Does anyone know an easy way to calculate the rolling 20 period average
or sum of a vector?

For instance:
x <- rnorm(1000)

y <- apply.subset(x,20,fun="sum")

The first element of y would contain the sum of elements 1 to 20, the
second element of y 
would contain the sum of elements 2:21, and so on.

I thought I had seen this on the list a year or so ago, but I couldn't
find anything in the archives.


Thanks in advance,
Whit

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