tapply changing order of factor levels?

Jim's advice is patently false. Please read ?tapply for correct details.

Counterexample:
y <- rnorm(6)
x <- factor(rep(factor(letters[1:3],lev = letters[3:1]),2))
x
[1] a b c a b c
Levels: c b a
tapply(y,x,mean)
c          b          a 
 0.4545897 -1.0544782  0.4682773   

Bert Gunter
Genentech Nonclinical Statistics

-----Original Message-----
From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-project.org] On
Behalf Of jim holtman
Sent: Wednesday, May 06, 2009 6:57 AM
To: Chirantan Kundu
Cc: r-help at r-project.org
Subject: Re: [R] tapply changing order of factor levels?

The result of 'tapply' is just a named vector and the names are in
alphabetical order.  If you want them printed in a different order, then you
have to specify it.  Since you have the order in 'mylevels', this will work:
str(mysummary)
int [1:3(1d)] 4 3 2
 - attr(*, "dimnames")=List of 1
  ..$ : chr [1:3] "IN0019800021" "IN0020020064" "IN0020020155"
mysummary[mylevels]
IN0020020155 IN0019800021 IN0020020064
           2            4            3

Hi,

Does tapply change the order when applied on a factor? Below is the code I
tried.

mylevels<-c("IN0020020155","IN0019800021","IN0020020064")

mydata<-c("IN0020020155","IN0019800021","IN0020020064","IN0020020155","IN001
9800021","IN0019800021","IN0020020064","IN0020020064","IN0019800021")
myfactor<-factor(mydata,levels=mylevels)
myfactor
[1] IN0020020155 IN0019800021 IN0020020064 IN0020020155 IN0019800021
IN0019800021 IN0020020064 IN0020020064 IN0019800021
Levels: IN0020020155 IN0019800021 IN0020020064
summary(myfactor)
IN0020020155 IN0019800021 IN0020020064
          2            4            3

# Everything fine upto this point. The order of levels is maintained as it
is.

mysummary<-tapply(myfactor,mydata,length)
mysummary
IN0019800021 IN0020020064 IN0020020155
          4            3            2

# Now the order has changed.

Is this the expected behavior? Any idea on how to avoid the change in
order?

Regards,
Chirantan

____________________________________
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and provide commented, minimal, self-contained, reproducible code.

Jim Holtman
Cincinnati, OH
+1 513 646 9390

What is the problem that you are trying to solve?

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______________________________________________
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and provide commented, minimal, self-contained, reproducible code.

tapply changing order of factor levels?

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