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
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