writing equations

Dear R-list members,

I have a data frame with two columns (X, Y). How can I write the following
equation (using nls)

log(X2/X1) ~ a + b * (Y1+Y2)/2 + c * (X1+X2)/2

To calculate the fitted values for each line (Xi) I have to use numbers in
consecutive lines (Xi, Xi+1, Yi and Yi+1).

What I do is build a lagged version of the data frame.  Since I tend to
use lagged data with different lags a lot, I'll generally predefine a lag
called tau (in your example tau=1) and then use it like so:

tau <- 1
theFrame <- data.frame(x1=X[1:(length(X)-tau)],
                       x2=X[(1+tau):length(X)],
                       y1=Y[1:(length(X)-tau)],
                       y2=Y[(1+tau):length(X)])

Now you can use these new lagged variables however you want in your formula.

Cheers,
Steve Boker
sboker at nd.edu
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