[R] count NAs per week

[Tagmarie]

Even though I work with R since a year or so I still struggle with simple
problems. I hope someone can help me with this. Been trying for days and am
a little frustrated now. 

I have a data frame somewhat like the one bellow:

dattrial<-data.frame(a=c(1,NA,rnorm(4,10)), Week=c(3,3,3,4,4,4))

I want to know how many NAs I have in week 3 and in week 4. 



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