Count of NAs colwise by a group [duplicate]

Answer 1

We can use aggregate

aggregate(.~grp, data=dat, FUN= function(x) sum(is.na(x)))

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Or with dplyr

library(dplyr)
dat %>%
    group_by(grp) %>%
    summarise_each(funs(sum(is.na(.)))

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Or using data.table

library(data.table)
setDT(dat)[, lapply(.SD,  function(x) sum(is.na(x))), grp]

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Or as @David Arenburg mentioned in the comments, rowsum is another option where we can do the group by operation while summing. We used + to coerce the logical matrix (is.na(dat)) to binary as the function will not work with logical class.

 rowsum(+(is.na(dat)), dat$grp)