Meaning of "I" in the R Language

Question

Naturally, I had little success searching Google for "R I", "I in R", and "R language I".

• The R help says "Change the class of an object to indicate that it should be treated ‘as is’.".
• My O'Reilly R book doesn't have an entry for it in its index.
• My Cambridge book essentially says, about "I(logdist^2)": "ensures that is taken as the square, rather than as an interaction of logdist".

Can someone explain the "interaction" comment? Can someone explain why "logdist^2" wouldn't be interpreted in the traditional way?

Answer 1

From p89 of R in a Nutshell

Caret(^) [is] Used to indicate crossing to a specific degree. For example:

y~(u+w)^2

is equivalent to

y~(u+w)*(u+w)

Identity function (I()) Used to indicate that the enclosed expression should be interpreted by it's arithmetic meaning. For example

a+b 

means that both a and b should be included in the formula. The formula:

I(a+b)  

means that "a plus b" should be included in the formula. See also ?AsIs()