Unexpected behavior of dplyr::case_when

This surprised me:

x = 1:2

  length(x)==1 ~ x,
  length(x)==2 ~ x[2]
#> [1] 2 2

It seems that case_when is evaluating each RHS and using that to determine the length of the result. Is there a way to avoid this? In my actual use case I am always expecting a character(1) so I subscript the result but it feels pretty hacky. It's easy to imagine situations where not every RHS is a valid expression in the current environment.

Any thoughts?

How do you expect the RHS to be evaluated? (Genuine question, I'm not quite clear!)

Couple of GH issues that might be illuminating and/or you might want to jump in on:

(plus StackOverflow thread related to the above)

I expected that the result of the case_when would be the same as the result of evaluating the relevant RHS. In my example, the second LHS is true so I expect the value to be x[2] which is 2. Instead, case_when seems to be evaluating every RHS, deciding that the result should have length 2, and extending the value to c(2, 2).

My actual use case was formatting a result which has varying number of values. Here is an example which is closer to what I was doing:

format_x = function(x) {
    length(x)==1 ~ as.character(x),
    length(x)==2 ~ paste(x[1], 'and', x[2])

#> [1] "2"
#> [1] "1 and 2" "1 and 2"

The repeated result for format_x(1:2) is confusing and not what I intended.

I don't see the relevance of the linked issues and SO, they all seem to have to do with NSE which I am not using here.

Oh, I thought you wanted to change the evaluation of the RHS.

case when is vectorized. The output of case_when() will always have the same length as x. You are checking for each element of x whether its length equals 1. This is nonsensical.

For your application you likely do not want to use case_when() but a normal if (...) else construct


Ah, <light goes on>. Thank you @hoelk.

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For your case there is an even simpler (but not very flexible) solution btw

paste(x, collapse = " and ")


My real use case is a little more complex. I rewrote it using switch(length(x)+1, ...) and it works as I wanted :slight_smile:. (Using length(x)+1 because the length can be 0.)

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