R help - Mar 2011 - Difference between the S-plus influence and R empinf functions

Hello everyone !

I am currently trying to convert a program from S-plus to R, and I am
having some trouble with the S-plus function called "influence(data, 
statistic,...)".

This function aims to "calculate empirical influence values and related 
quantities",
and is part of the Resample library that I cannot find for R.
However, 2 similar functions are available in R:
 - the lm.influence(model, ...) function,
 - the empinf(data, statistic,...)" function.
 
I didn't manage to use the lm.influence() function correctly, because it
needs a
linear model
as input (lm, glm), and what I have as input is a function (I don't know
well
R/S-plus languages, 

so I may be mistaken, but I believe lm.influence() is not what I should use for 
my problem ...?)
I have tried to use the R empinf() instead but I am stucked with a problem 
concerning the 

input argument "group" that I cannot translate in R.

Here is a copy of the S-plus "influence()" help concerning this
argument:
group : vector of length equal to the number of observations in data, for 
stratified sampling or 

multiple-sample problems. Sampling is done separately for each group (determined
by unique values 

of this vector). If data is a data frame, this may be a variable in the data 
frame, or expression 

involving such variables.

empinf() accepts an argument called "strata" but it doesn't seem
to correspond
to "group".

Below is a sample test showing my problem:

"testinflu" = function(data, weights) {    sum(data[,1]*weights) }
mydata <- cbind(c(1,2,3,4,5), c(1,1,1,1,0))

# In S-plus :
>  testinflu(data=mydata, weights=rep(1,length(mydata[,1])))
15
    
# In R:
>  testinflu(data=mydata, weights=rep(1,length(mydata[,1])))
15
    
    
# In S-plus :    
>  influence(data = mydata, statistic=testinflu)$L
          testinflu 
[1,] -2.000000e+000
[2,] -1.000000e+000
[3,] -1.776357e-013
[4,]  1.000000e+000
[5,]  2.000000e+000

# In R :
> empinf(data = mydata, statistic=testinflu)
[1] -2.000000e+00 -1.000000e+00  2.220446e-12  1.000000e+00  2.000000e+00
# ==> OK

# In S-plus :
> influence(data = mydata, statistic=testinflu, group = mydata[, 2])$L
     testinflu 
[1,]      -1.2
[2,]      -0.4
[3,]       0.4
[4,]       1.2
[5,]       0.0

# In R:
> empinf(data = mydata, statistic=testinflu, strata = mydata[, 2])
[1] -1.5 -0.5  0.5  1.5  0.0
# ==> NOT OK

So I have a few questions:
- has anyone already experienced the same kind of problem with the influence 
function ?
- is it possible to mimic the use of the "group" argument in empinf()
?


I have looked for answers on the web but couldn't find anythings really
helpful,
so if someone has an idea I would really appreciate it !! :)

Thanks,
Fred

ps : sorry for my broken English ...


      
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