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

Question

I've found R's ifelse statements to be pretty handy from time to time. For example:

ifelse(TRUE,1,2)
# [1] 1
ifelse(FALSE,1,2)
# [1] 2

But I'm somewhat confused by the following behavior.

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

Is this a design choice that's above my paygrade?

little strange design for ifelse given the fact that simple if else works. — 2sb, Jun 29 '12 at 03:55
ifelse is a vectorized function. They should be used for different tasks. — marbel, Jan 11 '14 at 05:07

score 124 · Accepted Answer · edited Apr 27 '18 at 16:35

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.

Perhaps what you really wanted for the second set of statements was `if (TRUE) c(1,2) else c(3,4)`. — Jonathan Chang, Aug 26 '09 at 16:33

score 78 · Answer 2 · answered Nov 18 '09 at 17:27

78

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

answered Nov 18 '09 at 17:27

Ken Williams

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@Ken, this works for me, even though I get what I need a constant warning `" Warning in if (req(inputval) == "All") { : the condition has length > 1 and only the first element will be used"` what should I do to get rid of this warning ? – user5249203 Jan 05 '18 at 21:23
1

@user5249203, the question and Ken's answer refer to the case where the condition is a single value, i.e., a vector of length 1. The warning indicates that `req(inputval)` has more elements. To get a single value the functions `any()` or `all()` might be useful. – Uwe Jul 20 '18 at 07:14

score 17 · Answer 3 · answered Apr 06 '18 at 15:03

17

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

answered Apr 06 '18 at 15:03

Cath

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IMHO, this is encouraging to misuse the vectorized `ifelse()` function in place of a control flow `if ... else ...` for assignment. If the condition is a single `TRUE` or `FALSE` value, I would prefer to write `a <- if (TRUE) c(1,2) else c(3,4)` or `if (TRUE) a <- c(1,2) else a <- c(3,4)` – Uwe Jul 20 '18 at 06:51
2

@Uwe though I don't think the difference in performance when using `ifelse` instead of `if`...`else` in case of a single condition can really be a problem and `ifelse` may be preferred in some cases inside code (simple guess here), I cannot disagree with you ;-). I just wanted to show a way with `ifelse`. – Cath Jul 20 '18 at 06:59
You could also slightly abuse `ifelse` and `list`s - `ifelse(TRUE, list(c(1,2)), list(c(3,4)) )[[1]]` – thelatemail May 25 '21 at 22:21

score 15 · Answer 4 · answered Jul 06 '19 at 16:45

15

use `if`, e.g.

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

answered Jul 06 '19 at 16:45

blueskyddd

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This is the only answer here that can actually provide the expected functionality of ifelse. – sus_mlm Jun 26 '20 at 15:24
Is there any URL about \`if\` ? – Takuro Ikeda Feb 09 '22 at 16:27
@TakuroIkeda see http://adv-r.had.co.nz/Functions.html – blueskyddd Mar 04 '22 at 16:56

score 9 · Answer 5 · answered Aug 28 '09 at 20:30

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.

score 4 · Answer 6 · answered Jul 07 '17 at 21:13

4

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)

answered Jul 07 '17 at 21:13

agenis

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score 4 · Answer 7 · answered Jul 25 '18 at 20:50

4

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

answered Jul 25 '18 at 20:50

bmonger

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score 2 · Answer 8 · answered May 20 '19 at 17:21

2

In your case, using if_else from dplyr would have been helpful: if_else is more strict than ifelse, and throws an error for your case:

library(dplyr)
if_else(TRUE,c(1,2),c(3,4))
#> `true` must be length 1 (length of `condition`), not 2

answered May 20 '19 at 17:21

Matifou

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score 0 · Answer 9 · answered Jul 15 '20 at 02:34

0

Found on everydropr:

ifelse(rep(TRUE, length(c(1,2))), c(1,2),c(3,4))
#>[1] 1 2

Can replicate the result of your condition to return the desired length

answered Jul 15 '20 at 02:34

SJGD

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

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