[R] Converting a Vector into a 2-level Factor

[ntyson at clovermail.net]

Hello Everyone,

Let's say I have a vector as follows:

vowels = c("aa", "aa", "ah", "ao", "eh)

I want to make a factor consisting of two levels: the things that are  
"aa" and those that are not "aa".  How would I do that?

I tried using factor(vowels, levels=c("aa", "~aa"), exclude=NULL), but  
this gives me '<NA>' or missing values, which is not what I want.  If  
there is no way to get two levels, then how do I convert the NAs to  
another symbol of my choosing?

Any suggestions are welcome.

Thanks.