[Rd] Very Long Expressions

[McGehee, Robert]

Thank you all for your reply. This was exactly what I was looking for.
Two quick points. One, as Peter Dalgaard pointed out, wrapping the
expression in a try() gives this error:

Error in eval(expr, envir, enclos) : evaluation nested too deeply: 
   infinite recursion / options(expression=)?
                                         ^^^
However, from the following posts, it seems that options(expressions=)
(plural) is what we're looking for instead. If so, this error message
should be corrected anyway.

Secondly, the ?options help (thanks for everyone who reminded me about
this), says that expressions can have values between 25...100000.

However, if the original example is set past 4995 on my computers, I
receive a stack overflow.
> eval(parse(text = paste(rep(1, 4996), collapse = "+")))
Error: protect(): stack overflow

I'm pointing this out not because I will ever approach anywhere near
this level of nesting, but perhaps that options(expression=5000+) might
not be meaningful anyway, (unless there are
processor/compilation-specific ways of getting rid of this stack
overflow). If so, one might considering lowering the range of valid
expressions values in the help file.

Thanks again, your replies are, as always, invaluable,
Robert

-----Original Message-----
From: Peter Dalgaard [mailto:p.dalgaard at biostat.ku.dk] 
Sent: Monday, January 24, 2005 5:36 AM
To: McGehee, Robert
Cc: r-devel at stat.math.ethz.ch
Subject: Re: [Rd] Very Long Expressions


"McGehee, Robert" <Robert.McGehee at geodecapital.com> writes:

> Greetings,
> I'm having some difficulties with evaluating very long expressions
> (Windows/Linux 2.0.1), as seen below, and would greatly appreciate any
> help, thoughts or work arounds. Let's say that I wanted to see what I
> would get if I added 1 to itself 498 times. One way of doing this
would
> be to evaluate the expression 1+1+1+... 
> 
> > eval(parse(text = paste(rep(1, 498), collapse = "+")))
> [1] 498
> 
> However, if we try this with 499+ items we get no answer:
> > a <- eval(parse(text = paste(rep(1, 499), collapse = "+")))
> > a
> Error: Object "a" not found
> 
> And if this eval is passed to any other function, that function exits
> without error and without returning and object.
> 
> So it seems that we've reached some upper limit of evaluation terms.
> While the parser is able to correctly create the long expression, eval
> does not successfully evaluate it.
 
> My problem is that since the evaluator does not return an object,
error,
> or warning, I'm not able to easily code around this problem. Also,
I've
> thought of no easy way to "count" the terms in the expression to see
> whether we've breached the upper limit or not. 

It's a bug. 1.9.1 had 

>  eval(parse(text = paste(rep(1, 499), collapse = "+")))
Error in eval(expr, envir, enclos) : evaluation nested too deeply: 
   infinite recursion / options(expression=)?

which also contains the hint about how to raise the limit.

You do see it if you do

> a <- try(eval(parse(text = paste(rep(1, 499), collapse = "+"))))
> a
[1] "evaluation nested too deeply: infinite recursion /
options(expression=)?"
attr(,"class")
[1] "try-error"

but that's obviously no excuse for not printing the message. The
problem appears still to be present in r-devel (the version at hand
was dated Jan.12, though).

> If I were able to see if the eval would work on a particular
expression,
> one thing I had considered was to make an eval.long wrapper that peels
> terms off the right hand side of an overly-long expression until every
> sub-expression is legal.

But do you *really* want to do it this way? Why? 

BTW, it's not really the length of the expression but its depth. The
parse tree for 1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 is really
(((((((1+2)+3)+4)+5)+6)+7)+8). So you get 7 levels of parentheses. You
can easily have less deeply nested parentheses:
((1+2)+(3+4))+((5+6)+(7+8))

With that sort of pattern, adding 500 terms requires a nesting no more
than 9 levels deep. Persuading R to nest that way is a bit tricky
though. 

-- 
   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