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

Martin Maechler maechler at stat.math.ethz.ch
Sat Aug 6 16:18:45 CEST 2016

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
and the problem was that the (*simplified*) gist of ifelse(test, yes, no)

  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.

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

Of course, an ifelse2()  should also be more efficient than
ifelse() in typical "atomic" cases.

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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