problem with while loop with next (was RE: [R] file connection, w hile, readLines and browser)

Tue, Oct 2, 2001 11:54 AM

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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problem with while loop with next (was RE: [R] file connection, w hile, readLines and browser)

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