A look at the tutorial might help here, but anyway:

Say you have that dataframe down there with the name myData (you should use 
dput() to give us the data btw),

then you can subset that by using myData[rows,columns], where left of the comma 
you define which rows you want, and right of the comma is which columns you 
want. If you leave it blank, all will be selected.

So you want all rows from the columns, which names are in aa, then you would 
write

myData[ , aa]

On 30.10.2012, at 15:09, alex_123 wrote:

> hello , I am new user in R . I have datafile (class = data.frame) which has
> 825 columns with unique column name i want extract 200 selected column from
> datafile how can I do this?
> my datafile look like..
>            Mi         RBN         RBF        nDB         nX
> 3   2.6225979  0.53132756 -0.80599902 -1.4471864 -0.5705269
> 10  0.4818746 -1.72143092 -2.19579027  2.0118824 -0.5705269
> 12  2.8519611  1.88298265  0.09614617  0.6282549 -0.5705269
> 20  0.6347834 -0.36977583  0.63255683  1.3200687 -0.5705269
> 
> 
> I want to extract data with unique name of column  , by  "Mi","RBF" etc . I
> have stored character vector of desired column names. eg  aa =
> c("Mi","RBF",..)
> 
> 
> 
> 
> 
> --
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> 
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