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Re: st: Selecting variables corresponds to observation numbers
From
Gordon Hughes <[email protected]>
To
[email protected]
Subject
Re: st: Selecting variables corresponds to observation numbers
Date
Tue, 08 Mar 2011 10:50:08 +0000
For me, the crucial lesson that I had forgotten was how easy it is to
extend a macro variable list by appending an additional variable.
But I was initially surprised that the method worked in my case since
the contents of varlist extends to ~500 variables, while the Section
18.3.4 of the User Guide says that the expression handler is limited
to 244 characters. The difference between extending a macro variable
(by adding something at the end) and evaluating the macro variable is
critical. Similarly, I did not know that referring to a macro that
does not exist - i.e. `varlist' in the first execution of -local
varlist `varlist' var`j' - in effect creates an empty macro of that name.
This is what I meant in referring to the subtle details of Stata's
macro language which are hard to fathom simply by reading the
manuals. Perhaps Kit Baum might like to consider a fuller treatment
of macros in a future edition of An Introduction to Stata Programming.
Gordon
------------------------------
Date: Mon, 7 Mar 2011 17:59:11 +0000
From: Nick Cox <[email protected]>
Subject: Re: st: Selecting variables corresponds to observation numbers
I agree, but let's make it easier. There were really two things to
learn in my example, the idea of a local macro and the idea of a loop:
local N = _N
forval i = 1/`N' {
local j = ind[`i']
local varlist `varlist' var`j'
}
keep `varlist'
Using `= ' to cut down on the code was at best a stylistic detail and
at worst something that might have obscured the code for learners.
Nick
On Mon, Mar 7, 2011 at 5:50 PM, Gordon Hughes <[email protected]> wrote:
> Thank you, Nick. This is a really nice solution that does the task much
> more efficiently than the best method I had come up (using index vectors in
> Mata).
>
> There is [sigh] so much to learn about how to get the best out of Stata's
> macro language.
>
> Gordon Hughes
> [email protected]
>
>> ------------------------------
>>
>> Date: Mon, 7 Mar 2011 00:05:36 +0000
>> From: Nick Cox <[email protected]>
>> Subject: Re: st: Selecting variables corresponds to observation numbers
>>
>> forval i = 1/`=_N' {
>> local varlist `varlist' var`=ind[`i']'
>> }
>>
>> keep `varlist'
>>
>> Nick
>>
>> On Sun, Mar 6, 2011 at 10:38 PM, Gordon Hughes <[email protected]>
>> 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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