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

[Dimitri Liakhovitski]

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