[R] a < b < c is alway TRUE

[lhomer]

----- Original Message -----
From: Thomas Lumley <tlumley at u.washington.edu>
To: Duncan Murdoch <murdoch at stats.uwo.ca>
Cc: <r-help at stat.math.ethz.ch>
Sent: Friday, July 06, 2001 8:29 AM
Subject: Re: [R] a < b < c is alway TRUE


> On Fri, 6 Jul 2001, Duncan Murdoch wrote:
> > In R like C, FALSE is 0 and TRUE is 1.  This is a bad thing, but it's
> > too late to change it now.  With that substitution, all the results
> > look reasonable:  0>1, 1<1 and 1>1 are all false, but 0<1 is true.
> >
> > Why a bad thing?  Because it leads to absurdities like this whole
> > thread has been discussing!
> >
> > What should they have done?  They should have done what Fortran,
> > Pascal, etc. do, and have a separate logical or boolean type that
> > isn't automatically converted to a numerical type.  In Pascal for
> > instance, "3 < 2 < 1" is flagged as a syntax error, because you can't
> > compare a boolean to an integer.  If you really want to do the weird
> > comparison that R is doing, you need to enter it as "ord(3 < 2) < 1",
> > and any reader will see that you're doing something weird.
>
>
> Well, actually R does have a separate logical type for precisely this
> reason. That's why, say
>   2+2==4
> evaluates to TRUE in R, not to 1,


> (2+2)==4
[1] TRUE
> ((2+2)==4)==1
[1] TRUE
>
....But you knew that....



 and why logical subscripts are different
> from integer subscripts
> For example
>   x[is.na(x)]
> returns the non-missing elements of x, but x[as.numeric(is.na(x))] returns
> as many copies of the first elements as there are missing elements.
>
> The feature/bug/wart in R that causes this whole discussion is that
> numerical operators in R try to coerce their arguments to numbers, so that
> if a or b is logical in
>   a<b
> we actually evaluate
>   as.numeric(a)<as.numeric(b)
>
> Also, we coerce going the other way in if() statements, to make
> if(length(x))
> work. This latter coercion is relatively recent and was done for
> compatibility with C programmers writing in S.
>
> Certainly a<b<c has no sensible use: if c is numeric it is nonense and if
> c is logical it is c & !(a<b).  I would even contend that it is a bad sign
> if you know what
> 3<2<1
> will evaluate to without careful thought or testing on a copy of R.
>
>
> -thomas
>
> Thomas Lumley Asst. Professor, Biostatistics
> tlumley at u.washington.edu University of Washington, Seattle
>
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