Re: st: RE: create local macros named after the values of a categorial variable

[Jeph Herrin <stata@spandrel.net>]

Seems like Oliver wants to create a 2xN array - a map between
city codes and integers 1..N. Why not do this as a matrix?

 levelsof citycode, local(ccodes)
 matrix input A = (`ccodes')

Now you can get the kth city code as, eg,

 local Ck=el("A",1,k)

or just loop over k to get them all.

hth,
Jeph




On 8/12/2010 11:44 AM, Nick Cox wrote:
> There is no context here. Although it is not the answer you seek, it's
> my experience that you rarely need to do what you are attempting. If you
> need the value of the ith observation of a variable you can always go
> something like
>
> `=`varname'[`i']'
>
> There is no gain in also putting that value in a local macro. That
> typically just adds another layer of indirection.
>
> If you told us why you think you need to do this, you would probably get
> better advice.
>
> Nick
> n.j.cox@durham.ac.uk
>
> Oliver Jones
>
> while working on a programming problem a came in need
> of creating a set of local macros that are
>    i) named after the values of a categorial variable,
>       e.g. city codes
> ii) contain the integer values from 1 to N, where N is
>       the number of distinct city codes
>
> I worked out to alternatives that do the job, but would
> very much appreciate to get any feedback telling me if
> there exists some third alternative that is faster
> (in terms of computing time).
>
> Here is an example of what I do:
> -------------begin example-----------------------------
> clear
> set obs 3
> local varname = "city_code"
> gen `varname' = _n
> expand = 4
> list, abbreviate(9)
>
> preserve
> ******
> *
> * Alternative 1 (sort and keep)
> display "Start alternative 1"
> sort `varname'
> by `varname': gen byte index = _n
> keep if index == 1
> quietly count
> forvalues i = 1/`r(N)' {
> 	local macro_name = `varname'[`i']
> 	local `macro_name' = `i'
> 	display "For City Code " `varname'[`i'] " the" ///
> 		" value of the local macro is ``macro_name''"
> }
> display "end"
> * end
> ***
>
> restore
> ******
> *
> * Alternative 2 (levelsof)
> display "Start alternative 2"
> levelsof `varname'
> tokenize `r(levels)'
> * Now we got one local macro for each of the
> * city_codes, named 1, 2, ... , N.
> * But we want to exchange the names
> * of the macros by their values (the city_codes)
> * and vice versa.
>
> * get the number of different city_codes
> local N_citys = 1
> while "``N_citys''" != "" {
> 	local ++N_citys
> }
> local --N_citys
> dis "{text}Number of distinct codes: {result}`N_citys'"
>
> forvalues i = 1/`N_citys'{
> 	local new_mac_name = ``i''
> 	local `new_mac_name' = `i'
> 	local `i'	//delete the macros we no longer need
> 	display "For City Code " `new_mac_name' " the" ///
> 		" value of the local macro is {result}``new_mac_name''"
> }
> display "end"
> * end
> ***
> ---------------end example-----------------------------
>
> The datasets for which this is needed are quite big: 30 million
> observations
> each year and I got datasets for 24 years.
> The number of macros to be created amounts to approx. 330, i.e. 330
> different
> city codes. But this number is not constant in each year.
>
> So far alternative 1 (sort and keep) needs about 6 minutes to run and is
> outperformed by alternative 2 (levelsof) which need about 1 minutes.
>
> *
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