[R] how to take this experiment with R?

[Gabor Grothendieck]

Suppose you just wanted variables 2,4 and 7 (which are in columns
3, 5 and 8, respectively).  Then you could do this:

   lm(X1 ~ X2 + X4 + X7, x)
or
   lm(X1 ~., x[,c(2,4,7)+1])


Date:   	Tue, 21 Sep 2004 12:27:38 +0800
From:   	rongguiwong <0034058 at fudan.edu.cn>
To:   	<r-help at stat.math.ethz.ch>
Subject:   	Re: [R] how to take this experiment with R?

ÔÚ 2004Äê9ÔÂ21ÈÕ ÐÇÆÚ¶þ 12:10£¬Gabor Grothendieck дµÀ£º
it works.
but i come across anather problem.
i just wnat to select 3 of the best indepent variables.
but the reslut from step(lm(X1 ~., x)) is :

Step: AIC= -37.64
X1 ~ X2 + X3 + X4 + X8 + X10 + X11 + X13 + X14 + X15 + X18 +
X21 + X22 + X25 + X26 + X27 + X29 + X30 + X31

Df Sum of Sq RSS AIC
<none> 11.014 -37.642
- X8 1 0.559 11.574 -37.165
- X27 1 0.867 11.881 -35.853
- X2 1 0.971 11.986 -35.416
- X18 1 0.978 11.992 -35.390
- X31 1 1.122 12.136 -34.793
- X10 1 1.400 12.414 -33.659
- X15 1 1.563 12.578 -33.005
- X29 1 1.608 12.622 -32.828
- X13 1 1.805 12.819 -32.055
- X30 1 1.880 12.894 -31.763
- X4 1 2.368 13.382 -29.906
- X14 1 2.495 13.509 -29.433
- X3 1 2.983 13.997 -27.659
- X22 1 2.999 14.013 -27.600
- X21 1 3.377 14.391 -26.271
- X25 1 4.323 15.338 -23.086
- X11 1 6.775 17.789 -15.672
- X26 1 7.232 18.246 -14.403

> You can express your model like this:
>
>
>
> lm(X1 ~., x)
>
>
>
> ?formula gives some help on formulas although the . notation
>
> above does not seem to be referred to there.
>
>
>
>
>
> Date:      Tue, 21 Sep 2004 11:52:04 +0800
>
> From:      rongguiwong <0034058 at fudan.edu.cn>
>
> To:      <r-help at stat.math.ethz.ch>
>
> Subject:      [R] how to take this experiment with R?
>
>
>
> i want to generate 30 independent variables and 1 dependent variable,each
> has
>
> 50 draws from a unit normal distribution.
>
> then, searching for the independent variables that together would do the
> best
>
> job for fitting the denpendent variabe.
>
>
>
> my way to generate the data is.
>
> x<-data.frame(matrix(rnorm(1550),c(50,31)))
>
> but is there more better way to do it?
>
>
>
> i want to use the followling is to search the model.
>
> model<-step(lm(X1~X2+X3+X4.....))
>
> but i don't know the to express the formula with lm function.i think there
> is
>
> a way the express it efficently.
>
> i try ?lm .but on result be found.
>
>
>
> any help is welcome!
>