[R-sig-hpc] randomForest and foreach plotting issue

[Matthew Gilbert]

Hi,

When I run randomForest sequentially I am able to plot the errors using 
plot() however when I try to perform randomForest within a foreach loop 
the resulting object throws errors when I try to plot it. mse seems to 
not be returned in the parallel structure. Has anyone run into this 
problem before? Below is an example of the issue

Thanks,
Matt

library(randomForest)
library(foreach)
library(doMC)
registerDoMC()

data(mtcars)
rf <- randomForest(mpg ~ ., mtcars, keep.forest=FALSE, ntree=100)
plot(rf)

rf.par <- foreach(ntree=rep(50, 2), .combine=combine, 
.packages='randomForest') %dopar% randomForest(mpg ~ ., mtcars, 
keep.forest=FALSE, ntree=ntree)
plot(rf.par)

Error in array(x, c(length(x), 1L), if (!is.null(names(x))) 
list(names(x),  :
   attempt to set an attribute on NULL

traceback()

7: array(x, c(length(x), 1L), if (!is.null(names(x))) list(names(x),
        NULL) else NULL)
6: as.matrix.default(y)
5: as.matrix(y)
4: ncol(y <- as.matrix(y))
3: matplot(1:x$ntree, err, type = type, xlab = "trees", ylab = "Error",
        main = main, ...)
2: plot.randomForest(rf.par)
1: plot(rf.par)