Why can't R's ifelse statements return vectors?

Question

The documentation for ifelse states:

ifelse returns a value with the same shape as test which is filled with elements selected from either yes or no depending on whether the element of test is TRUE or FALSE.

Since you are passing test values of length 1, you are getting results of length 1. If you pass longer test vectors, you will get longer results:

> ifelse(c(TRUE, FALSE), c(1, 2), c(3, 4))
[1] 1 4

So ifelse is intended for the specific purpose of testing a vector of booleans and returning a vector of the same length, filled with elements taken from the (vector) yes and no arguments.

It is a common confusion, because of the function's name, to use this when really you want just a normal if () {} else {} construction instead.

I bet you want a simple if statement instead of ifelse - in R, if isn't just a control-flow structure, it can return a value:

> if(TRUE) c(1,2) else c(3,4)
[1] 1 2
> if(FALSE) c(1,2) else c(3,4)
[1] 3 4

Note that you can circumvent the problem if you assign the result inside the ifelse:

ifelse(TRUE, a <- c(1,2), a <- c(3,4))
a
# [1] 1 2

ifelse(FALSE, a <- c(1,2), a <- c(3,4))
a
# [1] 3 4

yeah, I think ifelse() is really designed for when you have a big long vector of tests and want to map each to one of two options. For example, I often do colors for plot() in this way:

plot(x,y, col = ifelse(x>2,  'red', 'blue'))

If you had a big long vector of tests but wanted pairs for outputs, you could use sapply() or plyr's llply() or something, perhaps.

use `if`, e.g.

> `if`(T,1:3,2:4)
[1] 1 2 3

Sometimes the user just needs a switch statement instead of an ifelse. In that case:

condition <- TRUE
switch(2-condition, c(1, 2), c(3, 4))
#### [1] 1 2

(which is another syntax option of Ken Williams's answer)

Here is an approach similar to that suggested by Cath, but it can work with existing pre-assigned vectors

It is based around using the get() like so:

a <- c(1,2)
b <- c(3,4)
get(ifelse(TRUE, "a", "b"))
# [1] 1 2