R help - Oct 2007 - Factor levels.

I have factors with levels ``Unit", "Achieved", and
"Scholarship"; I
wish to replace these with
"U", "A", and "S".

So I do

	fff <- factor(fff,labels=c("U","A","S"))

This works as long as all of the levels are actually present in the  
factor.  But if ``Scholarship'' is absent
(as if often is) then I get an error.

I can do a workaround such as

	fff <-
factor(c("U","A","S")[fff],levels=c("U","A","S"))

but this seems kludgy to me.

Is there a sexier way?

			cheers,

				Rolf Turner


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Would

levels(fff) <- c("A","S","U")

not work?  Can you send an example?

-Christos 
> -----Original Message-----
> From: r-help-bounces at r-project.org 
> [mailto:r-help-bounces at r-project.org] On Behalf Of Rolf Turner
> Sent: Wednesday, October 03, 2007 12:58 AM
> To: r-help list
> Subject: [R] Factor levels.
> 
> 
> I have factors with levels ``Unit", "Achieved", and 
> "Scholarship"; I wish to replace these with "U",
"A", and "S".
> 
> So I do
> 
> 	fff <- factor(fff,labels=c("U","A","S"))
> 
> This works as long as all of the levels are actually present 
> in the factor.  But if ``Scholarship'' is absent (as if often 
> is) then I get an error.
> 
> I can do a workaround such as
> 
> 	fff <-
factor(c("U","A","S")[fff],levels=c("U","A","S"))
> 
> but this seems kludgy to me.
> 
> Is there a sexier way?
> 
> 			cheers,
> 
> 				Rolf Turner
> 
> 
> ######################################################################
> Attention:\ This e-mail message is privileged and 
> confidenti...{{dropped}}
> 
> ______________________________________________
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> PLEASE do read the posting guide 
> http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
> 
>

Rolf Turner wrote:
> I have factors with levels ``Unit", "Achieved", and
"Scholarship"; I
> wish to replace these with
> "U", "A", and "S".
>
> So I do
>
> 	fff <- factor(fff,labels=c("U","A","S"))
>
> This works as long as all of the levels are actually present in the  
> factor.  But if ``Scholarship'' is absent
> (as if often is) then I get an error.
>
> I can do a workaround such as
>
> 	fff <-
factor(c("U","A","S")[fff],levels=c("U","A","S"))
>
> but this seems kludgy to me.
>   
Does it even work? (What if it is the first or the 2nd level that is 
absent?)

The canonical way is

factor(fff, levels=c("Unit", "Achieved",
"Scholarship"),
labels=c("U","A","S"))
> Is there a sexier way?
>
> 			cheers,
>
> 				Rolf Turner
>   

-- 
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  c/ /'_ --- Dept. of Biostatistics     PO Box 2099, 1014 Cph. K
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~~~~~~~~~~ - (p.dalgaard at biostat.ku.dk)                  FAX: (+45) 35327907

On 03-Oct-07 04:57:33, Rolf Turner wrote:
> I have factors with levels ``Unit", "Achieved", and
"Scholarship"; I
> wish to replace these with
> "U", "A", and "S".
> 
> So I do
> 
>       fff <-
factor(fff,labels=c("U","A","S"))
> 
> This works as long as all of the levels are actually present in the  
> factor.  But if ``Scholarship'' is absent
> (as if often is) then I get an error.
> 
> I can do a workaround such as
> 
>       fff <-
factor(c("U","A","S")[fff],levels=c("U","A","S"))
> 
> but this seems kludgy to me.
> 
> Is there a sexier way?
>                       cheers,
>                               Rolf Turner
Maybe that particular issue could be worked round with something
derived (e.g. take the leading character in the strings) from

  labels=unique(fff)

but that doesn't work round what seems to be a loose specification
of the correspondence between "U" and "Unit", "A"
and "Achieved",
"S" and "Scholarship", in that factor(fff), unless told
otherwise,
will see the levels in the order "Achieved", "Scholarship",
"Unit"
but take the labels in the order "U", "A", "S".

For example, the following mimics your description but with
shorter strings, starting with a vector of "data":
> fff0<-rep(c("Uu","Aa","Ss"),5)
> factor(fff0)
 [1] Uu Aa Ss Uu Aa Ss Uu Aa Ss Uu Aa Ss Uu Aa Ss
Levels: Aa Ss Uu
> factor(fff0,labels=c("U","A","S"))
 [1] S U A S U A S U A S U A S U A
Levels: U A S

so now U->Aa, A->Ss, S->Uu

Maybe it would be enough for you to use

  labels=sort(substr(unique(fff0),1,1))

so that for:
> fff0<-rep(c("Uu","Aa","Ss"),5)
> factor(fff0)
 [1] Uu Aa Ss Uu Aa Ss Uu Aa Ss Uu Aa Ss Uu Aa Ss
Levels: Aa Ss Uu
> factor(fff0,labels=sort(substr(unique(fff0),1,1)))
 [1] U A S U A S U A S U A S U A S
Levels: A S U

while for:
> fff0<-rep(c("Uu","Aa",),8)
> factor(fff0)
 [1] Uu Aa Uu Aa Uu Aa Uu Aa Uu Aa Uu Aa Uu Aa Uu Aa
Levels: Aa Uu
> factor(fff0,labels=sort(substr(unique(fff0),1,1)))
 [1] U A U A U A U A U A U A U A U A
Levels: A U



Hoping this helps,
Ted.

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E-Mail: (Ted Harding) <Ted.Harding at manchester.ac.uk>
Fax-to-email: +44 (0)870 094 0861
Date: 03-Oct-07                                       Time: 07:40:54
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