R help - Oct 2001 - An example (was RE: file connection, while, readLines and bro wser)

Prof. Gentleman (and R-help),

Here's an example of what didn't work.  I still don't understand
why.

Function:
trycon <- function(file, n) {
  f.con <- file(file, open="rt")
  on.exit(close(f.con))
  i <- 0
  while( length(readln <- readLines(f.con, 1)) > 0 ) {
    x <- unlist(strsplit(readln, " "))
    if(length(x) <= 6 && x[3] == x[4]) next
    i <- i + 1
  }
  return(invisible())
}

Data file:
1 0.00 ABC ABC 1.00 0 
2 0.00 DEF DEF 1.00 0 CDE 1.00 0 XYZ 0.72 0 
3 0.00 abc abc 1.00 0 ghi 1.00 0 stu 0.72 0 lmn 0.80 0 

R call:
trycon("C:/home/data.txt", 4)
Error in while (length(readln <- readLines(f.con, 1)) > 0) { : 
	missing value where logical needed

The "if(...) next" line seems to be the source of the problem, but the
error
message and the behaviour of browser() really throw me off.  Can you shed
some light on this?  (I have a hunch that I'm missing something *real*
obvious...)

Regards,
Andy


> -----Original Message-----
> From: Robert Gentleman [mailto:rgentlem at jimmy.harvard.edu]
> Sent: Tuesday, October 02, 2001 9:50 AM
> To: Liaw, Andy
> Subject: Re: [R] file connection, while, readLines and browser
> 
> 
> On Tue, Oct 02, 2001 at 09:28:49AM -0400, Liaw, Andy wrote:
> > Dear R-help,
> > 
> > I have one more question about the functions mentioned in 
> the subject.
> > Again, the system info is:
> > platform i386-pc-mingw32
> > arch     x86            
> > os       Win32          (OK, it's NT4sp6)
> > system   x86, Win32     
> > status                  
> > major    1              
> > minor    3.1            
> > year     2001           
> > month    08             
> > day      31             
> > language R              
> > 
> > I defined a function like the following:
> > 
> > myfun <- function(file, maxline) {
> >   f.con <- file(file, open="rt")
> >   on.exit(close(f.con))
> >   while(length(readln <- readLines(f.con, 1)) > 0 ) {
> >     ## Do something with readln...
> >   }
> >   ## Do something more...
> > }
> > 
> > This didn't work for me.  It gave me an error at the while 
> statement, saying
> > "NA where logical needed".  I inserted a browser() right 
> inside the while
> > loop and have it step through the function.  It seems to be 
> stuck at the
> > first line of the file and never move down.  This behaviour 
> is different
> > from the call without the browser().  Anyone has ideas why 
> this is the case?
> > 
> 
>  It seems to work for me. Are you sure that's what you did? It is best
>  not to edit anything, get a simple example that shows the error and
>  post that. It makes for a lot less work for those of us that are
>  willing to look at it and it makes sure that we find the problem that
>  you are having.
> 
> 
> > What had worked for me is something like the following:
> >   repeat {
> >     readln <- readLines(f.con, 1)
> >     if(length(readln) == 0) break
> >     ## do something...
> >   }
> > 
> > Any suggestions are greatly appreciated.
> > 
> > Regards,
> > Andy
> > 
> > 
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> -- 
> +-------------------------------------------------------------
> --------------+
> | Robert Gentleman                 phone : (617) 632-5250     
>               |
> | Associate Professor              fax:   (617)  632-2444     
>               |
> | Department of Biostatistics      office: M1B28
> | Harvard School of Public Health  email: 
> rgentlem at jimmy.dfci.harvard.edu   |
> +-------------------------------------------------------------
> --------------+
> 
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