problem with while loop with next (was RE: [R] file connection, w hile, readLines and browser)
Luke Tierney
luke at nokomis.stat.umn.edu
Tue Oct 2 21:36:06 CEST 2001
It does look like `next' does not work properly with while loops in
versions through 1.3.1. We did some reorganizing of looping internals
in the development branch and that fixed the problem, so it will be
fixed in 1.4.
luke
On Tue, Oct 02, 2001 at 02:54:16PM -0400, Liaw, Andy wrote:
> Dear R-help,
>
> I think I have kinda isolated the problem I had to the following:
>
> i <- 0
> while( {i <- i + 1} < 5) {
> if(i < 3) next
> print(i)
> }
>
> This seems to go into an infinite loop. After I break the execution, i has
> the value 1.
>
> At the R prompt, if I start from i <- 0 and keep typing {i <- i + 1} < 5, it
> eventually evaluate to TRUE. So why does the while loop not work?
>
> (BTW, the same while loop runs as I expected in Splus 6.0.3 on WinNT.)
>
> Any hints? I'd really appreciate the help!
>
> Cheers,
> Andy
>
>
> > -----Original Message-----
> > From: Liaw, Andy [mailto:andy_liaw at merck.com]
> > Sent: Tuesday, October 02, 2001 10:30 AM
> > To: 'Robert Gentleman'
> > Cc: r-help (E-mail)
> > Subject: An example (was RE: [R] 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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--
Luke Tierney
University of Minnesota Phone: 612-625-7843
School of Statistics Fax: 612-624-8868
313 Ford Hall, 224 Church St. S.E. email: luke at stat.umn.edu
Minneapolis, MN 55455 USA WWW: http://www.stat.umn.edu
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