reference counting bug related to break and next in loops

-----Original Message-----
From: luke at stat.uiowa.edu [mailto:luke at stat.uiowa.edu] 
Sent: Wednesday, June 10, 2009 1:05 PM
To: William Dunlap
Cc: r-devel at r-project.org
Subject: Re: [Rd] reference counting bug related to break and 
next in loops

Thanks for the report.

It turns out that a similar issue arises in while() loops without
break/next being involved because the test expression is evaluated
after the final body evaluation.  After some discussion we decided it
was simplest both for implementation and documentation to have the
value of a loop expression always be NULL.  This is now implemented in
R-devel.

luke

On Tue, 2 Jun 2009, William Dunlap wrote:

One of our R users here just showed me the following problem while
investigating the return value of a while loop.  I added some
information
on a similar bug in for loops.  I think he was using 2.9.0
but I see the same problem on today's development version of 2.10.0
(svn 48703).

Should the semantics of while and for loops be changed 

slightly to avoid

the memory
buildup that fixing this to reflect the current docs would 

entail?  S+'s

loops return nothing useful - that change was made long ago to avoid
memory buildup resulting from semantics akin the R's 

present semantics.

Bill Dunlap
TIBCO Software Inc - Spotfire Division
wdunlap tibco.com

--------------------Forwarded (and edited) message

below---------------------------------------------------------
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I think I have found another reference counting bug.

If you type in the following in R you get what I think is the wrong
result.

i = 1; y = 1:10; q = while(T) { y[i] = 42; if (i == 8) { 

break }; i =

i + 1; y}; q
[1] 42 42 42 42 42 42 42 42  9 10

I had expected  [1] 42 42 42 42 42 42 42  8  9 10 which is 

what you get

if you add 0 to y in the last statement in the while loop:

i = 1; y = 1:10; q = while(T) { y[i] = 42; if (i == 8) { 

break }; i =

i + 1; y + 0}; q
[1] 42 42 42 42 42 42 42  8  9 10

Also,

i = 1; y = 1:10; q = while(T) { y[i] = 42; if (i == 8) { break };

i<-i+1 ; if (i<=8&&i>3)next ; cat("Completing iteration", 

i, "\n"); y};

q
Completing iteration 2
Completing iteration 3
[1] 42 42 42 42 42 42 42 42  9 10

but if the last statement in the while loop is y+0 instead 

of y I get

the
expected result:

i = 1; y = 1:10; q = while(T) { y[i] = 42; if (i == 8) { break };

i<-i+1 ; if (i<=8&&i>3)next ; cat("Completing iteration", i, "\n");
y+0L}; q
Completing iteration 2
Completing iteration 3
[1] 42 42  3  4  5  6  7  8  9 10

A background to the problem is that in R a while-loop 

returns the value

of the last iteration. However there is an exception if an 

iteration is

terminated by a break or a next. Then the value is the value of the
previously completed iteration that did not execute a break or next.
Thus in an extreme case the value of the while may be the 

value of the

very first iteration even though it executed a million iterations.

Thus to implement that correctly one needs to keep a 

reference to the

value of the last non-terminated iteration. It seems as if 

the current R

implementation does that but does not increase the reference counter
which explains the odd behavior.

The for loop example is

z<-{ tmp<-rep(pi,10);for(i in 1:10){ tmp[i]<-i^2;if(i==9)break ; if

(i<9&&i>3)next ; tmp } }

z

[1]  1.000000  4.000000  9.000000 16.000000 25.000000 36.000000
49.000000
[8] 64.000000 81.000000  3.141593

z<-{ tmp<-rep(pi,10);for(i in 1:10){ tmp[i]<-i^2;if(i==9)break ; if

(i<9&&i>3)next ; tmp+0 } }

z

[1] 1.000000 4.000000 9.000000 3.141593 3.141593 3.141593 3.141593
3.141593
[9] 3.141593 3.141593

I can think of a couple of ways to solve this.

1.       Increment the reference counter. This solves the 

bug but may

have serious performance implications. In the while example above it
needs to copy y in every iteration.

2.       Change the semantics of while loops by getting rid of the
exception described above. When a loop is terminated with a 

break the

value of the loop would be NULL. Thus there is no need to keep a
reference to the value of the last non-terminated iteration.

Any opinions?

-- 
Luke Tierney
Chair, Statistics and Actuarial Science
Ralph E. Wareham Professor of Mathematical Sciences
University of Iowa                  Phone:             319-335-3386
Department of Statistics and        Fax:               319-335-3017
    Actuarial Science
241 Schaeffer Hall                  email:      luke at stat.uiowa.edu
Iowa City, IA 52242                 WWW:  http://www.stat.uiowa.edu