st: ambiguity in -if- qualifier

["Yu Chen, PhD" <profyuchen@gmail.com>]

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
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