R help - Feb 2010 - group factor levels

Dear list,

I cannot find an elegant solution to this problem. I have a factor f
containing several levels (5) and I wish to create a new factor of the
same length with fewer levels (2). This new factor should therefore
group together some levels of the original data. Ideally this grouping
would be at random, i.e I would not group together the first 2 levels
of f, then the following 3, etc.

Below is a minimal example (my real problem has more levels, otherwise
I would do the operation manually...)

f <- factor(rep(sample(letters[1:5], 20, repl=TRUE), each=10))

# permute the levels in random order
disorder <- sample(levels(f), length(levels(f)))

# new levels matching the old ones
new.lev <- rep(LETTERS[1:2], length=length(disorder))

# associate old levels to new ones
groups <- split(disorder, new.lev)

# test each element of f for its new category
test <- lapply(groups, function(g) f %in% g)

# f2 is the new factor, initialized with f
f2 <- as.character(f)

# recursively modify f2
sapply(seq_along(test), function(ii) f2[test[[ii]]] <<- names(test[ii]))

# make it a factor
f2 <- factor(f2)

Any suggestions are very welcome, I must have missed something more obvious!

Best regards,

baptiste

Hi

r-help-bounces at r-project.org napsal dne 03.02.2010 14:52:02:
> Dear list,
> 
> I cannot find an elegant solution to this problem. I have a factor f
> containing several levels (5) and I wish to create a new factor of the
> same length with fewer levels (2). This new factor should therefore
> group together some levels of the original data. Ideally this grouping
> would be at random, i.e I would not group together the first 2 levels
> of f, then the following 3, etc.
> 
> Below is a minimal example (my real problem has more levels, otherwise
> I would do the operation manually...)
> 
> f <- factor(rep(sample(letters[1:5], 20, repl=TRUE), each=10))
> 
> # permute the levels in random order
> disorder <- sample(levels(f), length(levels(f)))
> 
> # new levels matching the old ones
> new.lev <- rep(LETTERS[1:2], length=length(disorder))
> 
> # associate old levels to new ones
> groups <- split(disorder, new.lev)
> 
> # test each element of f for its new category
> test <- lapply(groups, function(g) f %in% g)
> 
> # f2 is the new factor, initialized with f
> f2 <- as.character(f)
> 
> # recursively modify f2
> sapply(seq_along(test), function(ii) f2[test[[ii]]] <<-
names(test[ii]))
> 
> # make it a factor
> f2 <- factor(f2)
> 
> Any suggestions are very welcome, I must have missed something more 
obvious!

# order levels
f.t<-factor(f, levels=disorder)
# change levels
levels(f.t) <- new.lev
all.equal(f.t,f2)
[1] TRUE


Regards
Petr 

> 
> Best regards,
> 
> baptiste
> 
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