[R] Mapping factors to a new set of factors

[james.arnold at sssc.uk.com]

Thanks Jim,

I managed to get my function to work by putting it within an sapply. Unfortunately the performance was terrible, so I've gone with the suggestion of other posters to use recode in the car package.

Cheers,
James

-----Original Message-----
From: jim holtman [mailto:jholtman at gmail.com] 
Sent: 08 September 2009 18:37
To: Arnold, James
Cc: r-help at r-project.org
Subject: Re: [R] Mapping factors to a new set of factors

use 'ifelse'

# not tested; you supply data for the '%in%'
map <- function(x){
	ifelse(x %in% c('a','b'), "North",
	ifelse(x %in% c('c','d'), "South",
	ifelse(x %in% c('e', 'f'), "East",
	ifelse(x %in% c('g', 'h'), "West", NA))))
}

On Tue, Sep 8, 2009 at 12:01 PM, <james.arnold at sssc.uk.com> wrote:
> Hello,
>
> I am trying to map a factor variable within a data frame to a new variable whose entries are derived from the content of the original variable and there are fewer factors in the new variable. That is, I'm trying to set up a surjection.
>
> After first thinking that this would be a common operation and would have a quite simple interface, I can not seem to find one, nor any similar posts on this topic (please correct me if there is something).
>
> Therefore, I have written a function to perform this mapping. However, the function I have written doesn't seem to work with vectors greater than length 1, and as such is useless. Is there any way to ensure the function would work appropriately for each element of the vector input?
>
> mapLN <- function(x)
> {
>        Reg <- levels(df$Var1)
>        if (x==Reg[1] | x==Reg[2] | x==Reg[13] | x==Reg[17] | x==Reg[20] | x==Reg[23] | x==Reg[27]) {"North"} else
>        if (x==Reg[3] | x==Reg[5] | x==Reg[7] | x==Reg[14] | x==Reg[15] | x==Reg[24] | x==Reg[30]) {"East"} else
>        if (x==Reg[4] | x==Reg[6] | x==Reg[8] | x==Reg[9] | x==Reg[11] | x==Reg[16] | x==Reg[18] | x==Reg[21] | x==Reg[22] | x==Reg[25] | x==Reg[28] | x==Reg[29] | x==Reg[31]) {"West"} else
>        if (x==Reg[10] | x==Reg[12] | x==Reg[19] | x==Reg[26] | x==Reg[32]) {"South"} else
>        stop("Not in original set")
> }
>
> Many thanks,
> James
>
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-- 
Jim Holtman
Cincinnati, OH
+1 513 646 9390

What is the problem that you are trying to solve?

This E-Mail is confidential and intended solely for the use of the individual to whom it is addressed.  If you are not the addressee, any disclosure, reproduction, copying, distribution or other dissemination or use of this communication is strictly prohibited.  If you have received this transmission in error please notify the sender immediately by replying to this e-mail, or telephone 01382 207 222, and then delete this e-mail.

All outgoing messages are checked for viruses however no guarantee is given that this e-mail message, and any attachments, are free from viruses.  You are strongly recommend to check for viruses using your own virus scanner.  Neither SCRC or SSSC will accept responsibility for any damage caused as a result of virus infection.