[Rd] reference counting bug related to break and next in loops

From: William Dunlap <wdunlap_at_tibco.com>
Date: Tue, 02 Jun 2009 20:17:49 -0700


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-------------------------------------------------------------------

 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?



R-devel_at_r-project.org mailing list
https://stat.ethz.ch/mailman/listinfo/r-devel Received on Wed 03 Jun 2009 - 03:21:51 GMT

Archive maintained by Robert King, hosted by the discipline of statistics at the University of Newcastle, Australia.
Archive generated by hypermail 2.2.0, at Wed 10 Jun 2009 - 21:36:21 GMT.

Mailing list information is available at https://stat.ethz.ch/mailman/listinfo/r-devel. Please read the posting guide before posting to the list.

list of date sections of archive