[R] User defined function within a formula

[Kunshan Yin]

Hello, I have a question about the formula and the user defined function:

I can do following:
###Case 1:
> clotting <- data.frame(
+     u = c(5,10,15,20,30,40,60,80,100),
+     lot1 = c(118,58,42,35,27,25,21,19,18),
+     lot2 = c(69,35,26,21,18,16,13,12,12))
> g1=glm(lot1 ~ log(u) + poly(u,1), data = clotting, family = Gamma)
> dc=clotting
> dc$u=1
> predict(g1,dc)
          1           2           3           4           5
6           7           8           9
-0.01398929 -0.01398929 -0.01398929 -0.01398929 -0.01398929 -0.01398929
-0.01398929 -0.01398929 -0.01398929

However, if I just simply wrap the poly as a user defined function ( in
reality I would have my own more complex function)  then I will get error:
###Case 2:
> xpoly<-function(x,degree=1){poly(x,degree)}
> g2=glm(lot1 ~ log(u) + xpoly(u,1), data = clotting, family = Gamma)
> predict(g2,dc)
Error in poly(x, degree) :
  'degree' must be less than number of unique points

It seems that the predict always treat the user defined function in the
formula with I().  My question is how can I get the  results for Case2 same
as case1?

Anyone can have any idea about this?

Thank you very much.

Alex

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