[R] Can we get rid of && and ||?

[Prof Brian D Ripley]

On Thu, 13 Apr 2000, GB wrote:

> 
> I have some doubts about the implementation of the operators  '&&'  and
> '||'. I only discuss  '&&'  here. The reason: One of my students tried
> (slightly simplified here)
> 
> > x <- c(0.5, 1.2, -0.8, 0.7)
> > y <- x[(x > 0) && (x < 1)]
> > y
> [1]  0.5  1.2 -0.8  0.7

Where did he get the idea of using && from?  Only C programmers are likely
to think of it, and it will do what they expect.

> This was not his intention. He wanted
> 
> > y <- x[(x > 0) & (x < 1)]
> > y
> [1]  0.5  0.7
> 
> I suggested that his first attempt should render a "syntax error", since
> '&&'  only applies to scalar logicals. However, upon reading 

More precisely, it takes the first element of a vector (R has no scalars)
and coerces it to logical.

> the documentation, I found that 'a && b' is in fact equivalent to 
> 'a[1] & b[1]' (with one exception).
> 
> Therefore, '&&' seems to be of little use in  R  programming. You can
> almost always use  '&'  instead (This is in sharp contrast to the use of
> &  and  &&  in the  C  programming language). I can only see one
> advantage (admittedly important, though) with  '&&'. In
> 
> if (is.numeric(x) && min(x) > 0) ..... (Found in MASS3, p. 94)
> 
> you avoid  'min(x)' to be evaluated if  x  is non-numeric. This doesn't
> work with  '&', because then both expressions are evaluated before the
> comparison.

That is used a lot in base R.

> Provided there are no other differences, I suggest that either
> 
> a) '&&'  is made obsolete, and  '&'  is 'improved' so that  b  in
>    'a & b' is evaluated only if some component of  a  is TRUE, or

That would break S compatibility and a lot of correctly written code
(including in R itself).

> b) a warning (or syntax error) is given if  '&&'  is used with at least
>    one non-scalar argument.

That might be worthwhile.

-- 
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272860 (secr)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595

-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-
r-help mailing list -- Read http://www.ci.tuwien.ac.at/~hornik/R/R-FAQ.html
Send "info", "help", or "[un]subscribe"
(in the "body", not the subject !)  To: r-help-request at stat.math.ethz.ch
_._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._