Re: st: Selecting variables corresponds to observation numbers

[Nick Cox <njcoxstata@gmail.com>]

forval i = 1/`=_N' {
         local varlist `varlist' var`=ind[`i']'
}

keep `varlist'

Nick

On Sun, Mar 6, 2011 at 10:38 PM, Gordon Hughes <G.A.Hughes@ed.ac.uk> wrote:
> I would be very grateful if someone could suggest an efficient way of
> implementing the following task.
>
> I have a dataset with M observations and N variables where N >> M and the
> variables are named var1-varN.  In addition, I have an index variable ind
> which takes M unique values in the range 1..N.  I want to select the
> variables that correspond to the M index values.  For example, the data
> might be
>
> ind  var1  var2  var3  var4  var5
> 2     1      2      3      4      5
> 4     6      7      8      9      10
>
> so I want to create a dataset consisting of the following observations &
> variables
>
> ind  var2  var4
> 2     2      4
> 4     7      9
>
> However rather than 5 variables and 2 observations I have more like 5000
> variables and 500 observations.  I can do this using reshape or xpose and
> merging files, but this is very slow when N > 5000 and I want to repeat the
> exercise many times.  Another alternative is to use permutation matrices in
> Mata, since what I am trying to do is equivalent to shuffling rows and
> columns in some rather large matrices.  Still, I feel that there should be a
> cleverer way of doing it by manipulating varlists in Stata but I haven't
> come up with a solution.
>

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