r - Group function by two variables on data.table -

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my data looks this

students<-data.table(studid=c(1:6) ,faculty= c("it","science", "law","it","it","it"),                   sex=c("male","male","male","female","female","male"), wam=c(65,35,98,55,20,80))  studid   faculty    sex      ave_mark (wam) 1                male     65 2        science    male     35 3        law        male     98 4                female   55 5                female   20 6                male     80

i have used following code calculate averages

degrees[, mean(wam, na.rm=t),by=faculty][order(-v1)]

so headings are

faculty    vi              65      law        50

etc

any advice on how appreciated.

i break sex also

faculty    vi     vi            male   female         65     11 law        50     11

you try 

dcast.data.table(students, faculty~sex, fun.aggregate=mean, na.rm=true,                    value.var='wam') #    faculty female male #1:        37.5 72.5 #2:     law    nan 98.0 #3: science    nan 35.0

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