[R] Constructing variables conditional on two indicators

[Dimitris Rizopoulos]

have a look at ave() and/or tapply().

I hope it helps.

Best,
Dimitris


Reynaerts, Jo wrote:
> Dear R users
> 
> I have a data frame that looks like this:
> 
> m         j           X1         X2
> 
> 1          1
> 1          2
> 1          ...
> 1          J
> 2          1
> 2          2
> 2          ...
> 2          J
> .
> .
> .
> M         1
> M         2
> M         ...
> M         J
> 
> The data frame essentially looks like a panel: m and j are indicators where m is 1:M and j is 1:J (and M > J); X1 and X2 are variables.  I would like to construct and add new variables to the data frame that are m- and j- specific, e.g., compute X3 where X3 is the sum of X1 of all j in m, and this for each m.  The new variable X3 is then added to the data frame (and is the same for all rows with the same m indicator).
> 
> How can I program this efficiently without running loops?
> 
> Thanks in advance
> 
> Jo Reynaerts
> 
> Ph.D. student
> LICOS Centre for Institutions and Economic Performance
> Katholieke Universiteit Leuven
> Debériotstraat 34/3511
> B-3000 Leuven
> Belgium
> Jo.Reynaerts at econ.kuleuven.be<mailto:Jo.Reynaerts at econ.kuleuven.be>
> 
> 
> 
> 
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> 
> 
> 
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-- 
Dimitris Rizopoulos
Assistant Professor
Department of Biostatistics
Erasmus University Medical Center

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