Hi all,
I am playing around with latin squares, and wrote a recursive function
that searches for valid combinations.
Apart from the fact that there are very many, I run into troubles
beginning with size 10x10 because the recursion depth becomes too large
(max of 10x9-1=89 in this case).
Why is this a problem? Isn't there enough space allocated to the stack?
Can this be increased? The memory demand shouldn't be terrible, with
only minimal local variables (only set and the function params r,c,t - s
is local to a block called only once when a solution is found). Even if
variables aren't stored efficiently, a recursion depth of 100 shouldn't
consume more than a couple of kilobytes.
Is this a fundamental misunderstanding of the way R works?
Pascal
BTW: Is there a way to pass variables "by reference" in function
calls?
------
The function stripped-down to the essential looks like this:
latin.square <- function(t = 4)
{
latinCheck <- function(r,c,t)
{
set <- setdiff(LETTERS[1:t],c(m[r,],m[,c]));
for(i in set)
{
m[r,c] <<- i;
if(c<t)
{
latinCheck(r,c+1,t);
}
else
{
if(r<t)
{
latinCheck(r+1,1,t);
}
else # found a solution
{
s <-
paste(m[1,],collapse="",sep="");;
for(i in 2:t)
{
s <-
paste(c(s,"-",m[i,]),collapse="",sep="");
}
cat(s,"\n");
}
}
}
m[r,c] <<- NA;
}
latinSolutions <<- character(0);
fullset <<- LETTERS[1:t];
m <<- matrix(nrow=t,ncol=t);
m[1,] <<- LETTERS[1:t];
latinCheck(2,1,t)
}
l
Perhaps you could take the trouble to read the error message, which is Error in inherits(x, "factor") : evaluation is nested too deeply: infinite recursion? The evaluation depth is controlled by options(expressions=). Increasing it allows your code to run, albeit very slowly. On Tue, 25 Nov 2003, Pascal A. Niklaus wrote:> Hi all, > > I am playing around with latin squares, and wrote a recursive function > that searches for valid combinations. > Apart from the fact that there are very many, I run into troubles > beginning with size 10x10 because the recursion depth becomes too large > (max of 10x9-1=89 in this case). > > Why is this a problem? Isn't there enough space allocated to the stack?It is space for expressions. This is a safety check to avoid infinite recursion overrunning the C-level stack and crashing R (and thereby losing all the current work). There is no portable way to check the C stack size and usage that we know of.> Can this be increased? The memory demand shouldn't be terrible, with > only minimal local variables (only set and the function params r,c,t - s > is local to a block called only once when a solution is found). Even if > variables aren't stored efficiently, a recursion depth of 100 shouldn't > consume more than a couple of kilobytes.Why `shouldn't' it? Have you any idea of the storage requirements of R objects?> Is this a fundamental misunderstanding of the way R works?It is a misreading of a simple message, for sure. [...] -- Brian D. Ripley, ripley at stats.ox.ac.uk Professor of Applied Statistics, http://www.stats.ox.ac.uk/~ripley/ University of Oxford, Tel: +44 1865 272861 (self) 1 South Parks Road, +44 1865 272866 (PA) Oxford OX1 3TG, UK Fax: +44 1865 272595
>>>>> "Pascal" == Pascal A Niklaus <Pascal.Niklaus at unibas.ch> >>>>> on Tue, 25 Nov 2003 16:10:56 +0100 writes:Pascal> Hi all, I am playing around with latin squares, and Pascal> wrote a recursive function that searches for valid Pascal> combinations. Apart from the fact that there are Pascal> very many, I run into troubles beginning with size Pascal> 10x10 because the recursion depth becomes too large Pascal> (max of 10x9-1=89 in this case). Pascal> Why is this a problem? Isn't there enough space Pascal> allocated to the stack? Can this be increased? The Pascal> memory demand shouldn't be terrible, with only Pascal> minimal local variables (only set and the function Pascal> params r,c,t - s is local to a block called only Pascal> once when a solution is found). Even if variables Pascal> aren't stored efficiently, a recursion depth of 100 Pascal> shouldn't consume more than a couple of kilobytes. Pascal> Is this a fundamental misunderstanding of the way R Pascal> works? a slight one, at least: The recursion depth is limited by options(expressions = ...), i.e. getOption("expressions") which is 500 by default. We've had similar problem when drawing a somewhat large dendrogram (of less than 10000 end nodes still). I think we should consider increasing the *default* maximal recursion depth (from 500 to a few thousands) and even think about increasing the maximally allowed value for 'expressions' (which is 100000). Martin