[R] Multiple comparisons: its a trap!

Peter Dalgaard p.dalgaard at biostat.ku.dk
Mon Jul 26 06:47:07 CEST 2004

Gabor Grothendieck <ggrothendieck at myway.com> writes:

>  <Ted.Harding <at> nessie.mcc.ac.uk> writes:
> : 
> : On 25-Jul-04 Gabor Grothendieck wrote:
> : > Don't know how Python does it but its not the only one and
> : > I believe its often done like this. Rather than have a Boolean
> : > type, NULL is defined to be false and anything else is true.
> : > If the comparison is TRUE then the right argument is returned;
> : > otherwise NULL is returned.
> : This is weird, and I'm not sure what is being discussed here.
> We were discussing how some other languages string together comparison 
> operators without an intermediate  and  to connect them.  This discussion
> has nothing to do with R other than possibly to understand whether it
> could fit within the R framework.

Yes. The other side of the coin is that we do actually use the
TRUE/FALSE == 1/0  convention in places. E.g. (x>0)-(x<0) for the
sign of x, or x*(x>0) for x left-censored at 0. So changing the
current semantics is not really in the cards. Turning x<y<z  into a
syntax error is on the other hand quite simple (at least according to
5 seconds worth of googling for "yacc nonassoc") and we should
probably consider doing so.

   O__  ---- Peter Dalgaard             Blegdamsvej 3  
  c/ /'_ --- Dept. of Biostatistics     2200 Cph. N   
 (*) \(*) -- University of Copenhagen   Denmark      Ph: (+45) 35327918
~~~~~~~~~~ - (p.dalgaard at biostat.ku.dk)             FAX: (+45) 35327907

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