[R] efficient way of aggregating two variables at once

[John Christie]

Hi,
	I'm working out d' and therefore I'd like to aggregate two variables  
at once (hits and false alarms).  In  the raw data frame is subject,  
stimulus_presence, and response information.  I aggregate this into a  
table now currently by separating out the target_presence=true data  
and working out a hit rate and then workout out the false alarm rate  
from the target_presence=false data.  Then I cbind the "x" column  
from the false alarm aggregate to the hit rate data.  Then I can also  
append a dprime column if I wish.  I am just inquiring as to whether  
there is some facility I am missing for aggregating the variables  
simultaneously or just generating the d' in one step.