[R] Mapping factors to a new set of factors

[jim holtman]

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?