st: Re: re: st code skips based on conditions

[Clyde B Schechter <clyde.schechter@einstein.yu.edu>]

Yesterday, Dan Klein wrote:

"I am running some foreach tabulate loops the results of which are
accumulat ed by matrix commands. (see code below).
 I would like skip (at the point *** in the code) to the next level of
local levels based on a conditional test of the number
of observations contained in the scalar e(N) which is returned after
each tabulated combination of the foreach variables.
Have not been able to find anything (or even a lead) that could help me do this.

Any suggestions or ideas of how to go about this would be much appreciated.
"

and both Nick Cox and Maarten Buis advised him to use the -continue- command.

That will certainly work.  Some of us who lived through the birth of "structured programming" might argue that -continue- represents a "dangerous" style of programming.  While I think the word "dangerous" greatly overstates the case, you can accomplish the same thing with:

foreach [whatever] {
         [code to be done in all cases]
         if ![skip condition] {
               [code to be done only if skip condition is not met]
        }
}

Some advantages of this approach, not necessarily in order of importance, are:

1.  You don't have to learn a new command that may be infrequently needed.
2.  When you go back to your code 6 months from now, you won't have to remember what that command, which you have not used again since then, and had never used before, does.
3.  You won't confuse the meaning of -continue- with the possibly different meaning assigned to that word in other programming languages you may use.
4.  The semantics of the foreach loop is retained in its purest form, with each iteration having the same entry and exit points.
5.  If you later need to change the algorithm so that something else is actually done to all cases, but must be done after the code that is sometimes skipped, then you can just insert the additional code at the end of the foreach block without any other changes to the code.  If you used -continue- there, then the code surgery gets a bit more complicated.

It's just a matter of style and, to be honest, I do use -continue- myself.  But I generally reserve it for situations where the code that would go in the block begun by "if ![skip condition]" is itself long and complicated, and especially if that block itself contains multiple levels of indentation.

Clyde Schechter
Dept. of Family & Social Medicine
Albert Einstein College of Medicine
Bronx, NY, USA


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