Does the ternary operator exist in R?

Question

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!

Answer 1

if works like unvectorised ifelse if used in following manner:

`if`(condition, doIfTrue, doIfFalse)

The advantage of using this over ifelse is when the vectorisation is in the way (i.e I have scalar boolean and list/vector things as a result)

ifelse(TRUE, c(1,2), c(3,4))
[1] 1
`if`(TRUE, c(1,2), c(3,4))
[1] 1 2

Answer 2

Like everyone else said, use ifelse, but you can define operators so that you nearly have the ternary operator syntax.

`%?%` <- 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

It actually works if you define the operators without the % signs, so you could have

`?` <- 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"

(This works because the precedence of : is lower than ?.)

Unfortunately, that then breaks the existing help and sequence operators.

Answer 3

I would take a look at the ifelse command. I would call it even better because it is also vectorized. An example using the cars dataset:

> 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"

Answer 4

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

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

---

Just kidding, you can define c-style ?::

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

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

but you need brackets for assignment :(

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

---

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
  }
}       

You can get rid of brackets:

> 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

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

Answer 5

Your link points to an if statement.

> x <- 1
> if(x < 2) print("Less than") else print("Greater than")
[1] "Less than"

If your input variable is a vector, then ifelse might be more suitable:

> x <- 1:3
> ifelse(x<=2, "Less than or equal", "Greater than")
[1] "Less than or equal" "Less than or equal" "Greater than"   

---

To access the help page for if, you need to embed the if in backticks:

?`if`

The help page for ifelse is at:

`?ifelse`

Answer 6

It doesn't explicitly exist, but you can do:

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)

The difference between the two is that condition1 must be a logical vector of length 1, while condition2 must be a logical vector the same length as x, y, and z. The first will return either y or z (the entire object), while the second will return the corresponding element of y (condition2==TRUE) or z (condition2==FALSE).

Also note that ifelse will be slower than if / else if condition, y, and z are all vectors with length 1.