simple randomization question: How to perform "sample" in chunks
On Aug 20, 2009, at 11:22 AM, Tal Galili wrote:
Hello dear R-help group.
My task looks simple, but I can't seem to find a "smart" (e.g: non
loop)
solution to it.
Task: I wish to randomize a data.frame by one column, while keeping
the
inner-order in the second column as is.
So for example, let's say I have the following data.frame:
xx <-data.frame(a= c(1,2,2,3,3,3,4,4,4,4) ,
b = c(1,1,2,1,2,3,1,2,3,4) )
I would like to shuffle it by column "a", while keeping the order in
column
"b".
Here is my "not-smart" way of doing it:
# R example
xx <-data.frame(a= c(1,2,2,3,3,3,4,4,4,4) ,
b = c(1,1,2,1,2,3,1,2,3,4) )
randomize.by.column.a <- function(xx)
{
new.a.order <- sample(unique(xx$a))
new.xx <- NULL
for(i in new.a.order)
{
xx.subset <- xx[ xx$a %in% i ,]
new.xx <- rbind(new.xx , xx.subset)
}
return(new.xx)
}
randomize.by.column.a(xx)
# END of - R example
It was a bit confusing to read that you wanted to "keep the order in column "b"", but your code implies that you wanted to carry the b- values along with the sorted a-values. I think this achieves the same goal: xx[sample(1:nrow(xx)), ]
David Winsemius, MD Heritage Laboratories West Hartford, CT