> tapply(c(1,2,3,4,5,6),c("a","a","b","b","c","c"),mean)
  a   b   c 
1.5 3.5 5.5 

> Age <- c(3,4,5,3,4,5,6,6,4,4,5,6,3)
> Gender <- c('m','m','f','m','f','m','f','m','f','m','m','m','m')
> Hair <- c('y','g','b','b','a','r','g','b','b','b','g','y','b')
> tapply(Age, list(Gender, Hair), sum)
   a  b g  r  y
f  4  9 6 NA NA
m NA 16 9  5  9