[R] Re: problem with while loop with next
Luke Tierney
luke at nokomis.stat.umn.edu
Tue Oct 2 22:32:33 CEST 2001
On Tue, Oct 02, 2001 at 04:20:36PM -0400, Liaw, Andy wrote:
> Prof. Tierney,
>
> Thanks very much for the info.
>
> Why does the loop work if I move the assignment out of the condition? E.g.,
> the following works:
>
> i <- 0
> while(i < 5) {
> i <- i + 1
> if(i < 3) next
> print(i)
> }
>
> Regards,
> Andy
The bug in the code is that `next' causes a jump directly to
evaluating the body for the next iteration without first evaluating
the test expression. That happens to be OK with this way of writing
the loop since there are no side effects in the condition and the
condition is always true if the jump occurs.
luke
>
> > -----Original Message-----
> > From: Luke Tierney [mailto:luke at nokomis.stat.umn.edu]
> > Sent: Tuesday, October 02, 2001 3:36 PM
> > To: Liaw, Andy
> > Cc: r-help (E-mail)
> > Subject: Re: problem with while loop with next (was RE: [R] file
> > connection, w hile, readLines and browser)
> >
> >
> > 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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--
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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