[R] Multiple comparisons: its a trap!

[Peter Dalgaard]

Barry Rowlingson <B.Rowlingson at lancaster.ac.uk> writes:

> Liaw, Andy wrote:
> > Stupid me: fell into this trap:
> >
> >>0 == 0 == 0
> > [1] FALSE
> >
> 
>   Ouch!
> 
>   Python's comparison operators don't have this trap, since they
> unravel each comparison pair in a chain so that:
> 
>    (A op1 B op2 C)
> 
> becomes:
> 
>    (A op1 B) and (B op2 C)

[chop]

>   Of course old hand Fortran programmers understand all this since the
> second thing they learnt (after learning how to tap the space bar six
> times) was the order of precedence of operators...

SAS does likewise, at least in recent versions. Whether this kind of
syntactical exceptions is actually helpful is debatable. The problem
is that you get to teach people that comparisons are binary operators
except when they are not... 

I wonder how Python actually manages this; doesn't look like something
that is easy to implement in a yacc-style parser.

-- 
   O__  ---- Peter Dalgaard             Blegdamsvej 3  
  c/ /'_ --- Dept. of Biostatistics     2200 Cph. N   
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