Function to count NA values at each level of a factor

Question

I have this dataframe:

set.seed(50)
data <- data.frame(age=c(rep(\"juv\", 10), rep(\"ad\", 10)),
                   sex=c(rep(\"m\", 10), rep(\"f\", 10)),         


        

Answer 1

A data.table approach:

library(data.table)
DT <- data.table(data)
DT[, lapply(.SD, function(x) sum(is.na(x))) , by = list(age,sex,size)]
##    age sex  size length width height
## 1: juv   m large      5     4      4
## 2:  ad   f small      3     4      4

and the plyr equivalent using colwise and ddply

ddply(data, .(age,sex,size), colwise(.fun = function(x) sum(is.na(x))))
##   age sex  size length width height
## 1  ad   f small      3     4      4
## 2 juv   m large      5     4      4

You could always use a vector of column names for the by components

by.cols <- c('age', 'sex' ,'size')
# then the following will work....
DT[, lapply(.SD, function(x) sum(is.na(x))), by = by.cols]
ddply(data, by.cols, colwise(.fun = function(x) sum(is.na(x))))