## mark a section (Learn more)

> 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

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