Why is `poly` complaining about degree less than number of unique points?

Question

I am trying to generate orthogonal polynomials in R, but I keep getting an error I don't understand

> poly(1:1000, 50)
Error in poly(1:1000, 50) : 
  'degree' must be less than number of unique points

Surely the number of unique points is 1000? What does it mean? Is this a bug, and if so does anyone know I work around?

Edit: This appears to kick in for degree > 27 for any number of points - is this an undocumented limit?

Out of curiosity, why are you wanting to generate a 50th order polynomial? I sincerely hope it is not for modeling... — Justin, Aug 01 '13 at 16:00
No it's not for modelling, but now you put it like that, I'm feeling less surprised that it doesn't work for 50! I was so engrossed in what I was doing I forgot about what most people probably use this function for. (It's a bit complex to explain briefly, but it is to do with generating random analytic functions over an interval) — Corvus, Aug 01 '13 at 16:06

score 15 · Accepted Answer · answered Aug 01 '13 at 16:03

15

Numerical overflow. If you look at the code for poly, you'll see it's generating the individual polynomial terms as an intermediate step:

X <- outer(x, seq_len(n) - 1, "^")

and when n (the degree of the polynomial you want) is 50, the resulting terms go up to 1e132.

answered Aug 01 '13 at 16:03

Hong Ooi

Great thanks - I can't complain, as Justin pointed out above, this isn't really the target usage for this function! – Corvus Aug 01 '13 at 16:09

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