[R] recoding one variable into another - but differently for different cases

[Gabor Grothendieck]

You could create a lookup table or use recode in the car package.

Another possibility is to use a logical/arithmetic expression.  The
following expression says that

- if A is 1 then use the first term equals the coefficient, namely 1
if B ==1 and -1 if B == 2.
  Also, if A is not 1 then that term is zero and can be ignored.
- if A is 2 or 99 then the second or third terms are used analogously
- otherwise no terms are selected and the expression equals zero

   transform(Data, new =
      (A == 1) * ((B == 1) - (B == 2)) + (A == 2) * ((B == 2) - (B ==
1)) + (A == 4) * 99)

This could be reduced even more although at the expense of
understandability, e.g.

   transform(Data, new = ifelse(A > 2, 99 * (A == 4), (A == B) - (A != B)))

On Jan 22, 2008 12:25 PM, Dimitri Liakhovitski <ld7631 at gmail.com> wrote:
> Hello,
> I have 2 variables in my sample Data: Data$A and Data$B
> Variable Data$A can assume values: 1, 2, 3, and 4.
> Variable Data$B identifies my cases and can assume values: 1 and 2.
>
> I need to recode my variable Data$A into a new variable Data$new such that:
>
> People who are Data[Data$B %in% 1, ]  are recoded like this:
>
> Value on Data$A    Value on Data$new
> 1                             +1
> 2                             -1
> 3                             0
> 4                             99
>
> People who are Data[Data$B %in% 2, ]  are recoded like this:
>
> Value on Data$A    Value on Data$new
> 1                             -1
> 2                             +1
> 3                             0
> 4                             99
>
> I am having hard time doing this. Any help would be greatly appreciated.
> Dimitri
>
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