[Rd] ifelse() woes ... can we agree on a ifelse2() ?

[Duncan Murdoch]

On 06/08/2016 10:18 AM, Martin Maechler wrote:
> Dear R-devel readers,
> ( = people interested in the improvement and development of R).
>
> This is not the first time that this topic is raised.
> and I am in now state to promise that anything will result from
> this thread ...
>
> Still, I think the majority among us has agreed that
>
> 1) you should never use ifelse(test, yes, no)
>    if you know that length(test) == 1, in which case
>       	  if(test) yes else no
>    is much preferable  (though not equivalent: ifelse(NA, 1, 0) !)
>
> 2) it is potentially inefficient by design since it (almost
>    always) evaluates both 'yes' and 'no' independent of 'test'.
>
> 3) is a nice syntax in principle, and so is often used, also by
>    myself, inspite of '2)'  just because nicely self-explaining
>    code is sometimes clearly preferable to more efficient but
>    less readable code.
>
> 4) it is too late to change ifelse() fundamentally, because it
>    works according to its documentation
>    (and I think very much the same as in S and S-PLUS) and has
>    done so for ages.
>
> ---- and if you don't agree with  1) -- 4)  you may pretend for
>      a moment instead of starting to discuss them thoroughly.
>
> Recently, a useR has alerted me to the fact that my Rmpfr's
> package arbitrary (high) precision numbers don't work for a
> relatively simple function.
>
> As I found the reason was that that simple function used
>  ifelse(.,.,.)
> and the problem was that the (*simplified*) gist of ifelse(test, yes, no)
> is
>
>   test <- as.logical(test)
>   ans <- test
>   ans[ test] <- yes
>   ans[!test] <- no
>
> and in case of Rmpfr, the problem is that
>
>    <logical>[<logical>]  <-  <mpfr>
>
> cannot work correctly
>
>     [[ maybe it could in a future R, if I could define a method
>
>        setReplaceMethod("[", c("logical,"logical","mpfr"),
>                         function(x,i,value) .........)
>
>        but that currently fails as the C-low-level dispatch for '[<-'
>        does not look at the full signature
>      ]]
>
> I vaguely remember having seen proposals for
> light weight substitutes for ifelse(),  called
>  ifelse1() or
>  ifelse2() etc...
>
> and I wonder if we should not try to see if there was a version
> that could go into "base R" (maybe the 'utils' package, not
>      	      	   	     'base'; that's not so important).
>
> One difference to ifelse() would be that the type/mode/class of the result
> is not initialized by logical, by default but rather by the
> "common type" of  yes and no ... maybe determined  by  c()'ing
> parts of those.
> The idea was that this would work for most S3 and S4 objects for
> which logical 'length', (logical) indexing '[', and 'rep()' works.

I think your description is more or less:

    test <- as.logical(test)
    ans <- c(yes, no)[seq_along(test)]
    ans <- ans[seq_along(test)]
    ans[ test] <- yes[test]
    ans[!test] <- no[!test]

(though the implementation details would vary, and recycling rules would 
apply if the lengths of test, yes and no weren't all equal).

You didn't mention what happens with attributes.  Currently we keep the 
attributes from test, which probably doesn't make a lot of sense. In 
particular,

ifelse(c(TRUE, FALSE), factor(2:3), factor(3:4))

returns nonsense, as does my translation of your idea above.

That implementation also drops attributes. I'd say this definition would 
make more sense:

    test <- as.logical(test)
    ans <- yes
    ans[!test] <- no[!test]

(and this is suggested as an alternative in ?ifelse).  It generates an 
error in my test example, which seems reasonable.  It gives the "right" 
thing in

ifelse(c(TRUE, FALSE), factor(2:3), factor(3:2))

because the factors have the same levels.

The lack of symmetry between yes and no is slightly irksome, but I would 
think in most cases you could choose attributes from just one of yes and 
no to be what you want in the result (and use !test to swap the order if 
necessary).

>
> One possibility would also be to consider  a "numbers-only" or
> rather "same type"-only {e.g., would also work for characters}
> version.

I don't know what you mean by these.
>
> Of course, an ifelse2()  should also be more efficient than
> ifelse() in typical "atomic" cases.

I don't think it is obvious how to make it more efficient.  ifelse() 
already skips evaluation of yes or no if not needed.  (An argument could 
be made that it would be better to guarantee evaluation of both, but 
it's usually easy enough to do this explicitly, so I don't see a need.)

Duncan Murdoch

>
>
> Thank you for your ideas and suggestions.
> Again, there's no promise of implementation coming along with this e-mail.
>
> Martin Maechler
> ETH Zurich
>
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