[R] Mapping factors to a new set of factors

[james.arnold at sssc.uk.com]

Thanks Jorge,

That is what I was looking for.

Cheers,
James

From: Jorge Ivan Velez [mailto:jorgeivanvelez at gmail.com] 
Sent: 08 September 2009 18:41
To: Arnold, James
Cc: r-help at r-project.org
Subject: Re: [R] Mapping factors to a new set of factors

Hi James,

Take a look at the "recode" function in the "car" package. It might be useful in this case.

HTH,
Jorge

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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