Weighted mean with by function

Question

Trying to get weighted mean for a couple of categories want to use by(df$A,df$B,function(x) weighted.mean(x,df$C)) This doesn\'t work of course. Is there a way to do this using

Answer 1

Or simply recreate the calculation used by weighted.mean():

by(df,df$B,function(df)with(df,sum(A*C)/sum(C)))

df$B: gb
[1] 4
------------------------------------------------------------ 
df$B: hi
[1] 25.44444
------------------------------------------------------------ 
df$B: yo
[1] 3