[R] possibly Error in R version 2.12.1 (2010-12-16)

[peter dalgaard]

On Feb 2, 2012, at 21:24 , Frank Schwidom wrote:

> Hi, 
> 
> the following Code demonstrates an possibly Error in R
> (or you can explain me, why this happens, thanks in advance)

Looks like an effect of lazy evaluation: The value of i is not evaluated until after the loop has ended, at which point it will be 2. This is a feature, not an error, even if it confuses people at times...

-pd


> 
> Code:
> 
> #####################################
> 
> testClass <- function( stackData= c())
> {
> 
> list(
> 
>  write= function( ...)
>  {
>   sChain= ""
>   for( s in c( stackData, ...))
>   {
>    sChain= paste( sChain, '"', sub( '"', '\\\\"', s), '"', sep, sep='')
>   }
>   write( sChain, fHandle, append=TRUE)
>  },
> 
>  stackIt1 = function( ...)
>  {
>   testClass( stackData= c( stackData, ...))
>  },
> 
>  stackIt2 = function( ...)
>  {
>   tmp= c( stackData, ...)
>   testClass( stackData= tmp)
>  },
> 
>  getStack = function()
>  {
>   stackData
>  },
> 
>  NULL
> )
> }
> 
> to1= testClass()
> 
> for( i in 4:2)
> {
> to1= to1$stackIt1( i)
> }
> 
> print( all( rep( 2, 3) == to1$getStack())) # error!
> 
> to2= testClass()
> 
> for( i in 4:2)
> {
> to2= to2$stackIt2( i)
> }
> 
> print( all( 4:2 == to2$getStack())) # correct!
> 
> # what ist the difference between stackIt1 and stackIt2?
> # (error appears only by using an for loop)
> 
> "
>> version
> _
> platform       i486-pc-linux-gnu
> arch           i486
> os             linux-gnu
> system         i486, linux-gnu
> status
> major          2
> minor          12.1
> year           2010
> month          12
> day            16
> svn rev        53855
> language       R
> version.string R version 2.12.1 (2010-12-16)
> 
> Regards
> "
> 
> ##################################### End of Code
> 
> written in an R-File and called per source( '<Fname>.R')
> shows 2 subsequent outputs of 'TRUE', which is not ok
> in my mind
> 
> Thanks for your attention
> 
> ______________________________________________
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> PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.

-- 
Peter Dalgaard, Professor
Center for Statistics, Copenhagen Business School
Solbjerg Plads 3, 2000 Frederiksberg, Denmark
Phone: (+45)38153501
Email: pd.mes at cbs.dk  Priv: PDalgd at gmail.com