Re: st: Duplicate variable only if a previous variable has a certain value

[David de Jong <davidcdejong@gmail.com>]

Nick,

Your solution much more sensible and worked like a charm. The general
approach will be also be helpful to me in the future.

Thanks so much!

David
~
David C. de Jong, MA
Department of Clinical and Social Sciences in Psychology
University of Rochester
214 Meliora Hall
Rochester, NY 14627


On Fri, Nov 29, 2013 at 6:40 PM, Nick Cox <njcoxstata@gmail.com> wrote:
> The syntax for -generate- always starts
>
> generate newvar = ...
>
> where newvar is a new variable name.
>
> You are expecting Stata to evaluate the expression you give and use it
> as the new variable name. For that you need the extra scaffolding of
> `=  '
>
> generate `=subinstr(`v',"_w_sub400","corr",1)'  = ...
>
> But this would be more Stataish and easier to read:
>
> qui forval j = 21/164 {
>
>     gen rt`j'corr = rt`j'_w_sub400  if correct`j'int == 0
>
> }
>
> Think: what are the variable elements, what are the constant elements?
> And cycle over the variable elements along.
>
> Nick
> njcoxstata@gmail.com
>
>
> On 29 November 2013 23:27, David de Jong <davidcdejong@gmail.com> wrote:
>
>> I'm trying to duplicate a set of variables (response times for a set
>> of trials) so that the original value is reproduced only if the trial
>> response was correct (which is indicated by an earlier set of
>> variables). If the trial response was incorrect the duplicated
>> variable should be missing.
>>
>> Trials are numbered 21-164. Whether the trial was correct is stored in
>> variables correct21int-correct164int (0=correct, 1=incorrect, byte).
>> Response times are stored in variables rt21_w_sub400-rt164_w_sub400
>> (float).
>>
>> So, for trial 21, I want to create a new variable called rt21corr that
>> is equal to rt21_w_sub400 if correct21int equals 0.
>>
>> I've tried to do this with the following loop:
>>
>> qui foreach v of var rt21_w_sub400-rt164_w_sub400 {
>> generate subinstr(`v',"_w_sub400","corr",1) = `v' if
>> subinstr(subinstr(`v',"_w_sub400","int",1),"rt","correct","1") == 0
>> }
>>
>> In case -substr- wanted all of its constituent parts in quotes I tried this:
>>
>> qui foreach v of var rt21_w_sub400-rt164_w_sub400 {
>> generate subinstr("`v'","_w_sub400","corr",1) = `v' if
>> subinstr("subinstr("`v'","_w_sub400","int",1)","rt","correct","1") ==
>> 0
>> }
>>
>> Both of these return the error message:
>> ( invalid name
>>
>> Can anyone help me figure out how to make this syntax work, or, if
>> there is a more elegant way to do what I hope to do.
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