Hi Vassilis,

Try

test.df$y <- with(test.df, x1*w + x2*(1-w))
test.df

HTH,
Jorge


On Thu, Jan 6, 2011 at 8:33 AM, Vassilis <> wrote:

>
> 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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>
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