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

Here's a simple and efficient solution using data.table

library(data.table)
setDT(df)[, .(WM = weighted.mean(A, C)), B]
#     B       WM
# 1: hi 25.44444
# 2: gb  4.00000
# 3: yo  3.00000

Or using split and apply combination from base R

sapply(split(df, df$B), function(x) weighted.mean(x$A, x$C))
#      gb       hi       yo 
# 4.00000 25.44444  3.00000 

Or

library(dplyr)
df %>%
  group_by(B) %>%
  summarise(WM = weighted.mean(A, C))
# Source: local data frame [3 x 2]
# 
# B       WM
# 1 gb  4.00000
# 2 hi 25.44444
# 3 yo  3.00000