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

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 

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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 

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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.