[Rd] Very Long Expressions

[Uwe Ligges]

McGehee, Robert wrote:

> 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

See ?options and set:

   options(expressions = 1000)
   eval(parse(text = paste(rep(1, 499), collapse = "+")))

Uwe Ligges


> 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. 
> 
> 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.
> 
> Thus, to count to 600 I could just add the first 497 terms with the next
> 103 terms.
> 
>>eval(parse(text = paste(rep(1, 497), collapse = "+"))) +
> 
>  eval(parse(text = paste(rep(1, 103), collapse = "+")))
> [1] 600
> 
> But without an error or way of figuring out if the expression would even
> be evaluated, I'm not sure how to know when to start or stop the peeling
> process. It also may become more complicated when parentheses are
> introduced.
> 
> Any help would be greatly appreciated.
> 
> Thanks,
> Robert
> 
> platform i386-pc-mingw32
> arch     i386           
> os       mingw32        
> system   i386, mingw32  
> status                  
> major    2              
> minor    0.1            
> year     2004           
> month    11             
> day      15             
> language R              
> 
> Robert McGehee
> Geode Capital Management, LLC
> 53 State Street, 5th Floor | Boston, MA | 02109
> Tel: 617/392-8396    Fax:617/476-6389
> mailto:robert.mcgehee at geodecapital.com
> 
> 
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