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write() gotcha

5 messages · Brian Ripley, robin hankin, Roger Bivand

robin hankin
# 
Hi

I used write() the other day to save some results.

It seems that write() does not record the full precision of
the objects being written:


 > write(pi,file="~/f",ncolumns=1)
 > pi.saved <- scan("~/f")
Read 1 item
 > dput(pi)
3.14159265358979
 > dput(pi.saved)
3.141593
 > pi-pi.saved
[1] -3.464102e-07
 >


This difficulty was particularly difficult to find because pi.saved   
*looks*
the same as pi.



What's going on here?






 > R.Version()
$platform
[1] "powerpc-apple-darwin8.7.0"

$arch
[1] "powerpc"

$os
[1] "darwin8.7.0"

$system
[1] "powerpc, darwin8.7.0"

$status
[1] ""

$major
[1] "2"

$minor
[1] "4.0"

$year
[1] "2006"

$month
[1] "10"

$day
[1] "03"

$`svn rev`
[1] "39566"

$language
[1] "R"

$version.string
[1] "R version 2.4.0 (2006-10-03)"


--
Robin Hankin
Uncertainty Analyst
National Oceanography Centre, Southampton
European Way, Southampton SO14 3ZH, UK
  tel  023-8059-7743
Brian Ripley
#
On Mon, 18 Dec 2006, Robin Hankin wrote:

            

      
      
      
    

        
Why not save()?  It is the only way to preserve the results exactly.

            

      
      
      
    

        
See ?cat, for which write is a wrapper.

  
    

      
      
    

  


  


      

    

    

      
      

        


  

        

      robin hankin
    

    

      
      #
    

  


  

      
On 18 Dec 2006, at 08:50, Prof Brian Ripley wrote:

        

            

      
      
      
    

        
Professor Ripley

Thanks for this.  I'll use save() instead.

Also, could we clarify this issue in the write() manpage?  Although its
behaviour is  clear from the code, I'm sure other R users would
appreciate a hint or a warning that precision might be lost.  It cost
me a few hours of worry last night!



--
Robin Hankin
Uncertainty Analyst
National Oceanography Centre, Southampton
European Way, Southampton SO14 3ZH, UK
  tel  023-8059-7743

  
  


  


      

    

    

      
      

        


  

        

      Brian Ripley
    

    

      
      #
    

  


  

      
On Mon, 18 Dec 2006, Robin Hankin wrote:

        

            

      
      
      
    

        
Already done (via a xref to cat for the format used)!

  
    

      
      
    

  


  


      

    

    

      
      

        


  

        

      Roger Bivand
    

    

      
      #
    

  


  

      
On Mon, 18 Dec 2006, Robin Hankin wrote:

        

            

      
      
      
    

        
Use options(digits=) to control the output. ?write says write() is a 
wrapper for cat(), so:

options("digits"=16)
cat(pi, "\n")
write(pi,file="~/f",ncolumns=1)
pi.saved <- scan("~/f")
dput(pi.saved)
dput(pi)

looks OK