R help - Dec 2004 - Problem with stepAIC, "invalid second argument"

Dear all,

am I doing something wrong, or is this a bug (R 2.0.1 & 1.9.1,
linux&windows):

    > library(MASS)
    > df <- data.frame(x = rnorm(100), y = rnorm(100), z = rnorm(100))
    > f <- lm(x ~ y + z, data = df)
    > stepAIC(f, scope = list(upper = ~y * z, lower = ~1))
    Start:  AIC= -16.56
     x ~ y + z
    Error in eval(expr, envir, enclos) : invalid second argument
    >


I can avoid the problem doing the following, but in my application I do
not want to use attach / detach because of possible name conflicts:

    > attach(df)
    > f <- lm(x ~ y + z)
    > stepAIC(f, scope = list(upper = ~y * z, lower = ~1))
    Start:  AIC= 23.55
     x ~ y + z

    ...

    Call:
    lm(formula = x ~ 1)

    Coefficients:
    (Intercept)
        -0.1972
    > detach(df)


Thanks for any help,
Klaus Thul

Try ?df to see what you did wrong!

Always use conflicts() before posting to see what you have overridden.
R normally allows you to get away with masking objects, but in this case 
it is the R object that is masking yours.


On Sun, 26 Dec 2004, Klaus Thul wrote:
> am I doing something wrong, or is this a bug (R 2.0.1 & 1.9.1,
> linux&windows):
>
>    > library(MASS)
>    > df <- data.frame(x = rnorm(100), y = rnorm(100), z = rnorm(100))
>    > f <- lm(x ~ y + z, data = df)
>    > stepAIC(f, scope = list(upper = ~y * z, lower = ~1))
>    Start:  AIC= -16.56
>     x ~ y + z
>    Error in eval(expr, envir, enclos) : invalid second argument
-- 
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595