Re: st: Selecting variables corresponds to observation numbers

[Phil Clayton <philclayton@internode.on.net>]

In the example you've given, all you've done is drop the variables that don't 
have a corresponding entry in ind. 
 
Are you sure you don't want something more like this: 
ind  newvar 
2     2 
4     9 
 
If so, one slightly clunky way of doing it would be: 
 
gen newvar=. 
count 
foreach obs of numlist 1/`=r(N)' { 
  replace newvar=var`=ind[`obs']' in `obs' 
} 
drop var* 
 
Phil 
 
On Mon 07/03/11 09:38 , Gordon Hughes G.A.Hughes@ed.ac.uk sent: 
> 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. 
> Gordon Hughes 
> g.a.hughe 
> s@ed.ac.uk 
> * 
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>  
>  
 

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