Calculating the weighted mean of all numerical columns - r

Question

Example data:
library(data.table)
set.seed(1)
DT <- data.table(panelID = sample(50,50), # Creates a panel ID
Country = c(rep("Albania",30),rep("Belarus",50), rep("Chilipepper",20)),
some_NA = sample(0:5, 6),
some_NA_factor = sample(0:5, 6),
Group = c(rep(1,20),rep(2,20),rep(3,20),rep(4,20),rep(5,20)),
Time = rep(seq(as.Date("2010-01-03"), length=20, by="1 month") - 1,5),
wt = 15*round(runif(100)/10,2),
Income = round(rnorm(10,-5,5),2),
Happiness = sample(10,10),
Sex = round(rnorm(10,0.75,0.3),2),
Age = sample(100,100),
Educ = round(rnorm(10,0.75,0.3),2))
DT [, uniqueID := .I] # Creates a unique ID # https://stackoverflow.com/questions/11036989/replace-all-0-values-to-na
DT$some_NA_factor <- factor(DT$some_NA_factor)
I would like to calculate the weighted mean of all numerical columns, so I tried:
DT_w <- DT[,lapply(Filter(is.numeric,.SD), function(x) weighted.mean(DT$wt, x, na.rm=TRUE)), by=c("Country", "Time")]
But then it says:
Error in weighted.mean.default(DT$wt, x, na.rm = TRUE) :
'x' and 'w' must have the same length
I think I am perhaps misunderstanding the syntax. Am I doing this right?

Answer 1

Two issues:
when you use DT$wt that is an explicit call to the full wt column from the DT table - the by arguments won't work on it. The by arguments will only work on columns without the DT$ prefix.
The order of arguments for weighted.mean() is x first and w (weights) second - you seem to have this backwards
Fixing those two issues:
DT_w <- DT[,lapply(Filter(is.numeric,.SD), function(x) weighted.mean(x, w = wt, na.rm=TRUE)), by=c("Country", "Time")]
# runs without errors