Does the ternary operator exist in R?
ROperatorsR Problem Overview
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!
R Solutions
Solution 1 - R
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.
Solution 2 - R
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.
Solution 3 - R
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
Solution 4 - R
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"
Solution 5 - R
Just as a prank, you can redefine the ? operator to (almost) work like the ternary operator (THIS IS A BAD IDEA):
`?` <- function(x, y) { y <-substitute(y); if(x) eval(y[[2]], parent.frame()) else eval(y[[3]], parent.frame()) }
x <- 1:3
length(x) ? (x*2) : 0
x <- numeric(0)
length(x) ? (x*2) : 0
for(i in 1:5) cat(i, (i %% 2) ? "Odd\n" : "Even\n")
... But you need to put the expressions in parentheses because the default precedence isn't like in C.
Just remember to restore the old help function when you're done playing:
rm(`?`)
Solution 6 - R
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`
Solution 7 - R
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.
Solution 8 - R
I have written a small language extension which emulates C's conditional ternary operator in R. It can be installed as a package from here
The implementation is based on the answer given by @kohske, but I have made some changes so that it is robust to cases when the if_true and if_false arguments contain a colon, allows conditional statements to be chained and retains the base functionality of the ? operator.
I will refer to others' warnings about the dangers of redefining operators, but it's a neat example of how dynamic a language R can be!