group by in R, ddply with weighted.mean

Question

I am trying to do a \"group by\" - style weighted mean in R. With some basic mean the following code (using the plyr package from Hadley) worked well.

ddply         


        

Answer 1

Use an anonymous function:

> ddply(iris,"Species",function(X) data.frame(wmn=weighted.mean(X$Sepal.Length,
+                                                               X$Petal.Length),
+                                             mn=mean(X$Sepal.Length)))
     Species      wmn    mn
1     setosa 5.016963 5.006
2 versicolor 5.978075 5.936
3  virginica 6.641535 6.588
> 

This computes a weighted mean of Sepal.Length (weighted by Petal.Length) as well as unweighted mean and returns both.

Answer 2

Use summarise (or summarize):

ddply(iris, "Species", summarise, 
  wmn = weighted.mean(Sepal.Length, Petal.Length),
  mn = mean(Sepal.Length))