Re: st: ambiguity in -if- qualifier

[Nick Cox <njcoxstata@gmail.com>]

I don't think the one precise example here is puzzling in any sense.
Previous values of -mpg- are put in a new variable if and only
-foreign- is 1. This is calculated observation by observation.

You allude to different behaviour with -egen-. But the help for -egen- explains

"Explicit subscripting (using _N and _n), which is commonly used with
    generate, should not be used with egen; see subscripting."

That may illuminate your puzzlement.

Nick
njcoxstata@gmail.com


On 22 March 2014 21:26, Yu Chen, PhD <profyuchen@gmail.com> wrote:
> I think there is some ambiguity in the meaning and usage of the -if-
> qualifier. Generally, the command is performed on a subset that meets
> the -if- condition. However, a command may perform many tasks, and the
> subset for each task is not clear sometimes. For example, for the
> -generate- command, it seems to calculate the result of the expression
> on the full sample first, and then that result is assigned to a
> subsample that meets the -if- condition. However, for the -egen-
> command, the calculation is performed on a subset that meets the -if-
> condition, not the full sample, and then that result is assigned to
> the new variable on that subsample.
>
> For example, see the code below.
>
> sysuse auto
> gen mpg2=mpg[_n-1] if foreign==1
>
> Notice that observation number 53 has a value of 24 for mpg2. This
> indicates that the task of taking a lagged value is performed on the
> full sample first. Otherwise, this value should be missing. But -egen-
> works differently.
>
> There may exist other cases that have similar ambiguities. I would
> suggest that Stata have a clear rule to address this issue. If the
> rule is already out there, please tell me.
> Thank you very much.
>
> Yu Chen
> *
> *   For searches and help try:
> *   http://www.stata.com/help.cgi?search
> *   http://www.stata.com/support/faqs/resources/statalist-faq/
> *   http://www.ats.ucla.edu/stat/stata/
*
*   For searches and help try:
*   http://www.stata.com/help.cgi?search
*   http://www.stata.com/support/faqs/resources/statalist-faq/
*   http://www.ats.ucla.edu/stat/stata/