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

[Gabor Grothendieck]

Slight correction of the English:

- if A is 1 then the first term equals the coefficient of (A == 1).
  That is the first term equals 1 if B==1 and equals -1 if B==2.
  If A does not equal 1 then the first term is zero and can be ignored.
- terms 2 and 3 are interpreted analogously
- if A==3 (or other value) then the (A==?) part of each term equals
  zero so the entire expression is zero.

On Jan 22, 2008 2:08 PM, Gabor Grothendieck <ggrothendieck at gmail.com> wrote:
> 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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> >
>