[R] weighed mean of a data frame row-by-row

[Vassilis]

Dear list, 

This must be an easy one. I have a data frame like this one:

test.df	<- data.frame(x1=c(2,3,5), x2=c(5, 3, 4), w=c(0.8, 0.3, 0.5))

and I want to construct a weighted mean of the first two columns using the
third column for weighting; i.e.

y[1] = x1[1]*w[1] + x2[1]*(1-w[1])
y[2] = ...

One way to do this is to use a loop like

test.df$y	<-numeric(3)

with(test.df,
for(i in 1:length(w)) {
test.df$y[[i]]  <<- weighted.mean(c(x1[[i]],x2[[i]]),c(w[[i]],1-w[[i]]) ) }
)

My question is whether you can suggest a way to do the same without using a
`for' loop, a vectorized version of this snippet - My actual dataset is
large and it involves calculating the weighted mean of many columns. Such a
loop becomes ugly to write and quite slow.... 

Thanks in advance, 

Vassilis


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