[R] combining mathematical notation and value substitution

[Peter Dalgaard BSA]

Thomas Lumley <tlumley at u.washington.edu> writes:

> While the rules for constructing the tree are part of the language
> definition, the order of evaluation might not be.  In C, for example, the
> order of evaluation is not specified except to the extent that precedence
> constrains it (and for a few special operators like && and ||).
> 
> FOr an R example, if you do
> 	f(plot(a), plot(b))
> 
> it is clear that the plot commands must be evaluated before f() returns if
> their return values are actually used. It is not clear which order the
> plots() appear, and I would say that it shouldn't be part of the language
> definition.

...not to mention that lazy evaluation explicitly makes the order dependent
on the function body:

  f <- function(x,y) {x;y}
  f <- function(x,y) {y;x}

will produce the plots in different order when called as above. Notice
in particular that constructs like

  f(a<-b, a)

are playing with fire, unless you're really, really sure that the 1st
arg is evaluated before the 2nd.

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