count NAs per week

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.

Thanks for the reproducible example: there are many ways to do this
(aggregate, tapply, ave, etc.) but they are all based on the paradigm
of: break up your data by "Week" --> apply the function "function(x)
sum(is.na(x))" --> recombine. (See, inter alia, the JSS paper on the
plyr package) I'm not at a computer with R right now so this is a
little untested, but one way might be:

with(dattrial, tapply(a, Week, function(x) sum(is.na(x))))

Cheers,
Michael

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