R dcast aggregate function first non NA

Question

Could somebody tell me how to create an aggregate function for dcast that would return the first non-NA value?

Something along these lines (while the example is obviously not working)

wide<-dcast(long, usr_id ~ variable, value.var = "value", fun.aggregate = head, 1, na.rm=true)

The data could look like this:

set.seed(28)
long <- data.frame(
  usr_id= rep(1:2, each=4),
  variable= rep(c('M1','M2'), 4),
  value= sample(c(NA,1:2),2*4, replace=TRUE)
)

Answer 1

We can try

dcast(long, usr_id~variable, value.var="value",
   fun.aggregate= function(x) head(x[!is.na(x)], 1), fill=0)

data

set.seed(28)
long <- data.frame(usr_id= rep(1:3, each=3),
    variable= rep(paste0("M", 1:3), 3), value= sample(c(NA, 
           1:3),9, replace=TRUE))