal" "Less than or equal" "Greater than"
要访问if的帮助页面，你需要在反引号中嵌入if:
?`if`
ifelse的帮助页面在:
`?ifelse`
